sufficiently to emerge on the side of incidence. This scattering of the rays has been investigated by Eve, McLennan, Schmidt, Crowther and others. It has been found that the scattering for different chemical elements is connected with their atomic weight and their position in the periodic table. McCelland and Schmidt have given theories to account for the absorption of B rays by matter. The whole problem of absorption and scattering of particles by substances is very complicated, and the question is still under active examination and discussion. The negative charge carried by the B rays has been measured by a number of observers. It has been shown by Rutherford and Makower that the number of particles expelled per second from one gram of radium in equilibrium is about that to be expected if each atom of the Bray products in breaking up emits one ẞ particle.

Heat Emission of Radioactive Matter.-In 1903 it was shown by Curie and Laborde (52) that a radium compound was always hotter than the surrounding medium, and radiated heat at a constant rate of about 100 gram calories per hour per gram of radium. The rate of evolution of heat by radium has been measured subsequently by a number of observers. The latest and most accurate determination by Schweidler and Hess, using about half a gram of radium, gave 118 gram calories per gram per hour (53). There is now no doubt that the evolution of heat by radium and other radioactive matter is mainly a secondary phenomenon, resulting mainly from the expulsion of a particles. Since the latter have a large kinetic energy and are easily absorbed by matter, all of these particles are stopped in the radium itself or in the envelope surrounding it, and their energy of motion is transformed into heat. On this view, the evolution of heat from any type of radioactive matter is proportional to the kinetic energy of the expelled a particles. The view that the heating effect of radium was a measure of the kinetic energy of the a particles was strongly confirmed by the experiments of Rutherford and Barnes (54). They showed that the emanation and its products when removed from radium were responsible for about three-quarters of the heating effect of radium in equilibrium. The heating effect of the radium emanation decayed at the same rate as its activity. In addition, it was found that the ray products, viz. the emanation radium A and radium C, each gave a heating effect approximately proportional to their activity. Measurements have been made on the heating effect of uranium and thorium and of pitchblende and polonium. In each case, the evolution of heat has been shown to be approximately a measure of the kinetic energy of the a particles.

Experiments on the evolution of heat from radium and its emanation have brought to light the enormous amount of energy accompanying the transformation of radioactive matter where a particles are emitted. For example, the emanation from one gram of radium in equilibrium with its products emits heat initially at the rate of about 90 gram calories per hour. The total heat emitted during its transformation is about 12,000 gram calories. Now the initial volume of the emanation from one gram of radium is 6 cubic millimetres. Consequently one cubic centimetre of emanation during its life emits 2 X 10 gram calories. Taking the atomic weight of the emanation as 222, one gram of the emanation emits during its life 2 X 109 gram calories of heat. This evolution of heat is enormous compared with that emitted in any known chemical reaction. There is every reason to believe that the total emission of energy from any type of radioactive matter during its transformation is of the same order of magnitude as for the emanation. The atoms of matter must consequently be regarded as containing enormous stores of energy which are only released by the disintegration of the atom.

A large amount of work has been done in measuring the amount of the thorium and radium emanation in the atmosphere, and in determining the quantity of radium and thorium distributed on the surface of the earth. The information already obtained has an important bearing on geology and atmospheric electricity.

REFERENCES.-1. H. Becquerel, Comptes Rendus, 1896, pp. 420, 501, 559, 689, 762, 1086; 2. Rutherford, Phil. Mag., Jan. 1899; 3. Mme Curie, Comptes Rendus, 1898, 126. p. 1101; M and Mme Curie and G. Bémont, ib., 1898, 127. p. 1215; 4. Mme Curie, ib., 1907, 145. p. 422; 5. Thorpe, Proc. Roy. Soc., 1908, 80. p. 298; 6. Giesel, Phys. Zeit., 1902, 3. p. 578; 7. Giesel, Annal. d. Phys., 1899, 69. p. 91; Ber., 1902, p. 3608; 8. Rutherford and Boltwood, Amer. Journ. Sci., July 1906; 9. Debierne, Comptes Rendus, 1899, 129. p. 593; 1900, 130. p. 206; 10. Giesel, Ber., 1902, p. 3608; 1903, p. 342; 11. Marckwald, ib., 1903, p. 2662; 12. Mme Curie and Debierne, Comptes Rendus, 1910, 150. p. 386; 13. Boltwood, Amer. Journ. Sci., May 1908; 14. Rutherford, Phil. Mag., Feb. 1903, Soddy, ib., May 1903; 17. Rutherford and Soddy, ib., Nov. 1902; Oct. 1906; 15. Rutherford, ib., Jan. 1900; 16. Rutherford and 18. M and Mme Curie, Comptes Rendus, 1899, 129. p. 714; 19. Rutherford, Phil. Mag., Jan. and Feb. 1900; 20. Rutherford and Soddy, ib., Sept. and Nov. 1902, April and May 1903; Rutherford, Phil. Trans., 1904, 204A. p. 169; 21. Russ and Makower, Proc. Roy. Soc., 1909, 82A. p. 205; 22. Hahn, Phys. Zeit., 1909, 10. p. 81; 23. Rutherford, Phil. Mag., Nov. 1904, Sept. 1905; 24. Meyer and Schweidler, Wien. Ber., July 1905; 25. Antonoff, Phil. Mag., June 1910; 26. Cameron and Ramsay, Trans. Chem. Soc., 1907, P. 1266; Rutherford, Phil. Mag., Aug. 1908; 27. Cameron and Ramsay, Proc. Roy. Soc., 1908, 81A. p. 210; Rutherford and Royds, Phil. Mag., 1908, 16. p. 313; Royds, Proc. Roy. Soc., 1909, 82A. p. 22; Watson, ib., 1910, 83A. p. 50; 28. Rutherford, Phil. Mag., 1909; 29. Gray and Ramsay, Trans. Chem. Soc., 1909, pp. 354, 1073; 30. Rutherford and Soddy, Phil. Mag., Sept. and Nov. 1902; 31. Hahn, Proc. Roy. Soc., March 1905; Phil. Mag., June 1906; Ber., 40. pp. 1462, 3304; Phys. Zeit., 1908, 9. pp. 245, 246; 32. Hahn, Phil. Mag., Sept. 1906; 33. Godlewski, ib., July 1905; 34. Boltwood, ib., April 1905; 35. Strutt, Trans. Roy. Soc., 1905A.; 36. McCoy, Ber., 1904, p. 2641; 37. Soddy, Phil. Mag., June 1905, Aug. 1907, Oct. 1908, Jan. 1909: 38. Boltwood, Amer. Journ. Sci., Dec. 1906, Oct. 1907, May 1908, June 1908; 39. Boltwood, ib., April 1908: 40. Boltwood, ib., Oct. 1905, Feb. 1907; 41. Rutherford and Geiger, Proc. Roy. Soc., 1908, 81A. Proc. Roy. Soc., 1908, 81A. p. 280; 1910, 83. p. 404; 44. Boltwood p. 141; 42. Rutherford and Royds, Phil. Mag., Feb. 1909; 43. Dewar, and Rutherford, Manch. Lit. and Phil. Soc., 1909, 54. No. 6; 45. Bragg and Kleeman, Phil. Mag., Dec. 1904, Sept. 1905; 46. Rutherford, ib., Aug. 1906; 47. Geiger, Proc. Roy. Soc., 1910, 83A. P: 505; 48. Geiger, ib., 1910, 83A. p. 492; 49. Rutherford and Geiger, ib., 1908, 81A. pp. 141, 163; 50. Crookes, ib., 1903; 51. Regener, Verhandl. d. D. Phys. Ges., 1908, 10. p. 28; 52. Curie and Laborde, Comptes Rendus, 1904, 136. p. 673: 53. Schweidler and Hess, Wien. Ber., June 1908, 117; 54. Rutherford and Barnes, Phil. Mag., Feb. 1904.

General treatises are: P. Curie, Euvres, 1908; E. Rutherford, Radioactive Transformations, 1906; F. Soddy, Interpretation of Radium, 1909; R. J. Strutt, Becquerel Rays and Radium, 1904: W. Makower, Radioactive Substances, 1908; J. Joly, Radioactivity and Geology, 1909. See also Annual Reports of the Chemical Society. (E. Ru.)

RADIOLARIA, so called by E. Haeckel in 1862 (Polycystina, by C. G. Ehrenberg, 1838), the name given to Marine Sarcodina, in which the cytoplasmic body gives off numerous fine radiating pseudopods (rarely anastomosing) from its surface, and is provided with a chitinous "central capsule," surrounding the inner part which encloses the nucleus, the inner and outer cytoplasm communicating through either one or three apertures or numerous pores in the capsule. The extracapsular cytoplasm is largely transformed into a gelatinous substance (" calymma "), through which a granular network of plasm passes to form a continuous layer bearing the pseudopods at the surface; this gelatinous layer is full of large vacuoles," alveoli,” as in other pelagic Sarcodina (Heliozoa, q.v.), Globigerinidae, &c., among Foraminifera (q.v.). The protoplasm may contain oil-globules, pigment-grains, reserve-grains and crystals. There is frequently a skeleton present, either of silica (pure or containing a certain amount of organic admixture), or of “acanthin (possibly a proteid, allied to vitellin, but regarded by W. Schewiakoff as a hydrated silicate of calcium and aluminium); never calcareous or arenaceous. The skeleton may consist of spicules, isolated or more or less compacted, or form a latticed shell, which, in correlation with the greater resistance of its substance, is of lighter and more elegant structure than in the Foraminifera. The alveoli contain a liquid, which, as shown by Brandt, is rich in carbon dioxide, and in proportion to its abundance may become much lighter than sea-water; and possibly the gelatinous substance of the calymma is also lighter than the medium. In Acantharia the protoplasm at the base

of the projecting spines is often differentiated into a bundle of fibres converging on to the spines some way up (distally); these, comparable to the myonemes of Infusoria (q.v.), &c., and termed "myophrisks", possibly serve to drag outwards the surface and so extend it, with concurrent dilatation of the alveoli, and lower the specific gravity of the animal. In this group also a thick temporary flagellum "sarcoflagellum" may be formed, apparently by the coalescence of a number of pseudopodia. The pigmented mass or "phaeodium" in the ectoplasm of Phaeodaria, appears to be an excretory product, formed within the central capsule and passing immediately outwards; a similar uniform deposit of pigmented granules occurs in the Colloid species, Thalassicolla nucleata. The wall of the central capsule is simple in the Spumellaria, but formed of two layers. in the Nassellaria and Phaeodaria. In the Nassellaria the oscule is simply a perforated area, and a cone of differentiated fibres in the intracapsular cytoplasm has its base on it: it is termed the "porocone," and the fibres may possibly be muscular (myonemes). In Phaeodaria, the inner membrane at each oscule is prolonged through the outer into a tube (" proboscis "): the outer membrane of the principal oscule forms a large radially

name of Polycystina (1838), but without more than a very slight knowledge of a few living forms. T. H. Huxley in 1851 made the first adequate study of the living animal, and was followed by Joh. Müller in the same decade. E. Haeckel began his publications in 1862, and in two enormous, abundantly illustrated, systematic works, besides minor publications, has dealt exhaustively with the cytology, classification, and distribution of the class. Next in value come the contributions of Richard Hertwig (largely developmental), besides those of L. Cienkowsky, Karl Brandt and A. Borgert, while to F. Dreyer and V. Häcker we owe valuable studies on the physical relations of the skeleton.

Our classification is taken from Haeckel.

A. Spumellaria, Haeck. (Peripylaea, Hertwig). Central capsule perforated with numerous evenly distributed pores. Skeleton siliceous, latticed or of detached spicules, or absent. Form homaxonic or with at least three planes of symmetry intersecting at right angles, rarely irregular or spiral, sometimes forming colonies, i.e. with several central capsules in a common external cytoplasm.

striated circular plate, the "astropyle," or "operculum." The innermost shell of some with concentric shells may lie within the central capsule, or even within the nucleus; this is due to the growth of these organs after the initial shell is formed, so that they pass out by lobes through the latticed openings of the embryonic shell, which lobes ultimately coalesce outside the embryonic chamber, and so come finally to invest it (fig. III. 17). In some, a symbiosis occurs with Zooxanthella, Brandt, a Flagellate of the group Chrysomadineae, which in the resting state inhabits the extracapsular cytoplasm growing and dividing freely therein, and only (under study) becoming free and flagellate on the death of the host (fig. III. 4, 6-13). The Silicoflagellata or Dictyochidae, also possessing a vegetable colouring matter, but with a skeleton of impure silica (like that of Phaeodaria), may pass some of their lives in symbiosis with Radiolaria.

Living Radiolaria were first observed and partially described by W. J. Tilesius in 1803-6 and 1814, by W. Baird in 1830, and by C. G. Ehrenberg in 1831, as luminous organisms in the sea; F. J. F. Meyen in 1834 recognized their animal character and the siliceous nature of their spicules. Ehrenberg a little later described a large number of Nassellarian skeletons under the

FIG. II.-Eucyrtidium cranioides, Haeck. ; X150; one of the Nassellaria. Entire animal as seen in the living condition. The central capsule is hidden by the beehive-shaped siliceous shell within which it is lodged.

I. Skeleton of detached spicules, or absent.
Fam. 1.

Fam. 2.

COLLOIDEA. Skeleton absent. Thalassicolla, Huxl. (figs. 1. and III. 1); Thalassophysa, Haeck.; Collozoum, Haeck. (fig. III. 2-5, 15, 16); Actissa, Haeck.

BELOIDEA. Skeleton

spicular. Sphaerozoum, Haeck.; Raphidozoum, Haeck. II. Skeleton latticed or spongy-reticulate. Fam. 3.

SPHAEROIDEA. Skeleton homaxial, sometimes colonial. Collosphaera, Mull.; Haliomma, Ehrb.; Actinomma, Haeck. (fig. III. 17), showing concentric latticed shells, the smallest intranuclear, all connected by radial spines; Spongosphaera, Haeck. (fig. iv. 8); Heliosphaera, Haeck. (fig. III. 14).

Fam. 4. PRUNOIDEA. Skeleton a prolate spheroid or cylinder of circular section, sometimes constricted like a dice-box.

disposed on five successive zones of four on alternating meridians, the zones corresponding to equator, tropics and circumpolar circles on the globe; pores of central capsule in scattered groups. Fam. 1. ACTINELIDA. Spines numerous,

more

than

twenty, irregularly grouped. Litholophus, Haeck.; Xiphacantha, Haeck.

Fam. 2. ACANTHONIDA. Spines twenty, simple, usually equal. Acanthometra, J. Müll. (fig. iv. 6, 7); Astrolonche, Haeck.; Amphilonche, Haeck. (fig. 111. 18).

Fam. 3. SPHAEROPHRACTIDA: Spines equal, branching and often coalescing into a latticed shell, homaxonic.

Fam. 4. PRUNOPHRACTIDA: Branching spines coalescing into a latticed shell which is elongated and elliptical in at least one plane.

C. Nassellaria, Haeck. (Monopylaea, Hertw.). Silico-skeletal Radiolaria in which the central capsule is typically monaxonic (coneshaped), with a single perforate area (pore-plate) placed on the basal face of the cone; the membrane of the capsule, the nucleus single; the skeleton is extracapsular, and forms a scaffold-like or beehivelike structure of monaxonic form, a tripod or calthrop, a sagittal ring, or a combination of these.

Fam. 1. NASSOIDEA, Haeck. Skeleton absent. Cystidium,
Haeck.

Fam. 2.

PLECTIDA, Haeck. Skeleton formed of a single

branching spicule, a tripod or usually a 4radiate calthrop, its branches sometimes reticulate. Genera: Plagiacantha, Haeck.; Plegmatium, Haeck.

Fam. 3. SPYROIDEA. Shell latticed around the sagittal
ring ("cephalis "), sometimes with a lower
chamber added.

Fam. 4. BOTRIDEA, Haeck. Shell latticed, composed of
several chambers agglomerated without definite
order; a single central capsule. Genera:
Botryocyrtis, Haeck.; Lithobotrys, Haeck.
Fam. 5. CYRTOIDEA, Haeck. Skeleton a monaxonic or
triradiate shell, or continuous piece (beehive-
shaped). Genera: Halicalyptra, Haeck.;
Eucyrtidium, Haeck. (fig. 11.); Carpocanium,
Haeck. (fig. IV. 3).

Fam. 6. STEPHOIDEA, Haeck. Skeleton a sagittal ring
continuous with the branched spicule, and
sometimes growing out into other rings or
branches. Genera: Acanthodesmia, Haeck.;
Zygostephanus, Haeck.; Lithocircus, Haeck.
(fig. IV. 1).

D. Phaeodaria, Haeck. (Tripylaea, Hertw.). Radiolaria of cruciate symmetry, prolonged into tubular processes with three oscula to the central capsule, one inferior, the principal, and two symmetrically placed on either side of the opposite pole; skeleton of spicules, a network of hollow filaments, or a minutely alveolate shell, of a combination of silica with organic substance; extracapsular protoplasm containing in front of the large oscule an agglomeration of dusky purplish or greenish pigment ("phaeodium ").

Fam. 1. PHAEOCYSTIDA, Haeck. Siliceous skeleton absent or of separate needles. Genera: Aulacantha, Haeck.; Thalassoplancta, Haeck.

a

Fam. 2. PHAEOSPHAERIDA. Spicules united into latticed shell. Genera: Aulosphaera, Haeck. (fig. IV. 9); Auloplegma, Haeck.; Cannacantha, Haeck.

Fam. 3. PHAEOG ROMIDA, Haeck. Shell continuous, traversed by fine canals or finely alveolate, provided with at least one pylome. Genera: Challengeria, Wyv., Thomson; Lithogromia, Haeck.

Fam. 4. PHAEOCONCHIDA. Shell as in Phaeosphaerida, but of two symmetrical halves (valves), which meet in the plane of the three oscules (" frontal of Haeckel, who terms the plane of symmetry through the shells "sagittal"). Genera: Conchidium, Haeck.; Coelodendrum, Haeck. (fig. IV. 4).

The following passages may be repeated here from Sir E. Ray Lankester's article "Protozoa " in the 9th edition of this Encyclopaedia:

"The important differences in the structure of the central capsule of different Radiolaria were first shown by Hertwig, who also discovered that the spines of the Acanthometridea consist not of silica but of an organic compound (but see above). In view of this latter fact and of the peculiar numerical and architectural features of the Acanthometrid skeleton, it seems proper to separate them altogether from the other Radiolaria. The Peripylaea may be regarded as the starting-point of the Radiolarian pedigree, and have given rise on the one hand to the Acanthometridea, which

b.

9

13

FIG. III.-Radiolaria. 1. Central capsule of Thalassicolla nucleata, Huxley, in radial section. a, the large nucleus (Binnenbläschen); b, corpuscular structures of the intracapsular protoplasm containing concretions; c, wall of the capsule (membranous shell), showing the fine radial pore-canals; d, nucleolar fibres (chromatin substance) of the nucleus. 2, 3. Collozoum inerme, J. Müller, two different forms of colonies, of the natural size. 4. Central capsule from a colony of Collozoum inerme, showing the intracapsular protoplasm and nucleus, broken up into a number of spores, the germs of swarm-spores or flagellulae; each encloses a crystalline rod. c, yellow cells lying in the extracapsular protoplasm. 5. A small colony of Collozoum inerme, magnified 25 diameters. a, alveoli (vacuoles) of the extracapsular protoplasm; b. central capsules, each containing besides protoplasm a large oil-globule. 6-13. Yellow cells of various Radiolaria: 6, normal yellow cell; 7. 8, division with formation of transverse septum; 9, a modified condition according to Brandt; 10, division of a yellow cell into four; 11, amoeboid condition of a yellow cell from the body of a dead Sphaerozoon; 12, a similar cell in process of division; 13, a yellow cell the protoplasm of which is creeping out of its cellulose envelope. 14. Heliosphaera inermis, Haeck., living example; a, nucleus; b, central capsule; c, siliceous basket-work skeleton. Two swarm-spores (flagellulae) of Collozoum inerme, set free

15.

from such a central capsule as that drawn in 4; each contains a crystal b and a nucleus a. 16. Two swarm-spores of Collozoum inerme, of the second kind, viz. devoid of crystals, and of two sizes, a macrospore and a microspore. They have been set free from central capsules with contents of a different appearance from that drawn in 4. a, nucleus. 17. Actinomma asteracanthion, Haeck.; one of the Peripylaea. Entire animal in optical section. a, nucleus; b, wall of the central capsule; c, innermost siliceous shell enclosed in the nucleus; c1, middle shell lying within the central capsule; c2, outer shell lying in the extracapsular protoplasm. Four radial siliceous spines holding the three spherical shells together are seen. The radial fibrillation of the protoplasm and the fine extracapsular pseudopodia are to be noted. 18. Amphilonche messanensis, Haeck.; one of the Acanthometridea. Entire animal as seen living.

of the protoplasmic body. a, the tri-lobed nucleus; b, the siliceous shell; c, oil-globules; d, the perforate area (pore-plate) of the central capsule. 4. Coelodendrum gracillimum, Haeck.; living animal, complete; one of the Tripylaea. a, the characteristic dark pigment (phaeodium) surrounding the central capsule b. The peculiar branched siliceous skeleton, consisting of hollow fibres, and the expanded pseudopodia are seen. 5. Central capsule of one of the Tripylaea, isolated, showing a, the nucleus; b, c, the inner and the outer laminae of the capsule wall; d, the chief or polar aperture; e, e, the two secondary apertures. 6, 7. Acanthometra claparedei, Haeck. 7 shows the animal in optical section, so as to exhibit the characteristic meeting of the spines at the central point as in all Acanthometridea; 6 shows the transition from the uninuclear to the multinuclear condition by the breaking up of the large nucleus. a, small nuclei; b, large fragments of the single nucleus; c, wall of the central capsule; d, extracapsular jelly (not protoplasm); e, peculiar intracapsular yellow cells. 8. Spongosphaera streptacantha, Haeck.; one of the Peripylaea. Siliceous skeleton not quite completely drawn on the right side. a, the spherical extracapsular shell (compare fig. III. 17), supporting very large radial spines which are connected by a spongy network of siliceous fibres. 9. Aulosphaera elegantissima, Haeck.; one of the Phaeodaria. Half of the spherical siliceous skeleton.

retain the archaic structure of the central capsule whilst developing a peculiar skeleton, and on the other hand to the Monopylaea and Phaeodaria, which have modified the capsule but retained the siliceous skeleton.

FIG. IV. Radiolaria. 1. Lithocircus annularis, Hertwig; one of the Monopylaea. Whole animal in the living state (optical section); a, nucleus; b, wall of the central capsule; c, yellow cells; d, perforated area of the central capsule (Monopylaea). 2. Cystidium inerme, Hertwig; one of the Monopylaea. Living animal. An example of a Monopylaeon destitute of skeleton. a, nucleus; b, capsule-wall; c, yellow cells in the extracapsular protoplasm. 3. Carpocanium diadema, Haeck.; optical section of the beehive-shaped shell to show the form and position

Archi-peripylaea. RADIOLARIA.

"The occasional total absence of any siliceous or acanthinous skeleton does not appear to be a matter of classificatory importance, since skeletal elements occur in close allies of those very few forms which are totally devoid of skeleton. Similarly it does not appear to be a matter of great significance that some forms (Polycyttaria) form colonies, instead of the central capsules separating from one another after fission has occurred.

"It is important to note that the skeleton of silex or acanthin does not correspond to the shell of other Sarcodina, which appears rather to be represented by the membranous central capsule. The skeleton does, however, appear to correspond to the spicules of Heliozoa, and there is an undeniable affinity between such a form as Clathrulina and the Sphaerid Peripylaea (such as Heliosphaera, fig. III. 14). The Radiolaria are, however, a very strongly marked group, definitely separated from all other Sarcodina by the membranous central capsule sunk in their protoplasm. Their differences inter se do not affect their essential structure. The variations in the chemical composition of the skeleton and in the perforation of the capsule do not appear superficially. The most obvious features in which they differ from one another relate to the form and complexity of the skeleton, a part of the organism so little characteristic of the group that it may be wanting altogether. It is not known how far the form-species and form-genera which have been distinguished in such profusion by Haeckel as the result of a study of the skeletons are permanent (ie. relatively permanent) physiological species. There is no doubt that very many are local and conditional varieties, or even merely stages of growth, of a single Protean species. The same remark applies to the species discriminated among the shell-bearing Reticularia. It must not be supposed, however, that less importance is to be attached to the distinguishing and recording of such forms because we are not able to assert that they are permanent species.

The streaming of the granules of the protoplasm has been observed in the pseudopodia of Radiolaria as in those of Heliozoa and Reticularia; it has also been seen in the deeper protoplasm; and granules have been definitely seen to pass through the pores of the central capsule from the intracapsular to the extracapsular protoplasm. A feeble vibrating movement of the pseudopodia has been occasionally noticed.

"The production of swarm-spores has been observed only in Acanthometra and in the Polycyttaria and Thalassicollidae, and only in the two latter groups have any detailed observations been made. Two distinct processes of swarm-spore production have been observed by Cienkowski, confirmed by Hertwig, distinguished by the character of the resulting spores, which are called 'crystalligerous' and 'isospores' (fig. III. 15) in the one case, and dimorphous or anisospores in the other (fig. III. 16). In both processes the nucleated protoplasm within the central capsule breaks up by a more or less regular cell-division into small

pieces, the details of the process differing a little in the two cases. In those individuals which produce crystalligerous swarm-spores, each spore encloses a small crystal (fig. 111. 15). On the other hand, in those individuals which produce dimorphous swarm-spores, the contents of the capsule (which in both instances are set free by its natural rupture) are seen to consist of individuals of two sizes, 'megaspores' and microspores,' neither of which contain crystals (fig. 111. 16). The further development of the spores has not been observed in either case. Both processes have been observed in the same species, and it is suggested that there is an alternation of sexual and asexual generations, the crystalligerous spores developing directly into adults, which in their turn produce in their central capsules dimorphous swarm-spores (megaspores and microspores), which in a manner analogous to that observed in the Volvocinean Flagellata copulate (permanently fuse) with one another (the larger with the smaller) before proceeding to develop. The adults resulting from this process would, it is suggested, produce in their turn crystalligerous swarm-spores. Unfortunately we have no observations to support this hypothetical scheme of a life-history. "Fusion or conjugation of adult Radiolaria, whether preliminary to swarm-spore-production or independently of it, has not been observed this affording a distinction between them and Heliozoa. Simple fission of the central capsule of adult individuals, preceded of course by nuclear fission, and subsequently of the whole protoplasmic mass, has been observed in several genera of Acantharia and Phaeodaria, and is probably a general method of reproduction in the group. In Spumellaria it gives rise to colonial Polycyttarian forms when the extracapsular protoplasm does not divide. "The siliceous shells of the Radiolaria are found abundantly in certain rocks from Palaeozoic times onwards. They furnish, together with Diatoms and Sponge spicules, the silica which has been segregated as flint in the Chalk formation. They are present in quantity (as much as 10%) in the Atlantic ooze, and in the celebrated Barbados earth' (a Tertiary deposit) are the chief components."

BIBLIOGRAPHY.-The most important systematic works are those of E. Haeckel, Die Radiolarien (1862-87), and the "Report on the Radiolaria of the Challenger" Expedition (vol. xviii., 1887), which contains full lists of the older literature. Among the most important recent studies we cite K. Brandt, Die Koloniebildenden Radiolarien" in Fauna and Flora des Golfes von Neapel, xii. (1885); A. Borgert in Zeitschrift f. Wissenschaftliche Zoologie, li. (1891), and Zoologische Jahrbücher (Anatomie), xiii. (1900); F. Dreyer in Jenaischer Zeitschr., xix. (1892); V. Hacker in Zeitsch. f. Wiss. Zool., lxxxiii. (1905). (M. HA.)

RADIOMETER. It had been remarked at various times, amongst others by Fresnel, that bodies delicately suspended within a partial vacuum are subject to apparent repulsion by radiation. The question was definitely investigated by Sir W. Crookes, who had found that some delicate weighings in vacuo were vitiated by this cause. It appeared that a surface blackened so as to absorb the radiant energy directed on it was repelled relatively to a polished surface. He constructed an apparatus in illustration, which he called a radiometer or lightmill, by pivoting a vertical axle carrying equidistant vertical vanes inside an exhausted glass bulb, one side of each vane being blackened and the other side bright, the blackened sides all pointing the same way round the axle. When the rays of the sun or a candle, or dark radiation from a warm body, are incident on the vanes, the dark side of each vane is repelled more than the bright side, and thus the vanes are set into rotation with accelerated speed, which becomes uniform when the forces produced by the radiation are balanced by the friction of the pivot and of the residual air in the globe. The name radiometer arose from an idea that the final steady speed of rotation might be utilized as a rough measure of the intensity of the exciting radiation.

The problem of the cause of these striking and novel phenomena at first produced considerable perplexity. A preliminary question was whether the mechanical impulsion was a direct effect of the light, or whether the radiation only set up internal stresses, acting in and through the residual air, between the vanes and the walls of the enclosure. The answer to this was found experimentally by Arthur Schuster, who suspended the whole instrument in delicate equilibrium, and observed the effect of introducing the radiation. If the light exerted direct impulsion on the vanes, their motion would gradually drag the case round after them, by reason of the friction of the residual air in the bulb and of the pivot. On the other

hand, if the effects arose from balanced stresses set up inside the globe by the radiation, the effects on the vanes and on the case would be of the nature of action and reaction, so that the establishment of motion of the vanes in one direction would involve impulsion of the case in the opposite direction; but when the motion became steady there would no longer be any torque either on the vanes or on the case, and the latter would therefore come back to its previous position of equilibrium; finally, when the light was turned off, the decay of the motion of the vanes would involve impulsion of the case in the direction of their motion until the moment of the restoring torque arising from the suspension of the case had absorbed the angular momentum in the system. Experiment showed that the latter prediction was what happened. The important part played by the residual air in the globe had also been deduced by Osborne Reynolds from observing that on turning off the light, the vanes came to rest very much sooner than the friction sidence is an illustration of Maxwell's great theoretical disof the pivot alone would account for; in fact, the rapid subcovery that viscosity in a gas (as also diffusion both of heat and of the gas itself) is sensibly independent of the density. Some phenomena of retardation in the production of the effect had led Sir G. G. Stokes and Sir W. Crookes to the same general conclusion.

The origin of these phenomena was recognized, among the first by O. Reynolds, and by P. G. Tait and J. Dewar, as a consequence of the kinetic theory of the constitution of gaseous media. The temperature of a gas is measured by the mean energy of translation of its molecules, which are independent of each other except during the brief intervals of collision; and collision of the separate molecules with the blackened surface of a vane, warmed by the radiation, imparts heat to them, so that they rebound from it with greater velocity than they approached. This increase of velocity implies an increase of the reaction on the surface, the black side of a vane being thus pressed with greater force than the bright side. In air of considerable density the mean free path of a molecule, between its collisions with other molecules, is exceedingly small, and any such increase of gaseous pressure in front of the black surface would be immediately neutralized by flow of the gas from places of high to places of low pressure. But at high exhaustions the free path becomes comparable with the dimensions of the glass bulb, and this equalization proceeds slowly. The general nature of the phenomena is thus easily understood; but it is at a maximum at pressures comparable with a millimetre of mercury, at which the free path is still small, the greater number of molecules operating in intensifying the result. The problem of the stresses in rarefied gaseous media arising from inequalities of temperature, which is thereby opened out, involves some of the most delicate considerations in molecular physics. It remains practically as it was left in 1879 by two memoirs communicated to the Phil. Trans. by Osborne Reynolds and by Clerk Maxwell. The method of the latter investigator was purely a priori. He assumed that the distribution of molecules and of their velocities, at each point, was slightly modified, from the exponential law belonging to a uniform condition, by the gradient of temperature in the gas (see DIFFUSION). The hypothesis that the state was steady, so that interchanges arising from convection and collisions of the molecules produced no aggregate result, enabled him to interpret the new constants involved in this law of distribution, in terms of the temperature and its spacial differential coefficients, and thence to express the components of the kinetic stress at each point in the medium in terms of these quantities. As far as the order to which he carried the approximationswhich, however, were based on a simplifying hypothesis that the molecules influenced each other through mutual repulsions inversely as the fifth power of their distance apart-the result was that the equations of motion of the gas, considered as subject to viscous and thermal stresses, could be satisfied by a state of equilibrium under a modified internal pressure equal in all directions. If, therefore, the walls of the enclosure held