is not established on good physical evidence; (4.) all species which however abundant in the lower series disappear in the upper-either altogether, or ascend into it through so very limited a space above the line of demarcation as to produce no general impress on the upper fauna;-when all this has been done, I have no doubt that the number of common species will fall considerably below ten per cent perhaps as low as the percentage shown by the Palæozoic series of North America or Bohemia; but not so low as the per centage shown by the Cambrian and Silurian rocks of the north of England.

ART. XXXV.-New method of disintegrating masses of Fossil Diatomacea; by Prof. J. W. BAILEY.

MANY masses of fossil Diatomaceæ are so strongly coherent, that they cannot be diffused in water, (for the purpose of mounting in balsam,) without a degree of mechanical violence which reduces to fragments many of the most beautiful and interesting forms. This is particularly the case with some specimens from the "infusorial deposits" of California. Some of these I endeavored to break up, by boiling in water and in acids, and also by repeated freezing and thawing when moistened, but without good results. in either case. At last it occurred to me that the adherence might be due to a slight portion of a siliceous cement which the cautious use of an alkaline solution might remove without destroying any but the most minute shells of the Diatoms. As the case appeared a desperate one, a "heroic remedy", was applied, which was to boil small lumps of the diatomaceous mass in a strong solution of caustic potassa or soda. This proved to be perfectly efficacious, as the masses under this treatment rapidly split up along the planes of lamination, and then crumbled to mud, which being immediately poured into a large quantity of water ceased to be acted upon by the alkali, and gave when thoroughly washed, not only all the large shells of the Diatoms in a state of unhoped for perfection, but also furnished abundance of the minute forms. Having obtained by this method highly satisfactory results from specimens from many localities I can confidently recommend it as an addition to our modes of research.

The following directions will enable any one to apply the process. Put small lumps of the mass to be examined into a test tube, with enough of a solution of caustic potassa or soda to cover them; then boil over a spirit lamp for a few seconds, or a few minutes, as the case may require. If the solution is sufficiently strong, the masses will rapidly crumble to mud, which must be poured at once into a large quantity of water, which after subsi

dence is removed by decantation. If the mass resists the action of the alkaline liquor a still stronger solution should be tried, as while some specimens break up instantly in a weak solution of alkali, others require that it should be of the consistence of a dense syrup. The mud also should be poured off as fast as it forms, so as to remain as short a time as possible in the caustic ley.

The only specimens which I have found not to give good results by the method above given, are those from Tampa Bay, Florida, and the infusorial marls from Barbadoes. In the masses from Tampa the lapidification is so complete, that the alkali destroys the shells before the lumps break up; and in the case of the Barbadoes marls the cementing material is calcareous, and requires a dilute acid for its removal. In applying the above process one caution is necessary, which is to thoroughly wash the shells with water, and not with acids, as the latter will cause the deposit of a portion of the dissolved silica and materially injure the beauty of the specimens. When the washings are no longer alkaline, the specimens may then be thoroughly cleansed by acids or by the chlorate process described in the last number of this Journal. (See vol. xxi, p. 145.)

ART. XXXVI.-On the non-existence of polarizing Silica in the Organic Kingdoms; by Prof. J. W. BAILEY.

It is now more than twenty years since Sir David Brewster announced the existence of polarizing or doubly refractive silica in the cuticle of Equisetum, and in that of some of the grasses. In Lindley's Natural System of Botany, the following account of Brewster's experiments is given. "On subjecting a portion of the cuticle of Equisetum hyemale to the analysis of polarized light under a high magnifying power, Brewster detected a beautiful arrangement of the siliceous particles, which are distributed in two lines parallel to the axis of the stem and extending over the whole surface. *** Brewster also observed the remarkable fact that each particle has a regular axis of double refraction. In the straw and chaff of wheat, barley, oats and rye he noticed analogous phenomena." (Quoted by Lindley from Grevill. Fl. Edinens., 214.)

In Quekett's Treatise on the Microscope, 3d ed., p. 358, directions are given for preparing the siliceous cuticle of Equisetum hyemale for microscopic examination, by boiling in strong nitric acid, and it is added that "in balsam it forms a beautiful object for polarized light." Similar directions are given for preparing the silica in the chaff of wheat, oats, &c.

As these statements are contained in the last editions of each of the above-mentioned works, it is evident that no contradiction of the error involved in them has been pointed out; yet, notwithstanding the high authority on which they rest, the statements so far as the polarizing action of the silica is concerned are wholly erroneous. If the cuticle of the above-mentioned plants is completely deprived of its carbonaceous tissues it will be found wholly devoid of action on polarized light, and any preparation of the cuticle which is found to affect polarized light will also be found to blacken when heated in concentrated sulphuric acid, and if then decarbonised by throwing into the hot acid solution a little chlorate of potassa, the residual silica shows no signs of action under the polariscope, either alone or with the selenite plate, although it still retains the forms of the cells, stomata, &c.

It is clear then that the error in the above statements has been caused by the imperfect removal of the dense carbonaceous tissues which are deposited beneath the silica. I have examined several species of Equisetum and a large number of plants of the grass tribe which are most remarkable for their siliceous cuticles, but have found no trace of any action upon polarized light, when the carbonaceous matter was removed. But it is unnecessary to resort to artificial preparations to prove the correctness of my statements. Nature has made her own preparations, and deposited them by myriads beneath every peat bog, where may be found not only the siliceous shells of the Diatoms, and the spicules of the fresh-water sponges, but also a large number of the siliceous parts of the grasses, sedges, &c. Ehrenberg has shown, (Berlin Monthly Reports, May, 1848) and I can confirm his statements, that the silica in these Phytolitharia, as well as in the Diatomaceæ, Polycistineæ and Spongiolites is not doubly refractive. He makes an exception in the case of the shell of Arachnoidiscus, but my own experiments prove that when properly cleaned this shell forms no exception. As I have shown above that the silica in the cuticle of the Equisetum and grasses, agrees with that in the lower tribes in characters, I think the couclusion is warranted, that doubly refractive silica has no existence in the organic world.

ART. XXXVII.—On the Pendulum experiments lately made in the Harton Colliery, for ascertaining the mean density of the Earth; by G. B. AIRY, Esq., F. R. S., Astronomer Royal.*

THE speaker commenced with remarking that the bearing of the experiments, of which he was about to give, a notice, was not limited to their ostensible object, but that it applied to all the bodies of the solar system. The professed object of the experiments was to obtain a measure of the density of the earth, and therefore of the mass of the earth (its dimensions being known); but the ordinary data of astronomiy, taken in conjunction with the laws of gravitation, give the proportions of the mass of the earth to the masses of the sun and the principal planets; and thus the determination of the absolute mass of the earth would at once give determinations of the absolute masses of the sun and planets. To show how this proportion is ascertained, it is only necessary to remark, that a planet, if no force acted on it, would move in a straight line; that, therefore, if we compute geometrically how far the planet moves in a short time, as an hour, and then compute the distance between the point which the planet has reached in its curved orbit, and the straight line which it has left, we have found the displacement which is produced by the sun's attraction, and which is therefore a measure of the sun's attraction. In like manner, if we apply a similar calculation to the motion of a satellite during one hour, we have a measure of the attraction of its primary. The comparison of these two gives the proportion of the attraction of the sun, as acting upon a body, at one known distance, to the attraction of a planet, as acting upon a body at another known distance. It is then necessary to apply one of the theoreins of the laws of gravitation, namely, that the attraction of every attracting body is inversely as the square of the distance of the attracted body; and thus we obtain the proportion of the attractions of the sun and a planet, when the bodies upon which they are respectively acting are at the same distance from both: and finally, it is necessary to apply another theorem of the law of gravitation, namely, that the attractions thus found, corresponding to equal distances of the attracted bodies, are in the same proportion as the masses of the attracting bodies (a theorem which applies to gravitation, but does not apply to magnetic and other forces). Into the evidence of these portions of the law of gravitation, the speaker did not attempt to enter: he remarked only that they rest upon very complicated chains of reasoning, but of the most certain kind. His only object was to show that the proportion of the masses of all bodies, which have planets or satellites re

* Proc. Roy. Inst. of Great Britain, Part V, p. 17.

volving round them, can easily be found-(the proportion for those which have no satellites is found by a very indirect process, and with far less accuracy); and that if the absolute mass of the earth be known, the absolute mass of each of the others can be found. As their dimensions are known, their densities can then be found. Thus it rests upon such inquiries as those on which this discourse is to treat, to determine (for instance) whether the planet Jupiter is composed of materials as light as water, or as light as cork.

The obvious importance of these determinations had induced philosophers long since to attempt determinations of the earth's density and two classes of experiments had been devised for it.

The first class (of which there was only one instance) is the attraction of a mountain, in the noble Schehallien experiment. It rests, in the first place, upon the use of the zenith sector; and, in the next place, upon our very approximate knowledge of the dimensions of the earth. [The construction of the zenith sector was illustrated by a model: and it was shown, that if the same star were observed at two places, the telescope would necessarily be pointed in the same direction at the two places, and the difference of direction of the plumb line, as shown by the different points of the graduated arc which it crossed at the two places, would show how much the direction of gravity at one place is inclined to the direction of gravity at the other place.] Now, from our knowledge of the form and dimensions of the earth, we know that the direction of gravity changes very nearly one second of angle for every hundred feet of horizontal distance. Suppose then, that two stations were taken ou Schehallien, one on the north side and the other on the south side, and suppose that their distance was 4000 feet; then, if the direction of gravity had not been influenced by the mountain, the inclination of the directions of gravity at the two places would have been about 40 seconds. But suppose, on applying the zenith sector in the way just described, the inclination was found to be really 52 seconds. The difference, or 12 seconds, could only be explained by the attraction of the mountain, which, combined with what may be called the natural direction of gravity, produced directions inclined to these natural directions. In order to infer from this the density of the earth, a calculation was made (founded upon a very accurate measure of the mountain) of what would have been the disturbing effect of the mountain if the mountain had been as dense as the interior of the earth. It was found that the disturbance would have been about 27 seconds. But the disturbance was really found to be only 12 seconds. Consequently the proportion of the density of the mountain to the earth's density was that of 12 to 27, or 4 to 9 nearly. And from this, and the ascertained density of the mountain, it followed