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spear-head, which is 19 inches long, and has segmental and circular openings in the blade. It is difficult to assign closely the period of their occurrence throughout the British area, as they have never been found in direct association with interments. There are iron specimens, presumably of the 5th, and 6th centuries, known on the Continent. By the close of the 14th century the forms had developed into an extraordinary variety, and, owing to the increased trade with the East, the shape, length, and decoration of the shaft underwent considerable modifications and changes. In the Admiralty Is. (Pacific) spear-heads are made of black obsidian, chipped in the manner of flint - flaking, and attached in an ingenious way to the head of the shaft by means of gummed threads. The natives of Queensland, Australia, use a very long slender-shafted spear, the point of which may be set in a reed, and is often barbed and poisoned. This lance is thrown by using a strong piece of wood deeply notched at one end, called a spear-thrower. Spearmint (Mentha viridis), a hardy naturalized plant frequenting marshy places. It has Creeping roots, and bears sessile, lanceolate leaves, which are acute, glabrous, and unequally serrated. The whole plant has a pleasant aromatic odor, and its leaves are used as a flavoring agent in cookery. See MINT. Spearwort, a name given to certain species of the genus Ranunculus on account of their narrow, tapering leaves. The creeping R. pusillus and R. reptans are among our spearworts. Special Sessions. In England, a court formed by two or more justices of the peace sitting together for the trial of important cases within their jurisdiction. In New York, a criminal court, consisting of three judges sitting together, without a jury, for the trial of misdemeanors and petty criminal cases. Specialty Debt. By the com: mon law, a debt secured by deed —i.e. by writing under seal-as distinguished from a simple contract debt, or from a debt of record, such as a judgment or recognizance. In most jurisdictions actions to recover specialty debts may be brought at any time within twenty years, whereas in the case of simple contract debts the period of limitation is six years. See SEAL. Species, one of the grades in biological classification—a group of individuals, fertile inter se, and resembling one another in certain distinctive hereditary characters which mark them off from other groups. Three criteria of wellSpecies established species are:—(1.) The distinctive specific characters in small pieces; , this opening is afterwards closed by a stopper, and the bottle or tube filled u with water as described. Wit solids that are affected by water, liquids such as petroleum or benzine may used, or the volume may be determined by the volumenometer, which also allows for pores in the substance not readily filled by liquids. This instrument consists of an air-tight vessel to enclose the solid, connected by a tube to a mercury reservoir, so that the pressure on the contained gas can varied. The increase in volume vl caused by a given diminution of pressure from P to Pl is read when the vessel is empty; when in accordance with Boyle's law the volume, v, of air in the vessel is found from the equatio, vP = (v1 + v)P1, whence v = # ; Repeating with the solid in position, a second and smaller value for v will be found, the difference giving the volume of the solid, so that if its weight is known the specific gravity can be calculated. The buoyancy methods, which are very varied in detail, depend upon the principle discovered by Archimedes, that a body immersed in a liquid is buoyed up by a force equal to the weight of liquid it displaces. Thus, if an object is weighed first in air, and then, when suspended by a thread in water, it weighs less the secon time, by an amount equal to the weight of water equal in volume to itself. This weight, divided as before into the Yo! of the object, gives the specific gravity. In applying this method to o: it is only necessary to find the loss of weight of an object both in the liquid and in water to obtain , the necessary weights of equal volumes of the two. Hy

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which the members share should show some degree of constancy from generation to generation, and they should not similarities directly acquired in each individual lifetime through the influence of similar conditions of life. (2.) The distinctions between one species, and another should always be of greater magnitude than the distinctive features which may characterize the members of a łaj (using the word family here to mean the rogeny of a pair). (3.) The memrs, of a species are normally fertile inter se, but not usually or not readily, with members of other species. In fact, the evolution of distinct species has in part depended on a restriction of the range of fertile intercrossing. species often means, however nothing more than a group of individuals whose common and distinctive features seem to naturalists important enough to warrant the use of a special name. It should be clearly understood that a species is a relative conception—a device for scientific convenience when we wish to include under one title all the members of a group of individuals who resemble one another in certain distinctive hereditary characters. But as resemblances which seem important to one naturalist may seem trivial to others, there are often wide differences of opinion as to how many species should be recognized in any particular case. hus Haeckel says of calcareous sponges that, as the naturalist likes to look at the problem, there are 3 species, or 21, or 289, or 5911 When we study a large number of more or, less similar organisms, we find that they can be arranged in groups. In each group there is, so to speak, a densely packed centre of closely similar individuals, and a more sparsely peopled periphery of more divergent forms. This big fact of observation may be expressed with precision by measuring particular characters throughout a large number of similar individuals, and Poio out a curve, known as the curve of frequency. It will then be seen that the great majority of the individuals measured occupy an area near the top of the curve, and that there are only a few at the two basal ends. Whether we call one of these groups a variety, a subspecies, a cies, or a genus, matters little. hese groups represent stages in an evolutionary process: they are never quite constant, , and often fade into one ... another, being linked by the divergent outliers or variants of each group.... . It remains to give an illustra

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tion of the different grades of classification. All the tigers are said to form the species. Felis tigris, of the genus Felis, in the family felidae, in the order Car. nivora, within the class Mammalia, in the series or phylum vertebrata.” The resemblances of all tigers are very close; well marked, though not so close, are the resemblances between tigers, lions, leopards, jaguars, pumas, cats, etc., which form the genus Felis; broader still are the resemblances between all members of the cat family Felidae, which includes, besides the..}. Felis, the cheetah (Cynaelurus), , the extinct “sabre - toothed tiger’ (Machoerodus), etc.; still wider the likenesses between all the cats, dogs, bears, and seals which form the order Carnivora; even more general are the affinities of structure which bind mammals together in contrast to birds or reptiles; and, finally, there are the common characters of all vertebrate or chordate animals. A list of about a score of definitions of ‘species’ will be found in Quatrefages's Darwin et ses Précurseurs français (1870). Romanes in his Darwin and after Darwin (vol. ii. 1895) reduced the number of logically possible definitions to five. Specification. See PATENTs. Specific Gravity, or RELATIVE DENsity, is the comparison of the heaviness of a substance with that of a standard substance, and may thus be defined as the number of times the weight of a certain volume of the substance contains the weight of the same volume of the standard. In the case of solids and liquids, water at the point of its maximum density—i.e. at 4° C.—is usually taken as the standard; while in the case of gases, air, or, better, hydrogen, measured under the same conditions of temperature and pressure as the gas in question, is employed. the result in either case is a ratio, the cific gravity, is independent of the actual volume, weights or system of weights and measures used, and is numerically equal to the absolute density, or weight of unit volume, if the unit of volume of the standard substance is of unit weight, as in the metric system is the case with water. Methods of Determination. — For solids and liquids two general principles are relied on to determine specific gravities—viz.: (1) by weighing measured volumes; (3) by measuring, buoyancy. In the case of liquids the measurement of their hydrostatic pressure can also be utilized. The methods employed for gases are in general similar in principle, and are described in the article on gas and vapor density. In

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drometers, Mohr's specific gravity.

balance, and the use of "heavy liquids' also depend on the principle of buoyancy. Hydrometers are of two kinds—viz. of fixed and variable immersion. Nicholson's hydrometer is an example of the first kind, and consists of a hollow brass cylinder with conical ends, provided with a weighted basket at the iower end to make it float upright, and a pan supported on a thin vertical wire at the upper end. The instrument is adjusted by weights on the pan so as always to be immersed to a fixed point on the wire. If the weights required to produce this when immersed in water and in the liquid of which the specific, gravity is sought are added to the weight of the in: strument, the , weights, of equal volumes of the liquid and of water are obtained, and hence the specific gravity. The specific gravity of a solid can also be

determined if it is first placed in the upper pan and then in the basket, the hydrometer being adjusted with weights each time; the difference between the weights used with and without the object in the upper pan gives its weight in air, while its weight in water is , equal to the difference of weights used when the body is in the pan and in the basket. The specific gravity is then calculated in the same way as before. Mohr’s specific grayity balance is on the same principle as the Nicholson hydrometer, a plummet or loaded glass weight being sunk, to the same point by placing riders or slop; weights on a balanced eam, from which the weight is hung by a fine wire. It is, however, much more sensitive and ...Y to work with, as the plummet and weights, are made of such sizes as to give specific gravities without calculation Specific gravity beads are fixed immersion hydrometers, and consist of small differently weighted bulbs that sink or swim in a !. according, to its density, which is that of the bead that remains in equilibrium, neither sinking nor floating. The same principle is made use of in determining the density of minerals, a heavy liquid ** adjusted b admixture with a light one till the fragments of the substance are just in equilibrium; the specific gravity of the liquid is afterward found by a hydrometer or otherwise. Suitable heavy liquids are methylene iodide, solutions of thallium and silver nitrates, mercury and potassium iodides. Hydrometers of variable immersion are usually light hollow

Hydrometers. A, Usual form. B, U. S. Internal Revenue pattern. glass spindles weighted by shot or mercury. The divisions on them are of unequal size, for as the volumes immersed are in

versely as the densities of the liquids, the spaces representin

equal, increments in density di. minish harmonically. Variable

Sikes's Hydrometer. A, Weight to be slipped on at c.

immersion hydrometers are made of special forms and sizes to test the specific o of particular liquids, such as alcohol, milk. urine. The U. S. Treasury Dept. in its internal revenue service uses a series of carefully adjusted #. drometers, for , determining the amount of alcohol in various distilled liquors. In Great Britain for a similar purpose Sikes's hydrometer is used. It is made of gilded brass, and is provided with weights (A) that can be placed to the stem to increase the range; the graduations represent .00175 difference in cific gravity, and are convertible into degrees under and over proof by graduated scales.

In using the hydrostatic pressure method of measuring specific gravities, the liquid is poured into one limb of a vertical U-tube and water into the other, care being taken that the less dense liquid is , not forced round the bend, and the heights of the balancing columns measured. Then as the product of the height of the liquid into its specific gravity is equal to the same product in the case of the water column the specific so of the liquid is found by dividing the height of its column into that of the water. With liquids that mix with water the columns are sucked up from different beakers into an "inverted U-tube by a branch tube at the bend, so that the liquids are separated by, an air space. The same principle is then applied.

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Specific Heat

In , general, substances vary considerably in specific gravity, Thus, ordinary liquids, with the exception of mercury, which has a specific gravity of 13.6, range from about .6 to 3, and homogeneous solids from under 1 to over 22. Figures for the specific gravities of the elements are given under ELEMENTs, and those of other substances under their specific headings. A table of the specific gravities of a few common substances , is given below. Full tables have been compiled by F. W. Clarke, and published by the Smithsonian Institution. The practical details of carrying out the various methods may be found in Kohlrausch's Physical Measurements, Ostwald's Physico - Chemical Measurements trans. Walker, ...} and Glaze

rook and Shaw's Practical Physics (1893). See also HYDROMeter.

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Sea-water . . Sulphuric acid Turpentine . . . .86–.89 ses (compared with water). ir . . . . .00129

Aluminium . . . . 2.7 Brass . . . . . . 8.4–8.7 Clay . . . . . . 1.8-2.6 Coal . . . . . . 1.2–1.7 Copper . . . . . . 8.9 Diamond . . . . . 3.5 Ebony . . . . . 1.1–1.2 Gold . . . . . . 19.3 Glass . . . . . . 2.4–3.4 Granite . . . . . 2.5–2.9 Ice - - - - - - .92 Iron . . . . . . 7.8 Lead . . . . . . 11.3 Marble . . . . . 2.5–2.8 Oak . . . . . . .85–.95 Pinewood . . . . .5 uartz - - - - - 2.65 Silver . . . . . . 10.6 usoar - - - - - 1.59 Sulphur . . . . 1.98–2.07 Tin . . . . . 7.3 Zinc . . . . . . 7.1 Liquids.

Alcohol . . . . . .80 Ammonia solution . . .88 Benzine . . . . . .89 Ether - - - - - .73 Glycerin . . . . . . . 1.26 Hydrochloric acid . . 1.27

ercury . . . . . 13.6 Milk . . . - - 1.03 Nitric acid - - - 1.56

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to, but differs widely with the material of which the body is composed. In order to express this difference, it is necessary to formulate a standard unit of heat. he one most commonly used is the quantity of heat that is required to raise the temperature of unit mass of water one degree. In metric units this is called a ‘calorie,’ and is the heat required to raise the temperature of 1 gram of water 19 c.; in British units the British Thermal Unit (B.TH.u.) is the heat required to raise 1 lb. of water 1* F. As the quantity of heat required to raise the temperature of a body varies somewhat with the temperature, the unit is usually specified to be measured at 15° C., or else the mean value between 0° and 100° C. is chosen; the two values, however, differ but slightly: instead of using a water standard, it is perhaps more scientific to express quantities of heat, in the absolute units of mechanical work into which the heat can be converted (see o but until the exact value of the ratio between the two is more definitel decided, the water standard will probably be retained, especially as in practice most measurements are compared with it. Methods of . Determination.— The method of determining specific heats most frequently used is the ‘method of mixtures.’ A known weight of the substance, best in small pieces, at a known high temperature is mixed with a known weight of water at a lower temperature, and the temperature of the mixture is taken. Then, as the heat given out by the hot substance in cooling to the tem: perature of the mixture is equal to that received by the cold water in being warmed up, the product of the specific heat of the substance into its mass and fall of temperature is equal to the product of the mass of the water into its rise of temperature, the specific heat of the water being, by definition, unity. Due allowance must be made for the heat required to warm, the vessel (calorimeter) in which the experiment is carried out, and for heat lost to the atmosphere; and the method needs to be further modified, if the substance and water interact, by , substituting, some other liquid for water. In this case, as in the case of liquids, in which a hot solid, such as iron or copper, is added to the liquid, the specific heat of the substance used instead of water requires to be known; and from the fact that the product of the specific heat into the change of temperature and mass of both substances is the same, the unknown specific heat can be calculated. is process requires considerable quantities

Specific Heat

of the substance in question in order to get an accurate result—a difficulty avoided in Bunsen's ice calorimeter. Ice calorimeters, which depend on the fact that to melt unif weight of ice requires approximately eighty times as much heat as is required to raise the same weight of water one de#. were invented by Black, but

id not give very good results, owing to the difficulty of measur# the amount of ice melted. This was obviated by Bunsen,

Bunsen's Calorimeter.

who measured the contraction caused by the melting of ice, this being equal to .09 c.c. per gram melted. In Bunsen's instrument a test tube is sealed into and enclosed by an outer tube, the lower part of which, together with a tube leading from it to a narrow §: is filled with mercury.

ater fills the upper part of the outer tube, and is frozen round the test tube; the whole apparatus is then immersed in snow, to revent the ice from being melted } outside heat. The substance of weight (w) is then dropped into the test tube, and the volume (v) o which the mercury recedes in the gauge is measured; the specific heat of the substance can be calculated from the rela

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is the fall of temperature of the substance. The rate at which a substance loses heat by radiation can also be utilized to measure its specific heat by noting the times that heated quantities of it, and of water take to cool through a given range under exactly similar circumstances. This method can, however, only be applied to liquids, the specific heat, bein given by dividing the product o the time taken by the substance into the mass of the water by the product of the time taken by the water into the mass of the liquid. In determining the specific heat of gases, not only do practical difficulties arise owing to the bulk of the gas, but also there is the theoretical difficulty that gases have a different specific heat according to whether they are allowed to expand when heated or not. This is due to the fact that when a body expands in the open air it does work in pushing back the atmosphere; and this work requires a correspondin amount of heat to be expended. This amount is negligible in the case of solids and liquids, but owing to the great expansibility it becomes appreciable in the case of gases. In order to measure the specific heat of gases at constant pressure, the gas is conducted through a worm heated to a known temperature, and is then passed through a spirally divided chamber immersed in the water in a calorimeter, which is warmed up at the expense of the heat of the gas. The calculation is performed as in the method of mixtures given above. The specific heat of gases at constant volume may be determined by Joly’s steam calorimeter, in which a large globe containing the compressed gas is immersed in steam, some of which is condensed in warming up the gas and globe. Allowance is made for the heat used in warming the globe by making a similar measurement with an empt globe, and the specific heat is calculated from the relation S.H. = W x 536 W1 x t water condensed, 536 is the latent heat of steam, wi is the weight of the gas, and t its rise of temperature. The ratio of the specific heats of a gas at constant volume should be as 1.66: 1 if the gas consists of simple particles. This is, however, rarely the case, the ratio varying from 1.42: 1 downward, except in the cases of gases such as argon, helium, and mercury. The reason is that internal work between the parts of the molecule has to be done in most gases, making an addition to both figures of the ratio, and thus Fo: mating it toward unity. e determination of this ratio, which may be carried out by indirect as well as by the above direct methods, affords valuable information as to the complexity of the gas molecule. With the notable exception of the specific heat of liquid hydrogen, which approaches 6, the specific heats of almost all substances are smaller than 1—i.e. are less than that of water. In the case of most of the solid elements they vary inversely as the atomic weight, the atomic heat, or product of atomic weight into specific heat being approximately 6.4. This fact is known as Dulong and Petit’s law, from its discoverers. It shows considerable divergences from exactness in some cases, particularly with elements of low atomic weight, such as carbon and silicon, in which the specific heat is too small, but

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, where w is the weight of

the divergence becomes less and less the ‘...r the temperature at which the element is measured. Dulong and Petit's law can be applied to a certain extent to some compounds, but the constancy is confined at best to compounds of similar classes. A knowledge of specific heats is utilized to determine high temperatures in Siemens's pyrometer, in which a metal cylinder is heated in the furnace and then dropped into water; then if the rise of temperature and mass of the water and the specific heat and , mass of the metal cylinder are known, the original temperature of the metal cylinder can be calculated. For a table of the specific heats of the elements see LEMENTs; and see further Edser's Heat (1899), Ostwald's Physico-Chemical Measurements and Kohlrausch's Physical Measturements. Specific Performance. In certain cases of breach of contract, where an action for damages would not be an adequate remedy, courts of equity will compel the actual or substantial performance of the contract by

the delinquent, party. This relief is only nown as specific rformance. here must

inadequacy of remedy at law; and, in general, there must be mutuality, that is, it must be such a case that if a bill had been filed against the plaintiff, the same relief could have been obtained against him. Equity will almost invariably decree specific performance of contracts for the conveyance or purchase of real property, but not generally for personal property unless, it is of a nature that cannot be obtained elsewhere, as in the latter case damages would not be adequate relief. Equity will usually enforce an award or compromise, and in some states, will enjoin a person who has contracted to give exclusive services to another and agreed not to work elsewhere during a certain period, as a singer, from rendering services for any one else during the term of the contract. Of course the plaintiff must perform his part of the contract. If great hardship will be occasioned to the defendant, and little corresponding benefit to the Fol. by specific performance, the court will refuse the relief. Equity will not decree specific performance of a contract where, it is impossible to enforce the decree, as to compel personal services, or where performance would be practically impossible. See CoNTRActs; EQUITY... Consult Pomeroy, Specific Performance. Spectacles are frames of metal, *Pio lenses of ground optical glass, and are aids for pre

serving sight or correcting defects of vision. Spectacle lenses are of two principal classes—spherical and cylindrical—and these in turn are either convex or concave. In some cases compound spectacle lenses are used where the person requires the one pair of glasses to suit both distance and reading. These are called bi-focal, and were first invented by Benjamin Franklin. Torric lenses form another combination; in them a cross cylinder is ground on one part of the surface, and a spherical curve on the other half of the lass. Pebble lenses are made rom rock crystal. The frames for supporting the two lenses . are of two forms—the ordinary spectacle frame with straight sides or curved arms extending to behind the ears, and those which by means of a spring keep in contact with the nose of the wearer, and are called eye-glasses or ‘pince-nez.” The latest design, the frameless spectacle, shows the minimum of bridge or spring in the centre and is in great contrast to the heavy tortoiseshell goggles or horn frames of a century ago. . Spectacles were probably first invented by the o Alhazen, an Arab writer, makes mention of them in the 11th cen...}. and Italian monks in Pisa nd Florence used them in the 13th century. Nuremberg carried on spectacle-making in 1842, and later the house of Dolland in London and other English makers vied with those of Paris. The industry was soon established in the United States, and now both frames and lenses are made of the highest grades and in large quantitles. There are fourgeneral conditions of eyesight which require spectacles. Presbyopia, or old sight, becomes manifest, after forty-five years of age, and is noticed when persons cannot read fine , print with comfort at fourteen inches distant. , Myopia, or , near sight, is caused by an over-development of the eyeball, and is noticed when, in order to see clearly, a short-sighted person has to hold his book or work closer to his face than is natural or comfortable. To correct this condition the weakest concave lens is used that will afford the best vision. Hypermetropia, or long-distance sight, is a condition caused by the under - development of the eye, and for this the strongest convex lens is used that will make the distant vision normal. Astigmatism is a condition of the eve which requires the most careful fitting of spectacles. It is a distortion of the image on the retina, caused by the curvature of the cornea ing, uneven. Nearly every eye exhibits traces more or

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