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separated is in all cases ice. This, however, is not the case. Down to the ratio 65 water and 35 alcohol, the ice spicula freely moving in the mother-liquid are easily recognized. If they are remelted they invariably reform at the same temperature. At ratios of 60 water to 40 alcohol and stronger, the aspect of the solid formed is quite different. The liquid may then acquire the consistency of Canada balsam, and yet, if kept still, it may remain perfectly transparent. On rubbing with the thermometerbulb or with a glass rod minute crystals are formed, the liquid becomes more mobile, resembling old honey. It offers a notable case of a condition of supersaturation producing a temporary colloid condition.

§ 98. Finding that this phenomenon was first noticeable at about the solution of the ratio 60 water to 40 alcohol, and reflecting that this is nearly the ratio of one molecule of alcohol to four of water, I made a spirit of exactly this molecular ratio, namely 39.07 alcohol and 60.93 of water by weight. On submitting this to the action of a cryogen, I found the remarkable result that nothing separated till the temperature -34° was reached, although both weaker and stronger solutions begin to solidify at higher temperatures. The solid formed in this case is perfectly white and opaque, and the temperature remains constant till the whole has become perfectly dry. This pearly aspect and the constancy of temperature throughout the solidification betray the cryohydrate. Both weaker and stronger spirits sink in temperature as they solidify the former until by the elimination of ice it presents a mother-liquor of the 4-hydrate composition, the latter by the elimination of liquid alcohol as the solid 4-hydrate is formed.

§ 99. When, therefore, a dilute alcohol (say 95 water to 5 alcohol) is cooled, ice separates out, the spirit becomes stronger and stronger, and the temperature lower and lower. When the ratio C2 HO+4H, O is reached the temperature -34° C. is reached, and the remainder of the solution is a cryohydrate of alcohol whose melting- and freezing-point is -34°, and whose composition is C2 H2O+4H2O.

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On the other hand, if a spirit a little stronger than C, HO+4H2O is subjected to cold, nothing separates till about -27° C. At this temperature the cryohydrate C, HO+4H2O begins to separate out, and as it relinquishes the alcohol the solidifyingpoint of the mother-liquor falls. Observe, the cryohydrate separates from alcohol at a temperature above its own melting-point when alone. It follows from this that solidification may commence at the same temperature in two spirits of different strengths, provided they are both close about the 4-molecule hydrate in composition. But while in the weaker the solid will be ice, in the stronger it will be the cryohydrate. The apparent anomaly

arises from the circumstance that the cryohydrate is soluble both in water and alcohol, and that ice is soluble in the liquid cryohydrate. We are here reminded of the phenomenon of the maximum density of water, which I have already endeavoured to connect with the formation of a cryohydrate of water (in § 36).

§ 100. For some reasons this melted cryohydrate of alcohol, or spirit of wine containing 60.93 water and 39.07 of alcohol, should be the standard or proof spirit in all alcoholometry. English proof spirit contains 50.5 per cent. by weight of alcohol. It accordingly should begin to solidify at -38° (about the freezing-point of mercury). At lower temperatures it becomes a pasty mass, but never perfectly brittle, on account of the unfreezable alcohol in excess above that of the cryohydrate. Spirits of 39 per cent. and under become perfectly solid before the temperature reaches -36°. The very fact that weaker spirits wholly solidify while stronger never do so is, I conceive, a complete proof of the existence of a cryohydrate. For if under all circumstances water alone solidified, alcohol would be left even from the weakest spirits, and total solidity could never be reached.

sugar,

§ 101. The above considerations, of course, only apply to the chemically pure substances alcohol and water; how far the caramel, fusel, essences &c. of commercial spirits, and the innumerable substances in wines and beers may affect the solidification is a matter for future research. It is quite possible, for instance, that in some rums the 10 per cent. alcohol above that required for the cryohydrate might find sufficient foreign matter present to form therewith a solid, and so allow the whole to solidify.

§ 102. I believe that the detection of this 4-molecule hydrate of alcohol reconciles the apparently contradictory results of former experimenters. Thus, looking on wine as a 10-per-cent. spirit of wine, M. Melsens obtained by a freezing-mixture 40 per cent. of ice. I judge from Table XI. that the temperature reached was -8°, if we make no allowance for loss of ice during manipulation.

§ 103. The evidence adduced by Messrs. Dupré, Page, and others points to the existence of at least one hydrate of alcohol; but I am not prepared to say that such hydrate is necessarily identical with the 4-molecule cryohydrate which we have been considering. It must be remembered in this connexion that the cryohydrates of solid salts contain more water than any other known hydrate; and the existence of the 4-molecule cryohydrate rather, I conceive, tends to show that the ordinary hydrate of alcohol contains less water, and is, as some of Dupré and others' experiments may be interpreted as showing, the 3-hydrate C2 H6O+3H2O.

Cryohydrate of Ether.

§ 104. Cryohydrate of Ether (?).-It is well known that when water and ethylic ether are shaken together mutual but only partial mixture ensues, the water taking up about one ninth of its volume of ether, and the ether about one thirty-sixth of its volume of water. Such an aqueous solution of ether when subjected to cold, solidifies at -2° C., without any separation of ether, into a dry solid consisting of ether and water. The compound exhibits an interesting feature, inasmuch as it exemplifies the effect of cooling on the luminosity of flame. The cryohydrate of ether may be solidified in a long test-tube, and thence removed, presenting the appearance of a candle. One end of this is cut off flat, and the whole placed upright on a plate. A light being applied at the top, the whole melts away as the ether burns. The flame is quite non-luminous. The ether is only free to burn as it is in the act of melting ice, and is so cooled. So it is well known that marsh-gas becomes luminous if heated before combustion. Thus ether (which is empirically olefiant gas plus water) and alcohol (which is empirically ether plus water) have flames of luminosity the less according as their ratio of potential water is greater. The cryohydrate of ether is in fact physically, as far as its luminosity is concerned, an alcohol.

Throughout this second part of this research I have been much indebted to my friend Mr. R. Cowper for his very zealous and skilful assistance.

XXXI. The Specific Heat of the Elements Carbon, Boron, and Silicon.-Part I. The Relation between the Specific Heat of these Elements in the free state and the Temperature. By Dr. H. FRIEDRICH WEBER, Professor of Physics and Mathematics. [Concluded from p. 183.]

B. Specific Heat of Graphite.

KOPP has put forward the theory that all modifications of

carbon possess the same specific heat; but he has not tested this theory by experiment. All the forms of carbon except diamond are more or less porous, and therefore absorb varying quantities of gas; hence an error is introduced in determining their specific heats. In order to test Kopp's hypothesis I carried out the following experiments. I first of all sought to determine whether any thermal change occurs when graphite and water are brought into contact, the initial temperature of each being the same. A piece of pure graphite weighing 3.51 * Ann. der Chem. und Pharm. Ser. 3. vol. cxli. p. 121.

grms. was placed in the inner of two long test-tubes, which were then sunk to the neck in the ice of an ice-calorimeter: a stopper of cotton-wool prevented the outer air from coming into contact with the graphite. As the graphite was gradually cooled down the motion of the mercury thread was carefully noted, with the result that the forward movement was equal to 0·11 division per minute. When the temperature of the graphite was 0° it was quickly removed and placed in the receiving-vessel of the calorimeter, and the movement of the mercury thread again carefully noted. The forward movement was exactly the same as that noticed before the introduction of the graphite; hence it follows that there is not the smallest evolution of heat from the contact of graphite with water. Another experiment proved that the amount of gas absorbed by 3 grms. of lamellar graphite did not alter the weight of the mass so much as one milligramme.

The following experiments were carried out with a piece of very pure lamellar graphite from Ceylon. The percentage of ash in this sample of graphite was 0.38; the numbers deduced for the specific heat will therefore hold good for absolutely pure graphite within 1 or 2 units in the fourth decimal place. I did not determine the hydrogen which was possibly present, as Kopp has found that the amount of this element in Ceylon graphite is exceedingly small, varying from 06 to 17 per cent.

a. Experiments carried on at ordinary Temperatures with the help of the Ice-calorimeter.

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$89. Remarks on Table-The above Table contains the whole of the salts which I have as yet examined fully. The interesting group of the chlorides of the alkaline earths, including magnesium and the no less interesting group of the perchlorides of aluminium and iron, have presented difficulties with which I am still contending. The same is the case with the nitrate of calcium and the chloride of copper, Cu Clą.

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From the evidence before us I think, however, that I may venture to enunciate the general law, that if we define as similar salts either (1) those which consist of the same acid united with bases belonging to the same chemical group (ex. Na, SO4, K2SO4), or (2) those which consist of the same base united with acids belonging to the same group (ex. KNOŽ, KClO), or (3) those whose bases belong to the same group, and whose acids belong to the same group-then, of similar salts, the one which produces the greatest cold when used in a freezing-mixture unites as a cryohydrate with the fewest molecules of water. And to the following law there seems to be only one pronounced exception: The temperature at which the cryohydrate is formed is the same as the temperature of the corresponding freezing-mixture. This latter law, however, has to be taken with reserve as far as those salts are concerned which, like AICI, and MgCl2, decompose water, and also in regard to those bodies which, like CaCl2, unite with water under the liberation of much heat. These I shall consider in my next communication to the Society.

Cryohydrate of Ethylic Alcohol.

§ 90. Of very great interest is the behaviour which is shown by mixtures of ethylic alcohol and water when deprived of heat. This interest extends itself in a practical direction, in consequence of the use of alcoholic liquids in regions of extreme cold. We have here at once a new element for consideration. The two liquids are miscible in all proportions. This means that any possible hydrate of alcohol is soluble at ordinary temperatures both in water and in alcohol. I shall use the word alcohol to denote absolute alcohol, C2 H¿O, the word "spirit" for a mixture of this with water.

and

§ 91. The fact so long known, that heat is liberated and volume finally lost when ethylic alcohol is mixed with water, has silently pointed to the conclusion that there must be at least one definite hydrate of alcohol. It is sufficiently clear that if one were forced to the alternative of relying either upon the amount of heat liberated or upon the loss of volume, the former rather than the latter would be the most trustworthy.

$92. Historical. A useful historical summary of much of what has been previously done in France in this direction of

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