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containing 292 molecules of water. Accordingly
Chromate, K, CrO4 Bichromate, K, Cr, O
$84. Ammonium Alum.-This body was examined on account of the large percentage of water which is held by the ordinary hydrated salt. The cryohydrate solidifies at −0.2. The part contained last to solidify was examined; of this, 4:38 15 0-2060 grm. of anhydrous alum. This shows 47 per cent. Of the immediately preceding crop of crystals, 5-2400 grms. gave 0-2220 of anhydrous alum, or 4.2 per cent. The first of these determinations indicates the relationship
Al, NH2SO4+261·4 H2 0.
§85. Perchloride of Mercury.-A saturated solution of corrosive sublimate solidifies at -0.2. The ultimate and penultimate portions were examined. Of the former, 6·0640 grms. gave 0.197, or 3.24 per cent.; of the latter, 4.977 grms, gave 0-164, or 3-29 per cent. The first indicates the formula HgCl2+ 450 H2 O.
$86. Oxalate of Ammonium.-This solidifies as a cryohydrate at 0°2. Of the last portion to solidify, 4:340 grms. contained 0.125, or 2.8 per cent. of the anhydrous salt. This agrees with the relation
NH, CO,+239-1H, O.
§87. Carbonate of Sodium.-After igniting the carbonate so as to decompose any bicarbonate, the saturated salt solidifies as a cryohydrate at -2°. Of the final portion, 6·4090 grms. contained 0.383 of Na, CO. This shows 5.97 per cent., or the relation
Na, CO, +92-75 H2O.
$88. The following Table shows at a glance the relation be tween the lowest attainable temperature when the salt is mixed with ice, the temperature of the solidification of the cryohydrate, the water-worth or aquavalent. Column 1 shows the salt employed, and the degree of hydration when associated with water of crystallization. Column 2 shows the temperature obtained when the salt is mixed with ice. Column 3 shows the tempeperature at which the cryohydrate separates. In column 4 are hown the number of molecules of water associated with one nolecule of the salt in the cryohydrate. It is here called 'water-worth." In column 5 are the percentages of the anhy rous salt which the final portions of the cryohydrates contained,
according to the analyses given above. The letters M.L show that the liquids here analyzed stood before solidification in the relationship of mother-liquors to the preceding crops of cryohydrate. In column 6 are analyses of the crop of cryohydrate immediately preceding the last cryohydrate. The salts are arranged according to the degree of cold attainable when the salt is used as a cryogen, i. e. in a freezing-mixture.
TABLE X.-Showing (1) the chemical formula of the salt, (2) the lowest temperature to be got by mixing the salt with ice, (3) temperature of solidification of the cryohydrate, (4) molecular ratio between anhydrous salt and water of its cryohydrate (water-worth or aquavalent), (5) percentage of anhydrous salt in portion of cryohydrate last to solidify, (6) percentage of anhydrous salt in crop of cryohydrate before the last. (5) (6) Percentage Percentage of anhy- of anhydrous salt drous salt in last cryo- in next to hydrate. last cryoM.L. hydrate.
Formula of salt.
K, Cro...... BaCl2+2H, O Sr, NO... MgSO +7H2O Zn50, +7 H2O KNO3 Na CO3 CuSO,+5H, 0 FeSO,+7H2O K2SO K2 Cr2 07 Ba2 NO, Na, SO+10H2O KCIO AI, NÃ ̧ 280+1211,0 HgCl2
(4) Tempera- Molecular Tempera-ture of soliratio or ture of dification of water-worth cryogen. cryohy- or aquavalent.
- 22 18 -17.5
- 16.5 16 - 13 -10.5 -10.2
§ 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 Cl.
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. KNO, 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 AlCl, and MgCl,, decompose water, and also in regard to those bodies which, like CaCl,, 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 HO, and the word "spirit" for a mixture of this with water.
§ 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
research is prefixed to a recent paper on the subject by M. Melsens, in the Annales de Chimie et de Physique, entitled "Sur la refroidissement et la congélation des liquides alcoholiques et des vins." According to M. Boussingault, frozen wines after thawing furnish an alcoholic liquid and are not therefore pure ice. According to M. Melsens, alcoholic liquids containing about 50 per cent. of alcohol by weight or by volume become at-30°C. viscid, syrupy, and sometimes opalescent. These represent commercial spirits such as rum, cognac, &c., and may be represented by the formula C, HO+3H, O, corresponding to the maximum condensation. According, again, to M. Melsens, when wine which has become semisolid through being exposed to cold of a freezing-mixture is drained through wire gauze or introduced into a turbine, nearly colourless ice free from alcohol is left. From frozen wine containing from 10 to 12 per cent. of alcohol, from 16 to 25 per cent. of pure ice was got by means of a screw press. By the same means a frozen red or white Burgundy yielded 40 per cent. of ice.
§ 93. In the first of a series of able researches on the physical properties of mixtures of water with the alcohols, Messrs. Dupré and Page (Proc. Roy. Soc. March 11, 1869) examined, amongst others, the quantity of heat developed on mixing alcohol and water in various proportions, the specifie heat of such mixtures, their capillarity, boiling-point, and their compressibility.
The following fragments of these experimenters' Tables include the critical values.
TABLE showing number of heat-units evolved from 5 grammes of mixtures resulting from mixing the percentages by weight of alcohol in column 1 with the complementary percentage of water. The asterisk shows the critical region.
In the next Table Messrs. Dupré and Page's numbers are given, showing the specific heats of such mixtures. The column 1 shows the percentage of alcohol, column 2 the specific heat, column A the difference between the observed and calculated specific heats.
Messrs. Dupré and Page epitomize one branch of their research as follows:-"The whole of the physical characters of mixtures of alcohol and water come to a maximum deviation from their theoretical values somewhere between 30 and 45 per cent. of alcohol by weight. The 30 per cent. nearly corresponds to the formula C2 HO+6H2O (29.87 per cent.); the 45 per cent. has approximately the formula C, HO+3H2O (46 per cent.)." The mean of these values is
C, HO+45 H, O.
§ 94. According to Rudberg (Pogg. Ann. vol. xiii. p. 496), the contraction is greatest when 55 volumes of alcohol are mixed with 45 volumes of water, or 43.6 weights of alcohol with 45 weights of water. This corresponds to the formula
§ 95. According to Bussy, alcohol not stronger than 33 Beaumé may be frozen by the evaporation of SO,. This strength is that of 78-29 per cent. of alcohol by weight, or
According to Marchand (Journ. für Chemie, vol. xxv. p. 253), when 1 part by weight of spirit is mixed with 1 part of snow, the depression of temperature depends upon the strength of the spirit, according to the following Table :
This is, I believe, the condition of the question as left by others.
§ 96. My own experiments.-The alcohol I used was shaken with dry carbonate of potassium and distilled from quicklime.
With this I made decimal mixtures ranging from 95 alcohol and 5 of water to 95 water and 5 of alcohol. These were submitted in turn to the action of a cryogen, in order first of all to see at what temperature the solidification of each mixture begins. For if, as has been supposed, and quite recently by Melsens again proved, a very weak (10 per cent.) spirit gives up only ice, and since alcohol is notoriously not to be solidified by our most powerful cryogens, it must follow that during the continuous solidification of a weak spirit the temperature must fall continually. Temperatures down to -19° were observed on the mercurial thermometer. For lower temperatures, demanding the employment of solid carbonic acid and ether, use was made of an alcohol thermometer, which was collated with the mercurial one at -22°. The Table gives the temperature at which the solution began to yield solid matter. What this solid matter consists of I have afterwards to consider. Columns A, and A, are the values of the first and second differences respectively.
TABLE XI.-Temperatures at which Solidification begins in Spirits of various strengths.
From this Table it is seen that the temperature of initial solidification sinks so regularly that the column of second differences shows for a long time the value 0.6. Only at lower temperatures, which of course cannot pretend to the same degree of accuracy, are serious variations visible. There is a rapid fall at the ratio 35 water to 65 alcohol; and at the ratio 30 water to 70 alcohol, I failed to effect solidification at 65° C.
§ 97. So free from discontinuity are these numbers, that one might be readily misled into the belief that the solid matter