M.\ WATKK AM) ITS COMPOUNDS !M) 



formation of ordinary gaseous solutions play no part whatever in the 

 formation of a solution having a definite boiling point ; that they do 

 act is shown from the fact that such constant gaseous solutions vary in 

 their composition under different pressures/' Therefore, it is not at 



;o For this reason ('the want of entire constancy of the composition of constant boiling 

 solutions with a ch-inge of pressure) nrmy deny the existence of definite hydrates formed 

 by volatile snlst inces for instance, by hydrochloric acid and water. They generally 

 argue as follows: If there did exist a constancy of composition, then it would net be 

 altered by a change of pressure. But the distillation of constant boiling hydrates is un- 

 doubtedly accompanied (judging by the vapour densities determined by Binean). like the 

 distillation of sal-ammoniac, sulphuric acid. Arc., by an entire decomposition of the 

 previous compound that is, these substances do not exist in a state of vapour, but 

 their products of decomposition (hydrochloric acid and water) are gases at the tempera- 

 ture of volatilisation, whi:-h dissolve in the volatilised and condensed liquids ; but the 

 solubility of gases in liquids depends on the pressure, and, therefore, the composition of 

 constant boiling solutions may, and even ought to, vary with a change of pressure, and. 

 further, the smaller the pressure and the lower the temperature of volatilisation, the 

 more likely is a true compound to be obtained. According to the researches of Koscoe 

 and Dittmar (1859), the constant boiling solution of hydrochloric acid proved to contain 

 18 p.c. of hydrochloric acid at a pressure of 3 atmospheres, 20 p.c. at 1 atmosphere, 

 and 28 p.c. at ^ of an atmosphere. On passing air through the solution until its 

 composition became constant (i.e., forcing the excess of aqueous vapour or of hydro- 

 chloric acid to pass away with the air), then acid was obtained containing about 

 20 p.c. at 100, about 23 p.c. at 50, and about 25 p.c. at 0. From this it is seen 

 that by decreasing the pressure and lowering the temperature of evaporation one 

 arrives at the same limit, where the composition should be taken as HC1 + 6H 2 O, which 

 requires 25'26 p.c. of hydrochloric acid. Fuming hydrochloric acid contains more than 

 this. 



The most important fact in evidence of the existence of definite compounds in acids 

 boiling at a constant temperature is the fall of tension. The gas loses its tension, does not 

 follow the law of Henry and Dalton with a diminution of pressure ; its solution oaly parts 

 with water ; the vapour tension of a volatile liquid in solution is less than its own or that 

 of the water combined with it. This loss of tension is a loss of movement brought about 

 by the action of the attraction existing between the water and the substance dissolved. In 

 the case already considered, as in the case of formic acid in the researches of D. P. 

 Konovaloff (note 47), the constant boiling solution corresponds with a minimum tension 

 that is, with a boiling point higher than that of either of the component elements. But 

 there is another case of constant boiling solutions similar to the case of the solution of 

 propyl alcohol, C.'-H^O, when a solution, undecomposed by distillation, boils at a lower 

 point than that of the more volatile liquid. However, in this case also, if there be 

 solution, the possibility cannot be denied of the formation of a definite compound in the 

 form C-,H s O-fH 2 O, and the tension of the solution is not equal to the sum of tensions 

 of the components. There are possible cases of constant boiling mixtures even when there 

 is no solution nor any loss of tension, and consequently no chemical action, because the 

 amount of liquids that, are volatilised is determined by the product of the vapour den 

 into their vapour tensions (Wanklyn), in consequence of which liquids whose boiling 

 point is above 100 for instance, turpentine and ethereal oils in general when distilled 

 with aqueous vapour, pass over at a temperature below 100. Consequently, it is not in 

 the constancy of composition and boiling point (temperature of decomposition) that the 

 signs of a clear chemical action should be seen in the above-described solutions of acids, 

 but in the great loss of tension, which completely resemble* the loss of tension ob- 

 >erved. for instance, in the perfectly-definite combinations of substances with water of 

 crystallisation (see later, note i'i.">). Sulphuric acid. H..SO,. as we shall learn later, is a!-o 

 decomposed by distillation, like HC1 + 6H. 2 O, and exhibits, moreover, all the signs of a 



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