98 PRINCIPLES OF CHKMJSTKY 



hydrate with 10 molecules of water, and in the case of sodium nitrate, 

 the cryohydrate ~' 9 with 7 molecules of water (i.e., 126 parts of water 

 per 85 of salt) should be accepted as established substances, capable of 

 passing from a solid to a liquid stare and conversely ; and therefore it 

 may be thought that in cryohydrates we have solutions which are not 

 only undecomposable by cold, but also have a definite composition which 

 would present a fresh case of definite equilibrium between the solvent 

 and the substance dissolved. 



The formation of definite but unstable compounds in the process of 

 solution becomes evident from the phenomena of a marked decrease of 

 vapour tension, or from the rise of the temperature of ebullition which 

 occurs in the solution of certain volatile liquids and gases in water. As 

 an example, we will take hydriodic acid, HI, a gas which liquefies on 

 a very considerable reduction of temperature, giving a liquid which 

 boils at - 20. A solution of it containing 57 p.c. of hydriodic acid is 

 distinguished by its great stability. If it be evaporated by heating, 

 the hydriodic acid volatilises together with the water in the same 

 proportions as they occur in the solution, so that the gas passes off 

 together with the aqueous vapour, and therefore such a solution may be 

 distilled unchanged, for the distillate will contain the same proportion 

 of hydriodic acid and water as was originally taken. The solution 

 boils at a higher temperature than water. The physical properties of 

 the gas and water in this case already disappear ; there is formed a 

 stable compound between water and the gas, a new substance which 

 has its definite boiling point. To put it more correctly, this is not the 

 temperature of ebullition, but the temperature at which the compound 

 formed decomposes, forming the vapours of the products of dissociation, 

 which, on cooling, re-combine. The above-described aqueous solution 

 boils at 127. Should a less amount of hydriodic acid be dissolved 

 in water than the above, then, on heating such a solution, water only 

 will at first be 'distilled over, until the solution attains the above- 

 mentioned composition ; it will then distil over unaltered. If more 

 hydriodic acid be passed into such a solution a fresh quantity of the 

 gas will dissolve, which, however, may be very easily removed. It 

 must not, however, be thought that those forces which determine the 



on the addition of ice deposit ice, which would occur if ice in intermixture with the- salt 

 were formed on solidification. 



I may add with regard to cryohydrates that, in investigating aqueous solutions of 

 alcohol (note 19), I concluded, on the basis of the specific gravity, that a compound, 

 C 2 H6O + 12H2O, existed, and a solution -of this composition completely solidifies on cool- 

 ing to 20, forming well-formed crystals, which melt at about 18, as was shown by 

 observations made by W. E. Tischenko and myself. This definite compound reminds 

 one of cryohydrates in many respects. 



59 See note 24. 



