100 FIUNril'LES OF CHEMISTRY 



every, but only at the ordinary, atmospheric pressure that a constant 

 boiling solution of hydriodic acid will contain 57 p.c. of the gas. At 

 another pressure the proportion of water and hydriodic acid will be 

 different. It varies, however, judging from observations made by Roscoe, 

 very little for considerable variations of pressure. This variation in 

 composition directly indicates that pressure exerts an influence on the 

 formation of unstable chemical compounds which are easily dissociated 

 (with formation of a gas), just as it influences the solution of gases, 

 only the latter is influenced to a more considerable degree than the 

 former/' 1 Hydrochloric, nitric, and other acids form solutions 1iarin</ 

 definite boiling points, like that of hydriodic acid. They show further 

 the common property, if containing but a small proportion of water, that 

 they fume in air. Strong solutions of nitric, hydrochloric, hydriodic, 

 and other gases are even termed ' fuming acids.' The fuming liquids 

 contain a definite compound, whose temperature of ebullition (decom- 

 position) is higher than 100, and contain also an excess of the volatile 

 substance dissolved, which (the substance) exhibits a capacity to com- 

 bine with water and form a hydrate, whose vapour tension is less than 

 that of aqueous vapour. On evaporating in air, this dissolved substance 

 meets the atmospheric moisture and forms a visible vapour (fumes) with 

 it, which consists of the above-mentioned compound. The attraction 

 or affinity which binds, for instance, hydriodic acid with water is 

 evinced not only in the evolution of heat and the diminution of vapour 

 tension (rise of boiling point), but also in many purely chemical rela- 

 tions. Thus hydriodic acid is produced from iodine and hydrogen 

 sulphide in the presence of water, but unless water is present this re- 

 action does not take place/' 2 



definite chemical compound. The study of the variation of the specific gravities of 

 solutions as dependent on their composition (see note 19) shows that phenomena of a 

 similar kind, although of different dimensions, take place in the formation of both H 2 SO 4 

 from H 2 O and SO 3 , and of HC1 + 6H. 2 O (or of aqueous solutions analogous to it) from HC1 

 and H 2 0. 



61 The essence of the matter may be thus represented. A substance A, either gaseous 

 or easily volatile, forms with a certain quantity of water, ?zHoO, a definite complex com- 

 pound AnH^O, which is stable up to a temperature t 3 higher than 100 3 . At this tempera- 

 ture it is decomposed into two substances, A + H 2 O. Both boil below t at the ordinary 

 pressure, and therefore at t they distil over and re-combine in the receiver. But if a 

 part of the substance AnfL^O is decomposed or volatilised, there still remains a portion of 

 undecomposed liquid in the vessel, which can partially dissolve one of the products of 

 decomposition, and that in quantity varying with the pressure and temperature, and 

 therefore the solution at a constant boiling point will have a slightly-different composition 

 at different pressures. 



62 For solutions of hydrochloric acid in water there are still greater differences in 

 reactions. For instance, strong solutions decompose antimony sulphide (forming hydro- 

 gen sulphide, H 2 S), and precipitate common salt from its solutions whilst weak solutions- 

 do not act thus. 



