450 Frederick Guthrie on Salt Solutions 



ing the residual body), and an acid containing 60 per cent, 

 is strengthened by boiling, until in both cases an acid con- 

 taining nearly 70 per cent, of HN0 3 is got, which boils at 

 120° 0. at 760 millims., this body was viewed as a definite 

 hydrate. Roscoe showed that this acid, if it be boiled at a 

 lower or higher pressure, is itself decomposed. So with hy- 

 drobromic, hydrochloric, and hydriodic acids, with sulphuric 

 acid, and with ammonia. We have here physical ratios con- 

 ditioned by pressure. Without doubt the cryohydrates 

 would vary in composition if they were formed at enormous 

 pressures ; for the variation effected by pressure in the 

 freezing-point of water is not likely to be precisely the same 

 as the variation in the solidifying of a salt out of a solution. 

 Trifling as these differences would assuredly be under all ■ 

 conditions of variation of artificial pressure, the thought is 

 not remote that wo may have to refer the extraordinary va- 

 riation in quantity of the elements of some perfectly well 

 crystallized and constantly composed minerals from what is 

 sometimes called the atomic ratio to the enormous pressure 

 attending their genesis. The fact that such composition may 

 be represented on paper by the complex manipulation of 

 ratios known amongst laboratory products, can only be ad- 

 mitted into the argument when their formation can be shown 

 in the laboratory by means such as may be fairly supposed 

 to exist in nature. In a word, under vast changes of tem- 

 perature and pressure, the gravimetric saturating quantities, 

 or " atomic weights," of elementary substances may vary. 

 And it is perhaps noteworthy that this conception, which 

 has urged itself upon us with rare force in the consideration 

 of the relations by weight of bodies in union at extremely 

 low temperatures, is the same as that which has been brought 

 forward by some of those who have examined the relation of 

 the elements when subjected to the high temperature and 

 abnormal pressure of the sun's mass, which has, as a doc- 

 trine of continuity, commended itself to some of the most 

 philosophic chemists, and which is certainly in harmony with 

 the conception of dynamic energy, whether of mass or mere 

 velocity. 



Our chemistry, as it is generally understood, is the chemistry 

 of one atmosphere pressure. 



Separation of Ice from Solutions of Mixtures of Salts 



§ 143. In III. §§ 109-122, I proved that on cooling a 

 saturated solution of two salts, the temperature of the cryo- 

 hydrate, when reached after separation of various quantities 

 of the constituents, was never sensibly lower than that of the 



