Affinity and Heat. 299 
“It must be observed, that recourse has always been had 
successively to the physical properties of air to solve so 
important a question. _ Biot went so far even as to adduce 
the equality between the refractive indices of air, and of 
the mean of the indices of oxygen and of nitrogen, as a 
proof in favour of the opinion now generally admitted. 
If, in fact, there had been found in a constant manner a 
measurable physical property, different in air and in a 
mixture of oxygen and nitrogen, the conclusion would 
have been immediately drawn, that air was a combination. 
There would have been a change of condition, owing to 
the mixture of the two gases, and the beautiful verification 
of Lavoisier would have been invalidated in an irrefutable 
manner. Theargument drawn from the proportions of the 
two gases, oxygen and nitrogen, which are not in atonic 
proportion in the air, is without value. One of the two 
elements might simply have been in excess over a combi- 
nation of the twoothers. There would have been mixture 
and combination, and, therefore, change of state for one 
portion of the air alone, which is not the case. 
Combination being thus characterised by change of state, 
and, therefore, well defined, what is the particular form of 
combination which merits more especially the name of 
solution? I say especially; for it’ would be impossible to 
say now whether water and sulphuric acid, acetic acid, 
chloride of calcium, and many other bodies, are combined 
with, or merely dissolved in one another. But there is a 
fundamental difference between the effects produced when 
potash is thrown into sulphuric acid, and the effect ob- 
served when common salt is placed in water. 
In both cases the change of state is manifest; but when 
sulphate of potash has been obtained after the combination 
of sulphuric acid and of potash, new chemical properties 
have become strikingly evident. It is, in fact, by these 
chemical properties that change of state is most clearly 
manifested at the time of energetic combination. In the 
second place, when the so/ution of common salt in water is 
affected, the chemical properties of the elements in presence 
remain virtually unchanged. First,a true fusion of the salt 
is noticed, which assumes the liquid state—a change of 
_ physical condition, accompanied most frequently by an 
absorption and disappearance of heat—then variation in 
the density, or phenomena of contraction, alteration in the 
volume of the elements, which, again, is a change of 
physical properties, which essentially characterises true 
solution. 
