﻿398 Royal Institution : — 



From this examination of chemical exchange, it will be obvious 

 that no operation of this kind can produce a change in the " atomi- 

 city " of an atom ; for every relation ruptured a new one is en- 

 tered into. But we have no reason to suppose that all chemical 

 action is of this kind ; and there are numerous phenomena which it 

 is very difficult to explain, except by the assumption that there is an- 

 other kind of chemical action, in which the number of relations of 

 an atom is increased or diminished. Such actions are those by 

 which we pass from one series of compounds to another. Thus the 

 ferrous salts are connected together by processes of exchange ; but 

 it is only by making new hypotheses that we can thus explain the 

 passage from the ferrous to the ferric salts. Similar relations exist 

 between the manganous salts, the manganic salts, the manganates, 

 and the permanganates, where a consideration of each group, apart 

 from the others, would lead us to a different atomicity for manga- 

 nese ; and many other examples might be given of the same kind. 

 The speaker considered it, in the meantime, to be better to regard 

 each such series separately rather than by an attempt to bring all 

 chemical processes under one class to endanger the stability of the 

 theory of chemical structure, which, while it is probably not de- 

 stined, in its present form, to remain as a permanent part of the great 

 edifice of the science, is certainly a most convenient scaffolding, not 

 easy to replace, and not hastily to be thrown down. 



Having thus seen what is meant by chemical structure, and how 

 we arrive at a knowledge of it by a study of the history of the sub- 

 stance, of the ways in which it may be formed and in which it may 

 be decomposed, we may now glance at the relations which exist 

 between the chemical structure of a substance and its physical and 

 physiological properties. We shall consider specially two of the 

 physical characters of matter, volatility and colour, and examine in 

 what way these are modified by the performance upon the substance 

 of certain specified chemical operations. The volatility of a sub- 

 stance depends upon two things : — 1st, the temperature at which 

 the substance boils under a particular pressure ; and 2nd, the change 

 of boiling-point produced by a change of pressure. In order, there- 

 fore, fully to know the volatility of a substance, its boiling-point 

 must be determined through a very great range of pressure. This 

 involves great labour ; and only a few substances have been thus 

 fully examined. Almost all we know on this interesting question 

 is due to the ingenious and patient experiments of Regnault. These 

 do not, as yet, furnish us with sufficient data to enable us to deduce 

 anything like a law. They show us, however, that a mere com- 

 parison of boiling-points under an arbitrarily selected pressure 

 (such as 760 millimetres, which happens to be the mean pressure of 

 the atmosphere) cannot lead us to a law, as the boiling-points of two 

 substances are frequently changed very unequally by a change of 

 pressure. 



Such comparisons of boiling-points have been made, and from 

 them have been deduced, especially by Kopp, a series of very in- 

 teresting and certainly not fortuitous coincidences. That dis- 

 tinguished chemist and physicist has shown that, in a very large 



