460 PRINCIPLES OF CHEMISTRY 



valency of the elements, and, in connection with them, the various numbers 

 of affinities. In reality, if a system of atoins or a molecule he g'ven, then in 

 it, according to the third law of Newton, each portion of atoms acts on the 

 remaining portion in the same manner, and with the same force as the 

 second set of atoms acts on the first. We infer directly from this considera- 

 tion that both sets of atoms, forming a molecule, are not only equivalent with, 

 regard to themselves, as they must be according to Dalton's law, but also that 

 they may, if united, replace each other. Let there be a molecule containing 

 .atoms A B C, it is clear that, according to Newton's law, the action of A on 

 B C must be equal to the action of B C on A, and if the first action is directed 

 on B C, then the second must be directed on A, and consequently then, where 

 A can exist in dynamic equilibrium, B C may take its place and act in a like 

 manner. In the same way the action of C is equal to the action of A B. In 

 one word every two sets of atoms forming a molecule are equivalent to each 

 other, and may take each other's place in other molecules, or, having the 

 power of balancing each other, the atoms or their complements are endowed 

 with the power of replacing each other. Let us call this consequence of an 

 evident axiom ' the principle of substitution,' and let us apply it to those 

 typical forms of hydrogen compounds which we have already discussed, and 

 which, on account of their simplicity and regularity, have served as starting-- 

 points of chemical argument long before the appearance of the doctrine of 

 structure. 



In the type of hydrofluoric acid, HF, or in systems of double stars, are 

 included a multitude of the simplest molecules. It will be sufficient for our 

 purpose to recall a few : for example, the molecules of chlorine, Cl^, and of 

 hydrogen, H^, and hydrochloric acid, HC1, which is familiar to all in aqueous 

 solution as spirits of salt, and which has many points of resemblance with 

 HF, H Br, HI. In these cases division into two parts can only be made in 

 one way, and therefore the principle of substitution renders it probable that 

 exchanges between the chlorine and the hydrogen can take place, if they are 

 competent to unite with each other. There was a time when no chemist 

 would even admit the idea of any such action ; it was then thought that the 

 power of combination indicated o, polar difference of the molecules in com- 

 bination, and this thought set aside all idea of the substitution of one com- 

 ponent element by another. 



Thanks to the observations and experiments of Dumas and Laurent fifty 

 years ago, such fallacies were dispelled, and in this manner the principle 

 of substitution was exhibited. Chlorine and bromine acting on many 

 hydrogen compounds, occupy immediately the place of their hydrogen, and 

 the displaced hydrogen, with another atom of chlorine or bromine, forms 

 hydrochloric acid or bromide of hydrogen. This takes place in all typical 

 hydrogen compounds. Thus chlorine acts on this principle on gaseous 

 hydrogen reaction, under the influence of light, resulting in the formation 

 of hvdrochloric acid. Chlorine acting on the alkalis, constituted similarly to 

 water, and even o'n water itself only, however, under the influence of light 

 and only partially because of the instability of HC1O forms by this principle 

 bleachirg salts, which are the same as the alkalis, but with their hydrogen 

 replaced by chlorine. In ammonia and in methane, chlorine can also replace 



