514 Lr. D. MendeUeff [May 31, 



ments are endowed with the power of replacing each other. Let us 

 call this consequence of an evident axiom " the principle of substitu- 

 tution," and let us apply it to those typical forms of hydrogen 

 compounds, which we have already discussed, and which, on account 

 of their simj)licity 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 simj^lest molecules. It will be 

 sufficient for our purpose to recall a few : for example the molecules 

 of chlorine, CP, and of hydrogen, H^, and hydrochloric acid, HCL, 

 which is familiar to all in aqueous solution as spirit of salt, and 

 which has many points of resemblance wdth HF, IIB2, 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 ex- 

 changes between tbe 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 a polar difference of 

 the molecules in combination, and this thought set aside all idea of 

 the substitution of one component element by another. 



Thanks to the observations and experiments of Dumas and 

 Laurent fifty years ago, such fallacies were dispelled, and in 

 this manner, this same principle of substitution w^as 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 hy- 

 drogen — reaction, under the influeace of light, resulting in the forma- 

 tion of hydrochloric acid. Chlorine acting on the alkalis, constituted 

 similarly to water, and even on water itself — only, however, under 

 the influence of light and only partially because of the unstability 

 of HCIO — forms by this princijile bleaching salts, which are the same 

 as the alkalis, but with their hydrogen replaced by chlorine. In 

 ammonia and in methane, chlorine can also replace the hydrogen. 

 From ammonia is formed in this manner the so-called chloride of 

 nitrogen, NOP, which decomposes very readily with violent explosion 

 on account of the evolved gases, and falls asunder as chlorine and 

 nitrogen. Out of marsh gas, or methane, CPP, may be obtained con- 

 secutively, by this method, every possible substitution, of which chloro- 

 form, CHCP, is the best known, and chloro-carbonic acid, CCP the 

 most instructive. But by virtue of the fact that chlorine and bromine 

 act, in the manner shown, on the simplest typical hydrogen compounds, 

 their action on the more complicated ones may be assumed to be the 

 same. This can be easily demonstrated. The hydrogen of benzole, 

 C^il^ reacts feebly under the influence of light on liquid bromine, but 

 Gustavson has shown that the addition of the smallest quantity of 



