84 Prof. Louis Henry on the Polymerization 



in the natural world, that exceptions prove the rule. True 

 exceptions, true anomalies, have no real existence ; these 

 apparent deviations in the effects of a general known cause 

 are due to the influence of other causes, acting simultaneously, 

 the existence of which remains momentarily hidden from view. 



Among the difficulties which are met with in the compara- 

 tive study of bodies, there are some which are peculiarly 

 embarrassing, and which it would be chimerical to try to 

 solve under present circumstances. But there are others, 

 and I believe a very considerable number, which are purely 

 artificial, without real foundation ; difficulties which w r ould 

 vanish of themselves if we only had a less imperfect know- 

 ledge of the bodies with which they occur — if, in fact, the 

 chemical signification of these bodies molecularly in relation 

 to others were only clearly and surely defined. 



This point of view, from which bodies should be studied, is 

 not absolutely new. Sulphur and the fatty acids, especially 

 acetic acid, are well-known examples of substances which 

 have been clearly distinguished in their various physical 

 states. Moreover, the determination of the relative molecules 

 of bodies in their various states also touches on a question of 

 still greater interest, viz. that of the chemical relationships of 

 the various physical states in which they occur. 



The molecular formulae at present in use refer to the gaseous 

 state, and the principal method of determining them depends 

 on the vapour-density. This process, simple and expeditious, 

 is undoubtedly excellent in all cases in w r hich it is applicable. 

 Others, however, are necessary for the determination of mole- 

 cules in a state other than that of a perfect gas. It is in the 

 complete study of bodies in their various conditions that we 

 should find indications sufficient for information as to their 

 molecular nature. The several circumstances which charac- 

 terize the chemical molecule, both chemically and physically, 

 are in intimate connection, and to determine this molecule we 

 can appeal to all connected therewith, either in its past, pre- 

 sent, or future : in its past, to examine its mode of formation 

 and the various circumstances bearing on its chemical origin ; 

 in its present, to carefully investigate the whole of its pro- 

 perties — physical, mechanical, and chemical ; in its future, to 

 study the reactions in which it may take part, the products of 

 its decomposition, its metamorphoses, &c. 



This complete study of bodies, from both the statical and 

 dynamical point of view, always gives, though it may be 

 imperfect, valuable information for solving our present pro- 

 blem. It is, however, necessary to say, that when I speak of 

 actual molecules and of actual molecular formulae, I do not 



