18 KEPOKT — 1887. 



sword to open the oyster of science would have been wanting to us if we 

 had not taken a step farther than Dalton did, in the recognition of the 

 distinctive nature of the elemental atoms. We now assume on good 

 grounds that the atom of each element possesses distinct capabilities of 

 combination ; some a single capability, others a double, others a triple, 

 and others again a fourfold combining capacity. The germs of this theory 

 of valency, one of the most fruitful of modern chemical ideas, were 

 enunciated by Frankland in 1852, but the definite explanation of the 

 linking of atoms, of the tetrad nature of the carbon atoms, their power of 

 combination, and of the'diflference in structure between the fatty and aro- 

 matic series of comjDOunds, was first pointed out by Kekule in 1857; though 

 we must not forget that this great principle was foreshadowed so long ago 

 as 1833 from a physical point of view by Faraday in his well-known 

 laws of electrolysis, and that it is to Helmholtz in his celebrated Faraday 

 lecture that we owe the complete elucidation of the subject ; for, whilst 

 Faraday has shown that the number of the atoms electrolytically deposited 

 is in the inverse ratio of their valencies, Helmholtz has explained this by the 

 fact that the quantity of electricity with which each atom is associated is 

 directly proportional to its valency. 



Amongst the tetrad class of elements, carbon, the distinctive element 

 of organic compounds, finds its place ; and the I'emarkable fact that the 

 number of carbon compounds far exceeds that of all the other elements 

 put together receives its explanation. For these carbon atoms not only 

 possess four means of grasping other atoms, but these four-handed carbon 

 atoms have a strong partiality for each other's company, and readily 

 attach themselves hand in hand to form open chains or closed rings to 

 which the atoms of other elements join to gi'asp the unoccupied carbon 

 hand, and thus to yield a dancing company in which all hands are locked 

 together. Such a group, each individual occupying a given position with 

 reference to the others, constitutes the organic molecule. When, in 

 such a company, the individual members change hands, a new combination 

 is formed. And as in such an assembly the eye can follow the changing 

 positions of the individual members, so the chemist can recognise in his 

 molecule the position of the several atoms, and explain by this the fact 

 that each an-angement constitutes a new chemical compound possessing 

 difierent properties, and account in this way for the decompositions which 

 each differently constituted molecule is found to undergo. 



Chemists are, however, not content with representing the arrangement 

 of the atoms in one plane, as on a sheet of paper, but attempt to express 

 the position of the atoms in space. In this way it is possible to explain 

 certain observed differences in isomeric bodies which otherwise baffled our 

 eff'orts. To Van t'Hoff', in the first instance, and more recently to 

 Wislicenus, chemistry is indebted for work in this direction, which throws 

 light on hitherto obscure phenomena, and points the way to still further 

 and more important advances. 



It is this knowledge of the mode in which the atoms in the molecule 

 are arranged, this power of determining the nature of this arrangement, 



