TKANSACTIONS OF SECTION B. 663 



Meyer, we shall do well in this, as indeed in all similar cases in science, to remem- 

 ber the danger pointed out in Bacon's aphorism, that ' The mind delights in spring- 

 ing up to the most general axioms, that it may find rest, but after a short stay here 

 it disdains experience,' and to bear in mind that it is only the lawful union of 

 hypothesis with experiment which will prove a fruitful one in the establishment of 

 a systematic inorganic chemistry which need not fear comparison with the order 

 which reigns in the organic branch of our science. And here it is well to be 

 reminded that complexity of constitution is not the sole prerogative of the carbon 

 compounds, and that before this systematisation of inorganic chemistry can be 

 effected we shall have to come to terms with many compounds concerning whose 

 constitution we are at present wholly in ignorance. As instances of such I would 

 refer to the finely crystalline phospho-molybdates, containing several hundred atoms 

 in the molecule, lately prepared by Wolcott Gibbs. 



Arising out of Kekule's theory of the tetrad nature of the carbon atom, came 

 the questions which have caused much debate among chemists : (1) Are the four 

 combining units of the carbon atom of equal value or not ? and (2) Is the assump- 

 tion of a dyad carbon atom in the so-called non-saturated compounds justifiable 

 or not ? The answer to the first of these, a favourite view of Kolbe's, is given 

 in the now well-ascertained laws of isomerism ; and from the year 1862, when 

 Schorlemmer proved the identity of the hydrides of the alcohol radicals with the so- 

 called radicals themselves, this question may be said to be set at rest ; for Lossen 

 himself admits that the existence of his singular isomeric hydroxylamine deri- 

 vatives can be explained otherwise than by the assumptiou of a difference between 

 each of the combining units of nitrogen, and the differences supposed by Schreiner 

 to hold good between the methylethyl carbonic ethers have been shown to have 

 no existence iu fact. With respect to the second point the reply is no less definite, 

 and is recorded in the fact, amongst others, that ethylene chlorhydrin yields on 

 oxidation chloracetic acid, a reaction which cannot be explained on the hypothesis 

 of the existence in ethylene of a dyad carbon atom. 



Passing from this subject, we arrive, by a process of natural selection, at more 

 complicated cases of chemical orientation — that is, given certain compounds which 

 possess the same composition and molecular formuhB but varying properties, to 

 find the difference in molecular structure by which such variation of properties is 

 determined. Problems of this nature can now be satisfactorily solved, the number 

 of possible isomers foretold, and this prediction confirmed by experiment. The 

 general method adopted in such an experimental inquiry into the molecular 

 arrangement or chemical constitution of a given compound is either to build up the 

 structure from less complicated ones of known constitution, or to resolve it into 

 such component parts. Thus, for example, if we wish to discriminate between 

 several isomeric alcohols, distinguishing the ordinary or primary class from the 

 secondary or tertiary class, the existence of which was predicted by Kolbe in 

 1862, and of which the first member was prepared by Friedel in 1864, we have 

 • to study then- products of oxidation. If one yields an acid having the same 

 number of carbon atoms as the alcohol, it belongs to the first class and possesses 

 a definite molecular structure ; if it splits up into two distinct carbon compounds, 

 it is a secondary alcohol ; and if three carbon compounds result from its oxida- 

 tion, it must be classed in the third category, and to it belongs a definite molecular 

 structure, different from that of the other two. 



In a similar way orientation in the much more complicated aromatic hydro- 

 carbons can be effected. This class of bodies forms the nucleus of an enormous 

 number of carbon compounds which, both from a theoretical and a practical point 

 of view, are of the highest interest. For these bodies exhibit characters and 

 possess a constitution totally different from those of the so-called fatty substances, 

 the carbons atoms being linked together more intimately than is the case in the 

 latter-named group of bodies. Amongst them are found all the artificial colouring 

 matters, and some of the most valuable pharmaceutical and therapeutical agents. 



The discovery of the aniline colours by Perkin, their elaboration by Hofmann, 

 the synthesis of alizarin by Graebe and Liebermann, being the first vegetable 

 colouring matter which has been artificially obtained, the artificial production of 



