608 



SCIENCE. 



[N. S. Vol. IX. No. 226. 



ties for condensation, whether by polymeri- 

 zation or by union of two or more molecules 

 with separation of water or ammonia. The 

 so-called double and triple union between 

 carbon atoms only exceptionally leads to 

 spontaneous polymerization, while with sil- 

 icon this latter is apparently the rule. The 

 important carbouyl group, C ^ O, the char- 

 acteristic group of organic acids, aldehydes 

 and ketones, shows but little tendency to 

 polymerize, while organic hydroxyl com- 

 pounds are usually stable and do not spon- 

 taneously give rise to ethers or acid anhy- 

 drides. The silicon analogue of carbonyl, 

 Si = 0, on the contrary, appears to poly- 

 merize with great ease. The ethers of 

 carbonic acid are well known, but the 

 metasilicic ethers, those of the type SiO 

 (OE,)^. appear to exist only as polymers. 

 The silicic acids, too, show a marked ten- 

 dency to condense by dehydration and pass 

 spontaneously into complex bodies. It is 

 easy to see what would have been the re- 

 sult if carbon behaved like silicon. In- 

 stead of the innumerable sharply defined 

 organic acids, aldehydes, ketones and alco- 

 hols, each produced by a definite synthetic 

 process, each reaction would give rise to an 

 almost inextricable mixture of condensation 

 products, carbon dioxide would be a solid 

 like silica, and organic chemistry would be 

 scarcely further advanced than is the chem- 

 istry of silicon. This tendency of carbon 

 compounds to simplicity in reaction, each 

 molecule acting as if it were alone present, 

 has been, therefore, an important factor in 

 facilitating the growth of organic chemistry. 



4. Another feature of carbon, which plays 

 an important part, is the ease with which 

 intermediate or transition products can be 

 formed. It is much easier to limit reactions 

 in the case of carbon compounds than in 

 others. Compare, for example, the action of 

 chlorine on CH, and SiH^. 



5. The tendency to dissociation, both hy- 

 drolytic and electrolytic, is in general less 



marked among carbon compounds, whence 

 it is easier to control the course of a reac- 

 tion and to exclude changes of a sponta- 

 neous nature. Finally, the carbon com- 

 pounds show but little tendency to the 

 formation of so- called molecular addition 

 products, of which the metal ammonias, the 

 double salts and the compounds with water 

 of crj'stallization are examples, the rational 

 interpretation of which is difiBcult. 



A full consideration of the peculiarities 

 of carbon which have facilitated the synthe- 

 sis of such vast numbers of organic com- 

 pounds would be beyond the scope of this 

 address. The above are the most impor- 

 tant, and their relative absence in the ma- 

 jority of elements explains largely the 

 backward state of our knowledge of them. 

 Our inability to determine the true molecu- 

 lar weight of insoluble and non-volatile 

 substances ; the difficulty of limiting reac- 

 tions so as to obtain intermediate products ; 

 of preventing condensations ; of separating 

 mixtures and identifying their constituents 

 by such simple methods as melting- and 

 boiling-point determinations ; of building 

 up step by step ; of dissecting atom by atom ; 

 of explaining molecular compounds — these 

 are hindrances which can only be overcome 

 by greater perfection of our experimental 

 methods, and which often render the study 

 of the constitution of inorganic bodies a 

 problem of great difficulty, even in the case 

 of many of the simplest. 



At the very time that the organic struc- 

 tural formula was beginning to turn the 

 attention of organic chemists away from a 

 further development of theory to a greater 

 elaboration of details the Englishman New- 

 lands was publishing papers which con- 

 tained the germ of the Periodic Law. In 

 1865 Kekule announced his theory of the 

 benzene ring; in 1864 ISTewlands showed 

 that if the elements be arranged in the 

 order of their atomic weights ' the eighth 

 element, starting; from a given one, is a kind 



