TRANSACTIONS OF SECTION B. 



711 



at more exact defiuitions of the characters of the relations of the elements, and 

 ultimately of their respective offices in nature. Among inquiries of this kind 

 the comparative study of the elements carhon and silicon appears to me to 

 possess the highest interest. Carhon, whether combined with hydrogen, oxygen, 

 or nitrogen, or with all three, is the great element of organic nature, while silicon, 

 in union with oxygen and various metals, not only forms about one-third of the 

 solid crust of the earth, but is unquestionably the most important element of 

 inorganic nature. The chief functions of carbon are those which are performed at 

 comparatively low temperatures ; hence carbon is essentially the element of the 

 present epoch. On the other hand, the activities of silicon "are most marked at 

 very high temperatures ; hence it is the element whose chief work in nature was 

 performed in the distant past, when the temperature of this earth was far beyond 

 that at which the carbon compounds of organic life could exist. Yet between these 

 dominant elements of widely different epochs remarkably close analogies are 

 traceable, and the characteristic differences observed in their relations with other 

 elements are just those which enable each to play its part eflfecti^ely under the 

 conditions which promote its greatest activity. 



The chemical analogies of the two tetrad elements carbon and silicon are 

 most easily recognised in compounds which either do not contain oxygen, or which 

 are oxygen compounds of a very simple order, and the following table will recall a 

 few of the most important of these, as well as some which have residted from the 

 fine researches of Friedel, Crafts, and Ladenburg : — 



Some Silicon Analogues of Carhon Compounds. 



But these silicon analogues of carbon compounds are, generally, very different 

 from the latter in reactive power, especially in presence of oxygen and water. 

 For example, hydride of silicon, even when pure, is very easily decomposed, and, if 

 slightly warmed, is spontaneously inflammable in air ; whereas the analogous marsh 

 gas does not take fire in air below a red heat. Again, the chlorides of silicon 

 are rapidly attacked by water affording silicon hydroxides and hydrochloric acid; 

 but the analogous carbon chlorides are little alfected by water even at com- 

 paratively high temperatures. Similarly, silicon-chloroform and water quickly 

 produce silico-formic acid and anhydride along with hydrochloric acid, while ordi- 

 nary chloroform can be kept in contact with water for a considerable time without 

 material change. 



Until recently no well-defined compounds of silicon were known including 

 nitrogen ; but we are now acquainted with a number of significant substances 

 of this class. 



Chemists have long been familiar with the fact that a violent reaction takes 

 place when silicon chloride and ammonia are allowed to interact. Persoz, in 1830, 

 assumed that the resulting white powder was an addition compound, and assigned to 

 it the formula SiCl^, 6 NH3, while Besson, as lately as 1892, gave SiCl^, SNITj. 

 These formulse only express the proportions in which ammonia reacts with the 

 chloride under different conditions, and give us no information as to the real nature 

 of the product ; hence they are almost useless. Other chemists have, however, 

 carefully examined the product of this reaction, but owing to peculiar difficultiea 



