1909] on Advances in KnoivUdge of Silicon. 645 



other considerations lead us to the conclusion that silica as we know 

 it is a molecularly condensed substance represented by the expression 

 (Si0.2)„, where the value of n is probably at least =6. 



Before leaving the consideration of the simple oxide, I should like 

 to show jou the elfect of radium emanations on a disc of colourless 

 quartz. This disc has been exposed by Sir William Huggins to 

 radium for a consideral)le time, and the brownish discoloration about 

 the centre of it is due to the action of the rays. Whether the latter 

 reduce a portion of the oxide and separate a minute film of ])ro\vn 

 silicon, or merely attack some trace of impurity in the quartz is not 

 yet known. I have to thank Sir William Huggins for allowing me 

 to show you this interesting specimen. 



The chemical analogies of CO2 and Si02 are very close in many 

 respects, for both act as acid anhydrides and combine with metallic 

 oxides and form similar salts. On the one hand we obtain the well- 

 known carbonates, such as common soda crystals, and on the other 

 silicates, such as sodium silicate. I need scarcely remind you that 

 ordinary window and bottle glass are mixtures of silicates of such 

 metals as calcium and sodium. 



When a soluble carbonate is treated with any moderately strong 

 acid CO2 gas is evolved ; but when a soluble silicate such as Na2Si03 

 is similarly treated no gas is evolved but a gelatinous substance 

 separates. Now this consists for the most part of the feeble acid 

 H2 SiOg, which parts with the elements of water gradually, if exposed 

 to air, and affords various lower hydrates, one of the most beautiful 

 being that which we meet with in Nature as the precious opal. 



Chloride and bromide of silicon are easily obtained by heating the 

 free elements in the respective halogens, and precisely correspond in 

 composition to the analogous carbon compounds, but, unlike the 

 latter, are intensely reactive to water, and in so far resemble tlie 

 chloride and bromide of a metal. 



If, however, hydrochloric acid gas, instead of chlorine, be passed 

 over heated silicon a very volatile liquid is obtained which is similar 

 in composition to ordinary carbon chlorofortn : — 



Ordinary Chloroform ... ... CH CI3 



Silicon Chloroform ... ... SiHClg 



Silicon chloroform has no anassthetic effects for a reason you will 

 easily appreciate, when I compare the action of water on the two 

 substances. Under ordinary conditions carbon chloroform is not 

 affected by moisture, hence its vapour can be taken into the lungs 

 unchanged and passes into the system producing its characteristic 

 effects. Silicon chloroform on the other hand is instantly destroyed 

 by moisture, producing free acid, therefore it cannot be inhaled. 

 Nevertheless, the products of the action of water upon it at ordinary 

 temperature are very similar to those which can be obtained by the 



