SILICON. 



383 



ment. No water must on any account com* in contact with the mixture in 

 the flask. If even a drop or two find its way in through the bent tube a 

 tremendous explosion will result, and then the fire generated will surely 

 prove disastrous. The experiment can be performed in a cheaper and less 

 dangerous fashion by dropping phosphate of lime into the basin. We 

 strongly recommend the latter course to the student unless he has had some 

 practice in the handling of these inflammable substances, and learnt caution 

 by experience. The effect when the experiment is properly performed is 

 very good, the smoke rising in a succession of coloured rings. 



SILICON is not found in a free state in nature, but, combined with 

 oxygen, as Silica it constitutes the major portion of our earth, and even 

 occurs in wheat stalks and bones of animals. As flint or quartz (see 





Fig. 387. (Phosphuretted hydrogen and marsh gas) Will-o'-the-Wisp. 



Mineralogy) it is very plentiful, and in its purest form is known as rock 

 crystal, and approaches the form of carbon known as diamond. W T hcn 

 separated from oxygen, silicon is a powder of greyish-brown appearance, 

 and when heated in an atmosphere of oxygen forms silicic " acid " again, 

 which, however, is not acid to the taste, and is also termed "silica," or "silex." 

 It is fused with great difficulty, but enters into the manufacture of glass in 

 the form of sand. The chemical composition of glass is mixed silicate of 

 potassium or sodium, with silicates of calcium, lead, etc. Ordinary window- 

 glass is a mixture of silicates of sodium and calcium ; crown glass contains 

 calcium and silicate of potassium. Crystal glass is a mixture of the same 

 silicate and lead. Flint glass is of a heavier composition. Glass can be 

 coloured by copper to a gold tinge, blue by cobalt, green by chromium, etc. 



