1 90 CHEMISTRY. 



QUESTIONS. 



249. How long has phosphorus been known ? What are its properties ? 

 How does it act physiologically? In what is it soluble? 250. Describe 

 some experiments with a solution of phosphorus. 251. How is amorphous 

 phosphorus obtained ? What is allotropism ? 252. What is the chief use 

 of phosphorus? 253. Detail the method of obtaining phosphorus. 254. 

 Where and in what state does phosphorus occur in animals ? How do an- 

 imals get it ? 255. What is the composition of phosphoretted hydrogen ? 

 What is its nature ? Explain a method of obtaining it. What name is 

 given to it when occurring in marshes ? 256. Mention another way of 

 making this gas. 257. What compounds does phosphorus form with oxy- 

 gen ? What is said of the acids ? 



CHAPTER XIV. 



SILICON AND BORON. 



258. Silicon. This element never occurs in nature in a 

 free state, but its compound with oxygen silicic anhydride, 

 SiO 2 is most important and abundant. Silicon itself is ca- 

 pable of existing in three allotropic forms, like carbon ; the 

 form corresponding to lampblack is a dark-brown powder, 

 destitute of lustre ; the diamond form is crystalline, and so 

 hard as to scratch glass. These substances are mere chem- 

 ical curiosities, and their preparation does not interest us. 

 United with oxygen it forms silicic anhydride, commonly 

 called silica ; this unites with the elements of water, form- 

 ing a true acid, which, when freshly prepared, appears like a 

 transparent jelly. On igniting, water is driven off and sil- 

 ica remains. Compounds of silicic acid and metallic oxides 

 are called silicates. 



259. Abundance of Silica. It is estimated that silica con- 

 stitutes about one sixth of the bulk of the earth. It ap- 

 pears in various forms and combinations. It is nearly pure 



