144 PRACTICAL LESSONS IN SCIENCE. 



parison with it as a measure of the capability of a country to 

 support life. Hence substances containing phosphate of lime are 

 of great value as fertilizers. The phosphate being practically 

 insoluble, is not easily distributed through the soil. But by 

 treating the phosphate with sulphuric acid it is transformed into 

 a soluble lime salt called superphosphate, which is easily distrib- 

 uted to the soil by water. It is said that in England alone more 

 than 500,000 tons of phosphate materials are used annually in 

 the manufacture of artificial manures. Phosphorus forms a 

 compound with hydrogen called phosphine, formula PH 3 . It is 

 somewhat analogous to ammonia. It has no economic value, 

 and is characterized as having the most villainous odor known. 



ARSENIC (As) is often classed with the metals because of its 

 metallic luster, but it forms no base with oxygen, and in many 

 respects is similar to phosphorus. It is sometimes found pure 

 in nature and sometimes as an oxide, but more commonly in 

 combination with various metals, as the arsenides of nickel, co- 

 balt and iron. The principal source of arsenic and its compounds 

 is arsenical pyrites, formula, FeAsS. Arsenic is also obtained as 

 a by-product in working ores of tin, nickel and cobalt. 



When arsenic is heated in the air it burns with a bluish flame, 

 giving off fumes which have the odor of garlic, and are very 

 poisonous. These fumes condense into a white solid called 

 arsenic trioxide, formula As 2 3 . It is the arsenic of the shops. 

 It is used in glass-making, in the manufacture of coloring mat- 

 ters; with water it forms a weak acid which forms salts, called 

 arsenites. An arsenical soap made of arsenite of potash, soap 

 and camphor is sometimes used by naturalists to preserve the 

 skins of birds and animals. Arsenite of copper, called Scheele's 

 green, is much used in coloring wall papers, feathers, muslin, etc. 



In quantities less than poisonous doses arsenic trioxide has pe- 

 culiar effects on the animal body; it seems to favor the deposition 

 of fat ; grooms use it to improve the appearance of their horses, 

 and in some parts of Europe men and women take it for the 

 same purpose. A solution of arsenite of potash is much used 

 in medicine as Fowler's solutions. Arsenic pentoxide, formula 



