276 GENERAL SCIENCE 



a number of uses for copper sulphate. For the preven- 

 tion of " smut " in wheat the seeds are wet with a weak 

 solution of it before sowing. 



" Bordeaux mixture," composed of a mixture of copper 

 sulphate and lime in water, is widely used as a fungicide ; 

 other sulphates are zinc sulphate, " white vitriol " ; iron 

 sulphate, " copperas " or " green vitriol " ; potassium 

 sulphate, used as a fertilizer for plants; magnesium 

 sulphate, " Epsom Salt " ; and ammonium sulphate, 

 which is used in large quantities as a fertilizer to supply 

 nitrogen to crops. The use of sodium chloride is well 

 known. 



Silver chloride and silver bromide are used in photog- 

 raphy, since they are chemically affected by light. 



Silver nitrate is used in the manufacture of photo- 

 graphic films and plates to produce the sensitive chloride 

 of silver. It is also used in the indelible marking of 

 linens, since the organic matter of cloth or skin converts 

 the nitrate to silver and produces the black stain. 



Ammonium nitrate is used in the manufacture of ex- 

 plosives. 



Calcium phosphate, which occurs as phosphate rock in 

 South Carolina, Tennessee, Florida, Idaho, Utah, Mon- 

 tana, and Wyoming, is used in the manufacture of fer- 

 tilizers. More than a million tons of this rock are being 

 used each year. To render the calcium phosphate more 

 active it is treated with sulphuric acid, which converts 

 the whole mass into the very soluble calcium-hydrogen 

 phosphate, and calcium sulphate. The mixture is sold 

 for fertilizer as " superphosphate." 



Electrolytes. The process of decomposing a compound 

 substance by means of an electric current is called elec- 

 trolysis. The substance decomposed is called an electro- 



