98 



FIELD GEOLOGY. 



Substances. Reaction. 



Nitrates. Give off fumes of nitric acid when con- 



centrated sulphuric acid is added. 



Phosphates. j In solution give yellow precipitates on 

 the addition of nitrate of silver or 

 molybdate of ammonium. These 

 reactions usually take some time. 



Chlorides. Give a white curdy precipitate when 



nitrate of silver is added to their so- 

 lutions. 



Fluorides. When treated with strong sulphuric 



acid, give off fumes of hydro-fluoric 

 acid, which roughen or etch glass. 



Silicates. Many silicates gelatinise when heated 



in concentrated acids. The silicate 

 should be finely pulverised. 



Bloiv-pipe. "The student ought to accustom him- 

 self to the use of the blow-pipe, as an instrument to aid 

 him in the determination of rocks much assistance 

 may be obtained in this way. No field geologist should 

 consider his outfit complete if it does -not include a 

 blow-pipe, with the requisite reagents."*" For the 

 simple experiments, the results of which are given in 

 the last column of the table, much apparatus is not re- 

 quired. " A blow-pipe of brass or German silver with 

 platinum point, an oil-lamp with a broad rectangular 

 wick, a piece of platinum wire, and a flat lump of wood- 

 charcoal about six inches long for a support. 



"The blow-pipe flame is produced by forcing a small 

 continuous stream of air through the flame of the lamp 

 * " Student's Manual of Geology." Jukes and Geikie. 1872. 



