CHAP, xvi BLOW-PIPE WORK KJ 



thing present in the rock cxcqtf the carbonate of lime, 

 of non-calcareous matter to dissolved 

 >< .1.1.! will usually be extremely small 



ite student ixMsesses chemical knowledge, he may 

 proceed to test the acid solution he obtains from a pul- 

 verised rock and may detect the bases; but as a rule 



d analysis can hardly be undertaken by the 

 geologist in the field. 



Some acquaintance with chemical reactions, indeed, 

 will tx: found of great service in ti fication of 



rocks and of their constituent minerals. It is commonly 

 the case that minerals about which the observer may be 

 doubtful are precisely those which, from their small sue, 

 are most difficult of separation from the rest of the rock 

 preparatory to analytical processes. The mineral apatite, 

 for example, occurs in minute hexagonal prisms which 

 on cross-fracture might be mistaken for ncphclinc, or 

 even sometimes for quartz. If, however, a drop of solu- 

 tion of molybdate of ammonia be placed upon the 

 crystal, a yellow precipitate will appear if it be apatite. 

 Nepheline, which is another hexagonal mineral likewise 

 abundant in some rocks, gives no yellow precipitate with 

 the ammonia solution, while if a drop of hydrochloric 

 acid be put over it, crystals of chloride of sodium or 

 common salt will be obtained. These reactions can be 

 observed even with minute crystals, by placing them 

 under the microscope and using an exceedingly attenu- 

 ated pipette for dropping the liquid on the rock. 



Ttsts. The chief chemical tests available 

 for the field-geologist are those which he can r> 

 with the blow-pipe. These he will find to be simple, 



