212 



DETERMINATION OF ALKALIES IN MINERALS. 



36. In the analysis of many minerals, the characters of which 

 lead to the supposition of the presence of the alkalies, it is 

 useless to precede the quantitative determination by one of a 

 qualitative character, especially as the steps to be followed to 

 separate the alkalies are the same in both cases, and to proceed 

 in this way is economy of time. From my own experience 

 concerning the constitution of the silicates there are doubtless 

 but a very few of them without an aj3preciable quantity of al- 

 kalies in their constitution ; and an easy method to examine 

 with certainty the composition of such small quantities of al- 

 kaline chlorides must add to our analytical knowledge. With 

 a little experience the method about to be described will be 

 found very available. 



37. To ascertain the nature of the alkalies present in the 

 chlorides before proceeding to separate them we abstract a 

 quantity so small as not sensibly to affect the weight of the 

 mass. The smallest piece of the dried mass, that need not 

 exceed one fiftieth of an inch in diameter, is placed on a slip 

 of glass, and to it is added a drop of a watery solution of very 

 pure chloride of platinum,* 

 not too concentrated, and the 

 plate gently warmed ; if pot- 

 ash be present, a yellow de- 

 posit soon takes place, which 

 by the microscope will be 

 seen to consist of octahedral 

 crystals of the double chloride 

 of potash and platinum ; the 

 evaporation should be con- 

 tinued very gently with a 

 heat not exceeding 120° to 

 130° until the liquid begins 

 to dry on the edge; if this be now examined under the micro- 

 scope and soda be present, beautiful needle-shaped crystals Will 

 be seen, both formed and forming, with an oblique angle of 

 termination, the crystals frequently having re-entering angles, 

 as represented in the figure. The border of liquid on the glass 

 is the place to observe these crystals, and that while the pro- 



* The alcoholic 

 be used. 



solution does not give perfect results and should not 



