THE METHODS OF MICROCHEMICAL QUALITATIVE ANALYSIS 317 



reduce the danger of corrosion of the lens by the acid vapor. If 

 a considerable period of time is occupied in a series of tests, the 

 cover glass should be removed at intervals and the objective 

 thoroughly wiped off and cleaned with lens paper moistened with 

 water, dried and a new cover glass and glycerine applied. 



It is always preferable to have a cheap objective set aside, 

 especially for hydrofluoric acid work, so as not to run the risk of 

 ruining an expensive lens. 



For supports upon which to perform the tests, celluloid slips 

 will be found convenient. The chief difficulty arises when gently 

 heating the preparation, to cause development of the crystal 

 forms, since nitrocellulose is very inflammable. Slips of cellulose 

 acetate are therefore far preferable but are at present not com- 

 mercially obtainable. 



Glass object slides coated with a film of "zapon" varnish, 

 allowed to dry, and a second coat applied, yield good results when 

 carefully prepared, but require as great care in heating as cellu- 

 loid slips. 



A better device consists in coating glass object slides with 

 "Bakelite," and heating in an oven to the temperature directed 

 by the Bakelite Company for the particular grade of "Bakelite" 

 used. Slides thus coated can be warmed without danger and 

 yield good results. 



Whenever a critical case arises involving the detection of 

 minute amounts of silica, titanium or zirconium, etc., it is best 

 to have recourse to cellulose nitrate or acetate slips so as to pre- 

 clude the possibility of error due to pores or fissures in the var- 

 nished surface of a glass slide. 



Decompositions by means of hydrofluoric acid are best per- 

 formed upon small pieces of platinum foil or in the tiny platinum 

 spoons shown in Fig. 157, page 296. Subsequently the material 

 can be transferred to cellulose slips or varnished slides for 

 study. 



In selecting slips made from cellulose compounds, only such 

 pieces should be chosen as are not badly scratched and grooved, 

 and which are as nearly colorless as possible. Deep yellow slips 

 are not suitable since in testing for sodium or for silica we depend 



