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 
m c rcialiy T Tbl^hiabl e. 
Glass object slides coated with a film of ^^zapon” varnish, 
allbwed 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 sodiurn or for silica we depend 
