6 BACTERIOLOGICAL APPARATUS 
or 0.13 to 0.17 mm. thick. No. 2, zig to zie in. or 0.17 to 0.25 mm. 
thick. No. 3, zd to sy in. or 0:25 to 0.50 mm. thick. 
No. 2 cover glasses are the best for general use since they satisfy 
the demands of most objectives. No. 1, may be used with oil immersion 
objectives. The objectives of the different makers are standardized 
for the following thicknesses of cover glasses: 
LOSS oe eee eee eee eee .15-20 mm. 
Leitz oc cee cece eee eee 17-20 mm. 
Bausch & Lomb....... 0.0... eee eee .18-20 mm. 
Spencer Lens Co...... 0.0... ccc ee ees .18-20 mm. 
Cleaning of Apparatus. Clean apparatus is the first essential for 
successful laboratory work in bacteriology. All apparatus whether 
new or old should be carefully washed and rinsed before using. This 
removes any free acid or alkali, both of which are very detrimental! to 
the growth of bacteria. Glassware should be washed as clean as pos- 
sible in tap water with laboratory brushes. These are now constructed 
in many shapes and sizes which enables one to reach the interior of most 
flasks. After this is done and if any dirt, etc., remains in the apparatus, 
it should be taken out by the cleaning solution. This is accomplished 
by soaking the apparatus, if glassware, in the cleaning solution which 
is contained in a lead bath. After it has stood for a time, it may be 
taken out, the cleaning solution allowed to run back into the original 
container. The clean.ng solution may be used again and should not be 
thrown away. 
The cleaning of slides and cover glasses deserves some special atten- 
tion. They should be boiled in dilute alkali to saponify any oil or 
grease which may be on them. After they have been washed in water 
they should be boiled in cleaning solution for twenty minutes. Then 
each one should be carefully rinsed in distilled water and stored in ether 
or dry, as preferred. 
Cleaning Solution. This has the following composition: 
Sodium or potassium chromate (KeCreQ7)..... 6 gms 
Concentrated sulphuric acid (H2SO4).......... 6.¢.¢ 
Water... ccc ccc cee eee nes 100. ¢.c 
KeCreO7+4H 2804 = Ke804+Cre(S04)3+4H20+30. 
This is a strong oxidizing reagent. Its action may be accelerated 
by heat. Care should be used in handling it since it is very destructive 
