100 BUTTER-MAKING. 
account of its relative harmlessness, its cheapness, and efficiency. 
While bichromate of potash is relatively efficient in its 
preservative effect, and not so poisonous as some of the others, 
it does not give as general satisfaction as does corrosive sub- 
limate (mercuric chloride), unless relatively greater precau- 
tions are taken. If the composite samples preserved with 
bichromate of potash are left standing in the light very long, 
a leathery scum forms on the top, which is very difficult to 
dissolve in the sulphuric acid. This is claimed to be due to 
the reducing influence of light on chromate solutions. If too 
Fic. 59.—Composite Fic. 60 —Composite samples and rack 
sample bottle. to hold sample jars. 
much bichromate of potash is added, the sulphuric acid added 
digests the curd with difficulty. When the sulphuric acid is 
added the curd is precipitated into a heavy, gray-colored coag- 
ulum, which dissolves with difficulty in the acid. 
According to the authors’ experience, corrosive sublimate 
tablets can be highly recommended. The tablets contain a 
color, which, when dissolved, colors milk, so that it can readily 
be distinguished as not being fit for human food. The tab- 
lets are very poisonous, but are more efficient in their preser- 
vative effect than bichromate of potash. They can be obtained 
from any creamery-supply house. 
During the winter, when the samples are kept comparatively 
cold, less preservative is needed than in the summer. One 
