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 



FIG. 59. Composite FIG. 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 



