TO DETERMINE STRENGTH 327 



this machine, like any other, requires being kept well oiled. Too 

 liberal oiling while in motion, however, may cause particles of 

 fat to be thrown into the tops of the test bottles, thus introducing 

 a very appreciable error. 



In case of an accident causing acid to be spilled inside of the 

 iron frame, it is very unwise indeed, to allow it to remain. All 

 broken glass and every vestige of acid should be cleaned out 

 immediately and thoroughly, otherwise the acid will continue 

 slowly to act upon the iron, releasing fine dust-like particles 

 very disagreeable to breathe. The machine also, it may be known, 

 will, be rusted or corroded out rapidly under such conditions. 



Acid : Kind. — The acid used in the Babcock test is the com- 

 mercial sulfuric. The commercial quality is much cheaper in- 

 deed, than the pure, yet its impurities do not introduce any error 

 in the test. In case it becomes necessary to procure testing acid 

 from, or through a drug store, it is wise to insist, not only on 

 sulfuric acid, but for the commercial grade. Pure acid, in 

 addition to being far more expensive than is needed, is altogether 

 too strong to be used with comfort or safety. 



Strength of Acid. — Commercial sulfuric acid has a specific 

 gravity of about 1.82, or in other words, is 1.82 times as heavy 

 as water. Pure acid is heavier. The commercial grade is often 

 spoken of as 90 per cent strength. Thus in purchasing, the 

 buyer has amply described the article wanted by calling for com- 

 mercial sulfuric, specific gravity 1.82 or 90 per cent strength. 



To Determine Strength. — The commercial acid is compara- 

 tively cheap and varies in strength, not greatly, but sufficient to 

 interfere with the test made unless a little more or less of the 

 acid is used, according as it is needed. There are simple devices 

 on the market for the purpose of determining the strength of the 

 acid but these are easily broken, and after all, no more reliable 

 than a simple test or trial of the acid in question would be. To 

 test a new quantity of acid, thoroughly mix a sample of sweet 

 milk and load into four bottles and number them. Into bottle 

 No. 1 add three-quarters of the usual quantity of acid ; into bot- 

 tle ISTo. 2, seven-eighths ; into bottle JS^o. 3, the usual or ordinary 

 quantity as indicated by the acid measure, and into bottle ISTo. 4, 

 a quantity of acid slightly greater than the amount indicated 



