MILK AND MILK PRODUCTS 37 



Commercial agar almost always contains diatoms. One 

 characteristic form is Arachnoidiscus Ehrenbergii. (See figure 

 7.) The diatoms may be obtained by oxidizing the organic 

 material with a mixture of nitric and sulfuric acid or nitric and 

 hydrochloric acids. Moist materials should be well dried but 

 not powdered. The diatoms will be found by examining the 

 residue with a power of about loo diameters. 



Fig. 7. — Arachnoidiscus Ehrenbergii. X 100. The smaller oval diatoms are 

 a species of Cocconeis. 



Preservatives 



These are largely used, especially in the warmer season, 

 as a substitute for refrigeration. Many of them are sold under 

 proprietary names which give no indication of their composi- 

 tion. Preparations of boric acid and borax were at one time 

 the most frequent in use, but lately formalin, a 40 per cent, 

 solution of formaldehyde (methyl aldehyde), has come into 

 favor. Sodium benzoate is now in common use as a preserva- 

 tive of cider, fruit- jellies, and similar articles, and may, there- 

 fore, be found in milk. Salicylic acid is not so much employed 

 as in former years. Sodium carbonate is occasionally used to 

 prevent coagulation due to slight souring. Fluorids and 

 abrastol might be used. A mixture of boric acid and borax is 

 more efficient than either alone. The quantity generally used 

 is equivalent to about 0.5 gram of boric acid per liter. For- 

 maldehyde is the most effijcient antiseptic. In the proportion 

 of 0.125 gram to the Hter, it will keep milk sweet for a week. 



