N«. 6. DEPARTM-ENT OF AG-RICUtTUIiB. 653 



The quantity of potassium bichromate suggested above should 

 be euough to keep the milk sweet for a week. In case one finds 

 at any time that the amount does not prevent souring, then one 

 should use more. 



If milk is delivered that has firm clots of cream in it, then mix 

 the sample in the weigh can with a dipper and take out a small 

 portion which can be poured from one vessel to another until the 

 clots disappear, after which take out a tin cylinder full and transfer 

 to fruit-jar. 



At the end of a week, one has in each fruit-jar a sample of milk 

 which represents the milk delivered during that week. By test- 

 ing this one sample, one secures the same results he would secure 

 by testing the milk every day. This kind of a sample is known as 

 a ''composite sample." 



90. Testing Acidity of Milk and Cream. 



(1.) General Principles upon Which Acid Testing is Based. — The 

 method of testing acidity in milk or cream is based upon the chem- 

 ical action taking place between acids and alkalies. For example, 

 if to any acid we add an alkali we change the acid and alkali both 

 into a third compound, each of the others disappearing as acid or 

 as alkali. Thus, suppose to some lactic acid we add some solution 

 of caustic soda in just the right proportion, we then have neither 

 lactic acid nor caustic soda, but a new compound formed by the 

 union of lactic acid and sodium. To find out when a substance 

 is acid or alkaline or neutral, that is, neither acid nor alkaline, we 

 use some third substance, which is called an indicator. One very 

 useful substance to use as an indicator is a chemical compound 

 called ijlienol])lithaleln. When this substance is added to an alka- 

 line solution, it turns pink, while, in an acid or neutral solution, 

 it is colorless. For use 10 grams of phenolphthalein are dissolved in 

 300 c. c. of 90 per cent, alcohol, and a few drops of this are used. 

 Now, what use can be made of these facts in ascertaioing the 

 amount of acid in milk or cream? We will illustrate: To a measured 

 amount of cream we add some phenolphthalein as indicator, and 

 then to this cream add some caustic soda solution, so prepared that 

 we know just how much caustic soda it contains. We add the 

 caustic soda, stirring the cream after each addition, until finally 

 a pink color appears and does not go away oo continued stirring. 

 The appearance of the pink color means that enough caustic soda 

 has been added to combine with all the lactic acid in the cream. 

 Now, we know just how much caustic soda was required to equal 

 the acid in the cream and from this we know the amount of acid 

 in the cream. For dairy work, the caustic soda is prepared in 



