LEFFMANN-BEAM METHOD. 135 



The figures in parenthesis give the percentage of sulphuric 

 acid in the diluted acid, the other figures referring to the per- 

 centage of acid before dilution. 



Another method of determining the strength of acid is to 

 weigh about 1 gramme of acid in a basin and, after diluting 

 with water, to add an excess of strong ammonia. The solution 

 is evaporated on the water-bath and, when nearly dry, a little 

 more strong ammonia is added ; it is then dried to constant 

 weight at 100 C. (212 F.) and weighed. The weight divided 

 by 1-347 will give the weight of sulphuric acid in the sample ; 

 this, divided by the weight taken and multiplied by 100, will 

 give the percentage. There are slight errors in this method, due 

 on the one hand to slight loss of ammonia, and on the other to 

 the small residue left on ignition, but these usually compensate 

 each other sufficiently to be neglected. If very accurate results 

 are required the acidity of the dried residue to methyl-red, and 

 the residue on ignition, may be determined. 



Purified amyl alcohol, free from petroleum, specific gravity 

 0-815 to 0-818 at 15-5 C. (60 F.), which completely dissolves 

 to a clear liquid when mixed with an equal bulk of hydrochloric 

 acid ; this mixture must not become darker than sherry in 

 three days. 



Commercial hydrochloric acid. 



The amyl alcohol is mixed with an equal bulk of hydrochloric 

 acid for use ; it is best not to keep this mixture longer than a 

 few days. 



The Process Testing of Milk, Skim Milk, Buttermilk, 

 and Whey. Measure 15 c.c. each of the well-mixed samples 

 into test bottles, holding the point of the pipette against the 

 side of the neck, so that the liquid may run down, allowing 

 room for the air to escape. Add 3 c.c. of the mixture of amyl 

 alcohol and hydrochloric acid. Pour in, with care, 9 c.c. of 

 sulphuric acid, so that it washes down any particles of milk on 

 the neck of the bottle. Mix the contents of the bottle with a 

 rotatory motion ; a little practice is required to do this without 

 the liquid boiling over, owing to the heat evolved on mixing 

 sulphuric acid with water, but when the way is once learned 

 there is no difficulty in doing this. Fill up the bottles to the 

 zero mark with a mixture of one part of sulphuric acid to two 

 volumes of water, and place the bottles in the machine ; rotate 

 by turning the handle at 100 revolutions per minute for about 

 one minute, when the fat will separate in a clear layer. 



Read the fatty layer as follows : Note the position of the 

 lower surface of the fat (it is convenient to wait till it has fallen 

 to one of the main graduation lines), then note immediately 

 the position of the lowest point of the curve on the upper surface ; 



