EXAMINATION OF FATS. 



1417 



after running water of a constant temperature through the instrument 

 for some time, the fat is not evenly distributed on the surfaces of the 

 prism and the process must be repeated. The instrument should be 

 carefully adjusted by means of the standard fluid which is supplied. 

 As the index of refraction is greatly affected by temperature, care 

 must be used to keep it constant. (See fig. 2.) 



The following table can be used to convert the degrees of the instru- 

 ment into refractive indices: 



TABLE L a 



DETERMINATION OF IODIN ABSORPTION B5T HUBL ? S METHOD. b 



PREPARATION OF REAGENTS. 



lodin solution. Dissolve 25 grams of pure iodin in 500 cc of 95 per 

 cent alcohol. Dissolve 30 grams of mercuric chlorid in 500 cc of 95 per 

 cent alcohol. The latter solution, if necessary, is filtered, and then the 

 two solutions mixed. The mixed solution should be allowed to stand 

 twelve hours before using. 



Decinormal sodium thiosulphate solution. Dissolve 24.8 grams of 

 chemically pure sodium thiosulphate freshly pulverized as fineh 7 as pos- 

 sible and dried between filter or blotting paper, and dilute with water 

 to 1 liter at the temperature at which the titrations are to be made. 



Starch paste. One gram of starch is boiled in 200 cc of distilled 

 water for ten minutes and cooled to room temperature. 



Solution of potassium iodid. One hundred and fifty grams of potas- 

 sium iodid are dissolved in water and made up to 1 liter. 



a Winton, Conn. Expt. Sta., Kept., 1900, pt. 2, p. 143. 

 b U. S. Dept. Agr., Div. Chem. Bui. 46, revised, p. 50. 



