1412 FOODS AND FOOD ADULTERANTS. 



and weighed as tin oxid, or, as it was found more convenient, the 

 sulphid was washed free from hydrogen sulphid and sulphids, dissolved 

 in a saturated solution of ferric chlorid, and the resulting ferrous salt 

 titrated with potassium bichromate. a 



Any residue left upon filtering the soluble tin salt after fusion is 

 dissolved in nitric acid and made alkaline with ammonia. A blue 

 color indicates copper, which may be titrated with potassium cyanid, 

 or, in absence of lead, may be determined electrolytically. Lead is 

 indicated by yellow precipitate with potassium chromate in acetic acid 

 solution, and may be determined quantitatively as lead chromate. 



The filtrate from the original hydrogen sulphid precipitate may con- 

 tain iron, zinc, and phosphates. Brornin water is added and the solu- 

 tion boiled to destroy hydrogen sulphid and to oxidize the ferrous iron, 

 and, unless the solution has a distinct yellow color, sufficient iron is 

 added to give it that color, and then ammonium acetate is added to 

 the slightly acid solution to precipitate phosphate of iron and excess 

 of iron. The material is filtered, the precipitate washed, and from 2 

 to 3 cc of acetic acid added to the filtrate, which is treated with 

 hydrogen sulphid for precipitation of zinc. The zinc sulphid is col- 

 lected, ignited, and weighed as zinc oxid. Results are expressed in 

 milligrams of the metal per kilo of the original sample. 



EXAMINATION OF FATS. 



The methods employed for this purpose were those prepared by 

 Mr. Tolman for the association of official agricultural chemists. They 

 are incorporated here without change. They do not include the deter- 

 mination of chilling point, which was done by setting a narrow bottle 

 of melted fat in cold water and stirring till a turbidity set in. Although 

 this method is widely used technically in comparing fats of some uni- 

 formity, it is found of little value here, owing to the impurity of the 

 separated fats. 



In addition to Mr. Tolman, credit is due to Messrs. Chace and 

 Skinner for assistance in the examination of the fats. The micro- 

 scopical examination was made by Mr. Munson. 



DETERMINATION OF SPECIFIC GRAVITY. 



DETERMINATION AT 15.5. 



Determine the specific gravity of oils at 15.5 by the use of pycnom- 

 eter, Westphal balance, b or accurately graduated hydrometer. 



a Button's Volumetric Analysis, 8th ed., p. 373. 



b C. A. Crainpton, U. S. Dept. Agr., Div. Chem. Bui. 13, pt. 4, p. 438. 

 c Accurately made hydrometers reading from sp. gr. 0.900 to 0.940 at 15.5 will 

 satisfy every requirement of accuracy and speed. 



