A STUDY OF FOODS 45 



when these solvents evaporate, the fat or oil is left be- 

 hind. (3) A solution of osmic acid l stains fats brown or 

 black. 



Test for Starch. 2 An iodine solution 3 always turns starch 

 blue. If a large amount of starch is present in a food that 

 is being tested, a deep-blue color is produced upon the ad- 

 dition of a few drops of iodine ; if the percentage of starch 

 is small, the color will be light blue; the absence of a 

 blue color shows that starch is not present. 



Test for Grape Sugar. 4 Many different kinds of sugars 

 are found in foods; for example, cane sugar, beet sugar, 

 sugar of milk, and grape sugar. These sweet substances 

 differ more or less in chemical composition. In our physio- 

 logical study, grape sugar is the most important, and its 

 presence can be proved in the following way. A little of 

 the given food is put into a test tube, and hot water is added 

 to dissolve the sugar if present. Some blue Fehling's solu- 

 tion 5 is then added to the mixture in the test tube, and the 



1 Osmic acid is very expensive, and does not keep well in solution 

 unless the bottle in which it is contained is perfectly clean and is 

 Kept in the dark. A 1% solution gives more satisfactory results. 



2 See "Laboratory Exercises," No. 9. 



8 A quart (1000 cc.) of iodine solution is made by dissolving in 5 tea- 

 spoonfuls (40 cc.) of water one half teaspoonful (4 grams) of potas- 

 sium iodide and one fourth this amount (1 gram) of iodine. This 

 solution, when thoroughly mixed, should be diluted to make one quart 

 (1000 cc.). In a clean bottle this mixture will keep indefinitely. 

 From "Laboratory Exercises." Henry Holt & Co. 



4 See " Laboratory Exercises," No. 10. 



6 To make a quart (1000 cc.) of Fehling's solution, dissolve 3 tea- 

 spoonfuls (35.64 grams) of pure copper sulphate (blue vitriol) in a 

 little less than a half-pint (200 cc.) of water. Make a second solution 

 by dissolving in a pint (500 cc.) of water twelve heaping teaspoonfuls 

 (150 gr.) of Rochelle salt and 3 (5-inch) sticks of caustic soda (50 grams). 

 Mix the two solutions thoroughly, and dilute with enough water to 

 make a quart (1000 cc.). Fehling's solution does not keep for any 

 great length of time, and hence must be made up fresh a short time 

 before it is needed. It is more convenient to prepare it in small 



