THE VOLATILE PART OF PLANTS. 89 



distinction of this class of substances from all others, even In micro- 

 scopic observations. ^ 



1. Solution of iodine colors them intensely yellow or bronze. 



2. Warm and strong hydrochloric acid colors these bodies blue, 

 violet, or brown, or, If applied In large excess, dissolves them to a 

 liquid of these colors. 



3. In contact with nUrlc acid, especially when hot, they are stained a 

 deep and vivid yellow. Silk and wool, which consist largely of pro- 

 teids, are commonly dyed or printed yellow by means of nitric acid. 



4. A solution of mercunc nitrate in excess of nitric acid,* tinges them 

 of a deep red color. This test enables us to detect albumin, for exam- 

 ple, even where It is dissolved in 20,000 parts of water. 



6. With caustic soda and very dilute solution of copper sulphate, 

 successively applied, the protelds give a violet color which Is Intensi- 

 fied by 4arming. (Biuret test.) 



The Albumins are soluble in water; the solutions as 

 naturally occurring, unless very dilute, are coagulated by 

 heat. 



Egg Albumin. — The white of a hen's egg on drying 

 yields about 13 per cent of albumin in a state of tol- 

 erable purity. The fresh white of eggs serves to illus- 

 trate the peculiarities of this substance, and to exhibit 

 the deportment of the albuminoids generally toward the 

 above-named reagents. 



Exp. 42.— Beat or whip thejvhlte of an egg so as to destroy the deli- 

 cate transparent membrane In the cells of which the albumin is held, 

 and agitate a portion of It with water ; observe that it mostly dissolves 

 in the latter. The solution Is turbid from presence of globulin. 



Exp. 43.— Heat a part of the undiluted white of egg In a tube or cup. 

 At 166° F. it becomes opaque, white, and solid (coagulates), and is con- 

 verted into the Insoluble modification. A higher heat Is needful to 

 coagulate solutions of albumin, in proportion as they are diluted with 

 water. 



Exp. 44. — Add strong alcohol to a portion of the solution of albumin 

 of Exp. 42. It precipitates the albumin, which for a time remains solu- 

 ble in water, but later coagulates and becomes insoluble. 



Exp. 45.— Observe that albumin is coagulated by strong acids applied 

 in small quantity, especially by nitric acid. 



Exp. 46.— Put a little albumin, either soluble or coagulated, into each 

 of five test tubes. To one, add solution of iodine ; to a second, strong 

 hydrochloric acid ; to a third, nitric acid ; to a fourth, nitrate of mer- 

 cury, and to the last a few drops of solution of copper sulphate, and 

 then a little caustic soda or potash solution. Observe the characteristic 

 colorations that appear immediately, or after a time, as described 

 above. In the last four cases the reaction is hastened by a gentle heat. 



* This solution, known as MlUon's reagent, is prepared by dissolving 

 mercury in its own weight of nitric acid of sp. gr. 1.4, heating toward 

 the close of the process, and finally adding to the liquid twice its bulk 

 of water. 



