104 PROTEIDS. 



formed when urea is heated, ammonia being given off in the process. 

 The following formula expresses the change which takes place : 



2CON 2 H 4 - NH 3 = C 2 O 2 N 3 H 5 



Urea. Ammonia. Biuret. 



Since the rose-red color is produced by biuret, the reaction is also 

 called by this name. 



Crystallisation. While it is true that proteids as a class 

 are not crystallizable, or perhaps it would be more correct to say 

 have never been crystallized, still there are exceptions to this rule, 

 inasmuch as crystals of globulin or vitellin have been seen in the 

 aleurone grains of seeds and in the yolk of the egg of fishes and 

 amphibians. Egg-albumin, serum-albumin, and caseinogen have 

 also been made to crystallize. This may be demonstrated in the 

 following way : To a solution of egg-albumin, white of egg, add 

 half its volume of a saturated solution of sodium sulphate, pre- 

 cipitating the globulin, which is removed by filtration. In the 

 filtrate, exposed to the air for some days, during which evapora- 

 tion takes place, minute spheroidal globules and needles will form, 

 which are the crystals of the proteid. Acetic acid hastens the 

 crystallization and produces better crystals. 



Non-diffusibility. As a class, proteids are not diffusible ; 

 to this rule peptones are the exception. In order that this prop- 

 erty of proteids may be understood, it will be advantageous to the 

 student to describe the processes of diffusion and osmosis. If a 

 solution of common salt is placed in a vessel and water is care- 

 fully poured on the surface of the salt solution, the salt will pass 

 into the water, and in a short time the contents of the vessel will 

 be of the same composition throughout. This passage of the salt 

 into the water is diffusion, and in this instance the process takes 

 place very quickly. Not so, however, would be the case if a 

 solution of albumin was substituted for the solution of salt ; the 

 same phenomenon would occur, but would require a much longer 

 time. When liquids are separated by a membrane the diffusion 

 which takes place through it is osmosis. 



It is important to understand osmosis and the conditions under 

 which it takes place, as without this knowledge many of the proc- 

 esses which occur in the human body would be unintelligible ; at 

 the same time it must be said that osmosis does not occupy the 

 prominent place it once did in explaining phenomena connected 

 with absorption ; investigations have shown that the passage of 

 the products of digestion from the alimentary canal into the blood 

 is not a simple diffusion through a passive membrane, but that 

 cell-activity must be largely taken into account. 



Fig. 84 represents a jar, A, which contains distilled water. 

 Within this, resting on a tripod, is an osmometer, an expanded 

 glass vessel, B, closed by a piece of parchment, C, from the top 



