90 PHYSIOLOGICAL CHEMISTRY. 



called coagulation and the resultant product is so essentially 

 altered that it may no longer be brought into the original form, 

 or a similar form, by any known means. Some of the condi- 

 tions of coagulation have been explained above and illustrated 

 by experiments. While the simple or native egg albumin is 

 soluble in water the modified product is insoluble. It is, how- 

 ever, soluble in weak acids or alkalies, but is insoluble in solu- 

 tions of neutral salts. It follows, therefore, that while coagu- 

 lation or modification of a native albumin always follows on 

 heating, precipitation may not result. This depends on the 

 reaction of the mixture. Coagulation or modification on the 

 one hand and precipitation on the other are perfectly distinct 

 phenomena. In the case of egg albumin in solution, for ex- 

 ample, a precipitate forms on heating as long as the solution 

 is nearly neutral. In presence of salts the precipitation is more 

 complete. But if the original solution is alkaline modification 

 of the albumin takes place but without precipitation, as soluble 

 alkali albuminate is now formed. In presence of acid in 

 proper amount soluble acid albumin is formed. Although 

 often used synonymously the terms coagulation and precipita- 

 tion have here distinct meanings. 



The exact nature of the change which takes place when 

 native albumins are heated is not known. Hence the terms 

 u^ed in describing the phenomenon are somewhat indefinite. 

 They are " modified," or, to freely render a German expression, 

 " denatured." To bring them again into the original condi- 

 tion is not possible. White of egg may sometimes be mod- 

 ified or altered without becoming opaque, and the same is true 

 of clear blood serum. In both cases we have coagulation 

 without precipitation. 



ACID AND ALKALI ALBUMINS. 



These products represent the most important forms of the 

 coagulated modified albumin, and are now recognized as salts 

 of the albumin nucleus acting as an acid or basic ion. They 

 are usually secured by the action of acid or alkali in excess 

 on the native albumin, usually white of egg. 



