ALBUMINS OR PROTEINS. 121 



out. The reaction of the liquid is of the greatest importance. Complete 

 precipitation of the albumin can only occur when the solution is faintly 

 acid. Any excess of acid will hold some albumin in solution. Yet a 

 coagulation will take place on heating. The coagulated albumin has 

 combined with the acid. A so-called " acid-albumin " results. The 

 presence of alkali will prevent the coagulation of albumin for the same 

 reason. "Alkali-albuminates " are formed. The coagulated albumins are 

 insoluble in water and in neutral salt solutions. The readily soluble com- 

 binations of albumins with alkali and acids can be precipitated by salts. 



The coagulating temperature of the different albumins varies. Efforts 

 have often been made to utilize this fact in separating the various albu- 

 mins. The method is inefficient. For one thing the albumins in solution 

 have different effects upon one another, and then, again, the coagulating 

 temperature varies considerably with the composition of the solvent. 



We are also acquainted with other methods of effecting coagulation 

 besides that of heating. A number of albuminous bodies, for instance, 

 globin, myosin, and fibrinogen, will go over into the gel form simply on 

 standing. Many precipitants, like alcohol, acetone, metallic-salt solutions, 

 etc., will also produce the same result. The time required to effect coag- 

 ulation varies considerably. For instance, the albumins are not immedi- 

 ately changed to their insoluble forms, on being salted-out. They can be 

 filtered, put into solution again, and in this way purified, provided that 

 these processes are carried out in a short time. According to Ramsden, 1 

 coagulation has even been accomplished simply by shaking. 



The most varied albuminous bodies assume very analogous physical, 

 and even chemical, properties after being coagulated. They all form an 

 amorphous powder, which is insoluble in water and salt solutions. We can 

 easily appreciate the fact that such material when used as a starting-point 

 for our investigations of the albumins gives little guarantee of purity or 

 homogeneity. We know that the colloids possess the property of carrying 

 down other substances from solution when they themselves are precipitated. 

 This is also the case with the albumins. They also contain appreciable 

 amounts of ash. It is not yet certain whether ash is an essential constitu- 

 ent of albumin or not; we do know that the amount can be decidedly 

 diminished by dialysis or by other methods. 



As a rule, when we are investigating the composition of a new sub- 

 stance, and finally its constitution, the utmost precautions are used 

 to insure purity. To do this we usually resort to crystallization. By 

 re-crystallizing, and fractional crystallization, adhering substances are 

 removed. This accomplished, we analyse the substance in order to 

 determine its composition. The size of the molecule is obtained by a 

 molecular weight determination; and then by preparing a series of deriva- 



1 Arch. Anat. Physiol. 1894, 517. 



