ALBUMINS OR PROTEINS. 129 



These statements are sufficient to show .how little value should be 

 attached to the molecular weight determinations of such " compounds." 

 In individual cases we are not able to decide at present whether there is 

 an actual chemical combination, or whether the metal is merely held in 

 solution. No better results have been obtained by using halogen substitu- 

 tion products for the molecular weights. We are still without the neces- 

 sary knowledge and foundation for such work. 



It might be thought that the cleavage-products could be utilized for 

 determining the molecular weights. Unfortunately, we have not yet 

 sufficiently perfected our methods to utilize any individual, character- 

 istic, cleavage-product for such a determination. We must temporarily 

 content ourselves with approximations. 



If we take all the known facts concerning the size of the albumin molecule 

 into consideration, and critically examine them, we must conclude that no 

 definite statement can be made. It is, of course, possible that the molecu- 

 lar weight is as large as has been computed; on the other hand, it may be 

 much smaller. It is, therefore, practically useless to assign definite 

 formulae to individual proteins as long as the methods of molecular weight 

 determinations are still so indirect, and based upon so many unknown 

 factors. 



Direct determinations of the molecular weights of proteins have so 

 far been unsuccessful. The raising of the boiling-point method is not 

 applicable, because most of the albumins undergo changes on heating. 

 Determinations made up to the present by means of the lowering of the 

 freezing-point have not taken sufficiently into consideration the amount 

 of ash in the proteins. They are, therefore, practically worthless. 



Before discussing the decomposition products of the albumins, or con- 

 sidering the question of the constitution of the albumins, we will now 

 devote a little attention to the various kinds of proteins as far as they can 

 be characterized by our present methods. It is impossible now to classify 

 the large number of known proteins in accordance with purely chemical 

 principles. We are still forced to follow the old grouping. This classi- 

 fication is, however, only accepted as a matter of necessity. The farther 

 we proceed into the chemistry of the albumins, the more we learn of 

 properties which can be utilized to identify individual proteins. We can 

 even indicate a prospective classification of the proteins according to their 

 constituents in an objective and satisfactory manner. We shall presently 

 see that they are essentially composed of amino acids. These are of very 

 different kinds. We distinguish between the mono-amino and the di- 

 amino acids. The relative amounts of these two groups of amino acids 

 vary considerably in the different proteins. We know of proteins like silk, 

 elastin, etc., which are mainly composed of mono-amino acids, the di-amino 

 acids being of little importance. On the other hand, we have the prot- 



