THE VOLATILE PART OF PLANTS. 105 



them fully. Again, if we succeed in removing foreign 

 substances, it must usually be done by the aid of acids, 

 alkalies, salt-solutions, alcohol and ether, and there is 

 reason to believe that in many cases these reagents essen- 

 tially modify the properties and composition of the pro- 

 teids. These bodies, in fact, as a class, are extremely 

 susceptible to change and alter in respect to appearance, 

 solubility, and other qualities that serve to distinguish 

 them, without any corresponding change in chemical 

 composition being discoverable by our methods of anal- 

 ysis. On the other hand, the substances that have been 

 prepared by different experimenters from the same 

 sources, and by substantially the same methods, often 

 show decided differences of composition. 



Finally, the methods of analysis used in determin- 

 ing their composition are liable to considerable error, 

 and, if applied to the pure substances, are scarcely 

 delicate enough to indicate their differences with entire 

 accuracy. 



In the accompanying table (p. 106) are given the most 

 recent and trustworthy analyses of the various vegetable 

 albuminoids, and of the corresponding substances of ani- 

 mal origin. 



Eeferring to the analyses of Albumins we observe that 

 the egg-albumin differs from serum-albumin in contain- 

 ing about one per cent more of oxygen and one less of 

 carbon, while hydrogen, nitrogen and sulphur are prac- 

 tically the same. These two albumins have been very 

 thoroughly studied, their difference of composition is 

 well established, and they have positive differences in 

 their properties, so that there can be little doubt that 

 they are specifically distinct substances. Of the Vegeta- 

 ble Albumins none offer any reasonable guarantee of 

 purity. The composition of barley-albumin is near that 

 of the animal albumins, but it contains one-third less 

 sulphur. So far, then, as present data indicate, the vog- 



