COMPOSITION OF FOOD AND ACTION OF ENZYMES. 665 



be due, therefore, to an incomplete purification. In fact, it is stated 

 that solutions of some of the enzymes may be prepared (pepsin, 

 invertase, thrombin) which show ferment activity, but give no 

 proteid reactions. Much of the older work upon the composition of 

 supposedly purified preparations of enzymes is not accepted to-day, 

 on the ground that the evidence for the purity of the preparations is 

 insufficient. In spite, however, of the very great amount of attention 

 that has been paid to these substances in recent years, there is at 

 present no agreement as to their chemical structure. Some au- 

 thorities (Arthus) have gone so far as to suggest that the enzymes, 

 or more properly enzyme actions, are not due to definite material 

 substances, but are to be classified as forms of energy like heat, 

 electricity, etc. The suggestion is not very helpful, but it indicates 

 forcibly the present uncertainty regarding the real nature of these 

 bodies. 



