INTRODUCTION 5 



than it has hitherto been. Their preparation was no doubt due to the 

 possibility of the presence of other units than those above described, 

 which possibility will not be excluded until the quantity of products 

 isolated reaches 100 per cent. 



The amino acids composing the protein molecule are generally 

 referred to as the bricks, or foundation-stones, " Bau-steine " ; but as the 

 English translation of the German word is not entirely expressive of its 

 meaning, it is preferable to use the term unit or element for these 

 compounds in their relation to the proteins. 



The methods which have been employed for the purpose of de- 

 termining the composition of the protein molecule have been many 

 and various. They may be classified under four headings : 



(1) Fusion with alkali. 



(2) Oxidation with permanganate, chromic acid, etc. 



(3) Action of halogens. 



(4) Hydrolysis. 



Of these, the last, that of hydrolysis, has thrown most light on the 

 darkness of this complex problem. Hydrolysis has been effected by 

 the action of acids, of alkalies, and of the various proteoclastic enzymes 

 which occur in plants and animals, and is practically the only method 

 by which we have attained to our present knowledge. Proteins were 

 first hydrolysed by acids in 1820 by Braconnot, who used dilute sul- 

 phuric acid; between 1850 and 1875 hydrochloric acid was most fre- 

 quently employed as the hydrolysing agent by Ritthausen, Hlasiwetz 

 and Habermann and others, and from 1870 to 1880 Schiitzenberger 

 employed baryta water under pressure. The action of vegetable 

 enzymes on proteins has been studied chiefly by Schulze and his co- 

 workers, that of animal enzymes by Kiihne, Kossel, Kutscher, Drechsel, 

 and numerous other investigators. 



As the result of hydrolysis a complex mixture of all, or nearly all, 

 the previously mentioned units is obtained. These have been isolated 

 by various methods based upon the fractional crystallisation of the 

 compounds themselves, or of their copper, silver, and other salts. Only 

 when one or more of the amino acids occurred in somewhat large 

 amounts was their isolation and characterisation effected ; their amount 

 seldom reached a value higher than 20 per cent, of the total quantity, 

 and the remainder was represented by uncrystallisable syrups of un- 

 known nature. The products described by Schiitzenberger have been 

 shown by Hugounenq, Galimard and Morel to be mixtures of now 

 definitely known substances. 



