INTRODUCTION 7 



products, which can be readily estimated quantitatively without the 

 employment of the elaborate esterification method. Amongst these 

 cystine can be estimated by determination of the sulphur content of 

 the protein, and tyrosine by the method suggested by Millar (p. 38). 

 The cystine and tyrosine factors should therefore be of value. 



Much work remains to be done in the elaboration of quantitative 

 reactions which might serve for the characterisation of the proteins. 

 It is not necessary that these reactions should lead always to the 

 formation of definite chemical compounds ; it would be sufficient if 

 standard conditions could be chosen, under which a definite amount 

 of chemical reaction should take place. Similar methods have been 

 employed in the case of the fats, as an instance of which may be 

 cited the so-called Hiibl iodine number. This indicates approxi- 

 mately the number of double bonds in a given fat, but it is generally 

 admitted that the addition of iodine at the double bonds is accom- 

 panied by a certain amount of substitution. Under defined conditions, 

 however, the amount of iodine absorbed is a constant for each 

 particular fat. 



Similar reactions are required for the identification of proteins, 

 and the various suggestions which have been made in this respect 

 have received what will be, it is hoped, adequate consideration in the 

 following pages. 



