PREFACE TO THE FIFTH EDITION. 



I much regret the delay in publication which has resulted in 

 the book being out of print for over a year. But the pressure of 

 extra work during the absence of most of my colleagues on worthier 

 occupations has prevented me devoting the time necessary for the 

 complete revision that I desired. A good many of the methods 

 described in the last edition were already obsolete by 1917, and it 

 was therefore impossible to reprint the book and still claim that 

 it represented the most modern technique. 



I have added a considerable amount of new material, the most 

 important being a chapter on the properties of solutions in which 

 particular attention is paid to the hydrogen-ion concentration and 

 to the colloidal state : a chapter on the preparation and properties 

 of certain of the amino-acids : on the preparation and hydrolysis 

 of nucleic acid : sections on the asymmetric carbon atom and on the 

 theory of the polarimeter : on the action of intestinal bacteria on 

 proteins : on autolysis : on the action of oxidase systems and many 

 new quantitative methods related to enzyme action and blood, 

 urinary and gastric analyses. A good deal of this material has not 

 been published previously, but the methods have stood the test of 

 routine class work at Cambridge, and it is trusted that they will 

 not fail when tried elsewhere. 



With the exception of the exercises on the preparation of the 

 amino-acids and on the hydrolysis of nucleic acid, the book represents 

 the course in Practical Physiological Chemistry for Medical Students 

 at Cambridge.. It may be objected that the course is overburdened 

 with analytical exercises. They are inserted for two reasons that 

 seem important to my Chief, Prof. F. G. Hopkins, and to myself. 

 In the first place they have considerable educational value : one is 

 enabled to train the student to make accurate observations and 

 pay attention to the effect of variations in conditions on results. 



