132 



SCIENCE. 



[N. S. Vol. XXII. No. 553. 



Ibe regarded as of fundamental importance, 

 as it demonstrates beyond cavil just what 

 is possible in protein restriction under or- 

 dinary conditions. The periods of investi- 

 gation chosen were long enough to answer 

 objections to the results of some of the 

 earlier tests, and the values obtained for 

 the soldiers and athletes of about 55 grams 

 of protein metabolized daily will have to 

 be taken as practical standards. It doubt- 

 less remains true that for men at severe 

 work at low temperatures a large number 

 of calories are required in the food. An 

 instructive example of such dietaries is 

 given in the, recent publication by C. D. 

 Woods on the diet of Maine lumbermen, 

 where it is shown that the heat value of the 

 food consumed daily by men in the lumber 

 camps may amount to 6,000 or 8,000 

 calories. It would be interesting to ex- 

 periment in such cases on the replacement 

 of a good share of the protein by fat and 

 carbohydrates, 



A study of the Chittenden series of ex- 

 periments on men shows very clearly that 

 as far as the human organism, at any rate, 

 is concerned the old Liebig notion of the 

 source of muscular energy is without foun- 

 dation. As suggested above, experiments 

 with carnivorous animals do not apply to 

 man; it would be as justifiable to discuss 

 the food value of pentoses for man from 

 experiments on the feeding of straw to 

 cattle. It is true that for short periods, 

 or under special conditions, proteins may 

 serve man as the main or only source of 

 muscular energy, but evidently this is not 

 usually or normally the case. 



When the far-reaching importance of 

 the whole question is realized, and when it 

 is further remembered that considerable 

 internal work must be done to remove, es- 

 pecially, the products of protein metabol- 

 ism, I believe it will be granted that I am 

 right in placing this work of Chittenden 



among the most important recent achieve- 

 ments in physiological chemistry. 



The next topic of which I wish, to speak 

 very briefly deals with a problem even 

 older than that of the Liebig theory of the 

 source of muscular energy. Some years 

 before the organic chemistry of Liebig was 

 published Mulder had introduced the term 

 protein, and had even announced the essen- 

 tial composition of what he considered the 

 protein nucleus. His positive statements 

 led to extended investigations on the part 

 of others, and the work of many chemists 

 soon disclosed the fact that no one simple 

 nucleus may be assumed to exist in these 

 molecules and that they must be enormous- 

 ly complex. Ever since the early forties 

 the problem has been 'an extremely inter- 

 esting one, but it is only recently that it 

 has been seriously attacked from the second 

 side possible in such investigations. Up to 

 a period within five years the work done on 

 the protein question has been largely in the 

 way of analysis or disintegration, but now 

 we have the beginning of attempts at syn- 

 thesis or reconstruction of large groups. 

 Glycine and leucine had been known since 

 about 1820 as decomposition products of 

 glue and other bodies by action of acid. 

 Nearly thirty years later tyrosine was 

 added as obtained in about the same way, 

 and soon a few other individual substances 

 were listed among the products which 

 could be secured in various decompositions 

 of proteins. In the seventies systematic 

 methods of hydrolysis by alkalies and acids 

 were worked out, especially by Schiitzen- 

 berger and Hlasiwetz and Habermann. 

 Numerous products were recognized, but 

 at first these attracted no great attention, 

 as there remained always the possibility 

 that the amino acids and other compounds 

 found might be results of secondary reac- 

 tions. We can not infer much regarding 

 the structure of soft coal from the presence 

 of methane in the gas, or of benzene, tolu- 



