72 



THE POPULAR SCIENCE MONTHLY.— SUPPLEMENT. 



urea from its elements, the first instance of an 

 artificial exhibition of an organic body out of in- 

 organic elements. The work on uric acid is a 

 classic model to this day. 



Liebig's labors in animal physiology may be 

 divided into two parts : 1. The discovery by 

 chemical analysis of the component parts of va- 

 rious organs, and excreta ; 2. The endeavor to 

 explain the chemistry of physiological processes. 

 To the first part much has been contributed by 

 others besides Liebig and his school. Much as 

 these researches have extended our knowledge, 

 they would never have received the attention 

 accorded to the second part; this Liebig re- 

 served to himself, namely, to explain his ideas 

 of the significance of all these matters in the liv- 

 ing body. 



Physiology falls mainly into an anatomic- 

 physical and an organic-chemical division. With 

 the first Liebig scarcely troubled himself at all, 

 and in the second he was interested purely as a 

 chemist, but he taught the physiologists from his 

 laboratory how to observe chemical processes in 

 the organism, without ever having made a physi- 

 ological experiment. This was at once a strong 

 and a weak point with him. No physiologist 

 will deny that, in spite of this weakness, he exer- 

 cised an influence on the progress of physiological 

 science. The case was much the same as with 

 agricultural chemistry : he constructed a theory 

 from a purely chemical standpoint, and then ap 

 pealed to practice ; but his essays in the physio- 

 logical sphere were much more audacious, for he 

 had not command of the weapons — physiological 

 experiment ; at least I am not aware that he ever 

 himself made an experiment on a living being. 



The progress of every science depends upon 

 the discovery of facts, which may be called scien- 

 tific practice, and upon the conclusions deduced 

 from them — that is, on theory and practice. 

 They may be compared to the army and diplo- 

 macy in statecraft. Diplomacy wages no actual 

 warfare, but is not seldom the cause of it, and 

 the soldiers have to make experiment after ex- 

 periment, to marshal facts against facts, until it 

 appears which side is the stronger. 



In the realm of pure chemistry Liebig was 

 both soldier and diplomatist, indeed one of the 

 greatest generals. In the physiological sphere 

 he was only a diplomatist, but he was the cause 

 of many movements and conflicts, which have 

 cleared up various points in physiology, although, 

 when peace was concluded, some things were not 

 settled in accordance with the original demands. 

 The decision of Liebig's character induced him 



at once to state what he thought to be the case 

 without troubling himself about other claims. 

 Thus, for instance, it had been stated by La- 

 voisier that animal heat was derived from pro- 

 cesses of oxidation, from a sort of combustion of 

 the substances in the body containing carbon and 

 hydrogen. Dulong and Despretz proved by ex- 

 periment that this was nine-tenths true, that is, 

 capable of demonstration. Liebig tried to prove 

 by purely theoretical methods that all animal 

 heat was derived from chemical processes, and 

 did not, like many others, infer, from the experi- 

 ments of Dulong and Despretz, that the other 

 tenth was otherwise produced, for example, by 

 nervous influences ; did not even take the trouble 

 to investigate the remaining tenth at all, but 

 simply gave in his evidence to the opposite party, 

 that the heat of an animal kept in a box sur- 

 rounded with ice during experiment did not be- 

 come diminished by one-tenth. And proof to 

 the contrary has never yet been offered. 



As is well known, Liebig interested himself 

 no less in the nutrition of animals than in that 

 of plants. Notwithstanding that the food of dif- 

 ferent animals and man is so various, they are 

 chemically alike in their corporeal substance. 

 There is no essential difference between the flesh 

 of birds, oxen, or man ; cow's milk can be sub- 

 stituted for mother's milk ; and, physiologically 

 speaking, there is nothing impossible in the 

 myth that the founders of Rome were suckled by 

 a wolf. The transmutation of matter in an her- 

 bivorous animal, when fasting, differs in no re- 

 spect from that of a carnivorous animal, for so 

 long as he does not eat he must consume his own 

 flesh. What is, then, the common principle in 

 the thousand kinds of food, this unity in multi- 

 plicity? The mind of man has occupied itself 

 with these questions for ages. Just before Lie- 

 big's time, the experiments made by Magendie 

 and others on the subject of nutrition had pro- 

 duced results which only seemed to render the 

 question more obscure ; but six years ago Voit 

 said : 



" This obscurity was to be brilliantly illumi- 

 nated by one who is still among us. INo one will 

 accuse me of flattery if I mention the services of 

 one living and present, for they are so universally 

 acknowledged that they belong to history." 



It was Liebig who first expressed the decided 

 opinion that the animal must be provided by his 

 food with the chief elements in the blood, and 

 therewith of the body built up and nourished by 

 it, and that its original and essential constituents 

 are to be found only in vegetation, for the exist- 



