LIEBIG'S SCIENTIFIC ACHIEVEMENTS. 



T3 



ence of the carnivora implies that of the her- 

 bivora, and the herbivora that of vegetation. 

 Liebig said : 



" The nutriment of men and animals consists 

 of two substances, differing entirely in their com- 

 position. The one class, consisting of nitrogenous, 

 albuminous matter, serves to make blood, and to 

 form the solid structure of the body — these are 

 called plastic elements of nutriment. The others, 

 which are non-nitrogenous, fats and hydrocarbons, 

 are like ordinary fuel, and serve only for the pro- 

 duction of heat — they are called breathing material. 

 Sugar, starch, and gums, may be regarded as fibres 

 of wood transformed, and can be produced from 

 wood. Fat stands next to coal in the quantity of 

 carbon it contains. We heat our bodies as well as 

 our stoves with fuel containing the same elements 

 as wood and coal, but essentially distinguished 

 from them by their solubility into the Juices of the 

 body." 



It is upon this that Liebig's division of all nutri- 

 tive elements into blood-making or plastic, and 

 combustible or respiratory, material, is based, 

 and from its simplicity and clearness it has been 

 retained almost without exception to the present 

 day, although the definition more recently given 

 by Voit, from a physiological rather than a 

 chemical standpoint, is also very simple. Voit 

 classifies articles of diet, not according to their 

 possible effects, but according to their material 

 importance for the body, and states the question 

 thus : What must the food contain to prevent 

 the diminution of albumen, fats, salts, water, and 

 oxygen, in the transmutation of matter, and what 

 share has each article of diet in it ? Both men 

 and animals live exclusively upon the above- 

 named materials, and consume them in the inter- 

 vals between meals and when fasting. In order 

 that food may be really convertible by and ac- 

 ceptable to the organism, it must, as Voit justly 

 observes, contain also articles of luxury, that is, 

 materials which work upon certain nerves and 

 affect the activity of the digestive apparatus. 

 This definition embraces not only the solid, but 

 also the fluid and air-forming constituents of 

 food ; it is more complete and precise than Lie- 

 big's, but not so intelligible to every layman, so 

 that Liebig's will long maintain the field with the 

 public at large. Jt has, however, been proved, 

 by experiment, that even under the greatest 

 exertion no more blood-forming or strength- 

 producing elements are consumed than during 

 absolute rest, while a great deal more of the 

 other class, the heat-producing elements, are 

 consumed, though the heat of a healthy body is 

 rather diminished than increased. 



Another great service of Liebig's, on the sub- 

 ject of nutrition, is that he established the prin- 

 ciple that it depends not only on partaking of 

 blood-forming and heat-producing materials, but 

 that they should be taken in certain proportions 

 varying with circumstances. He also endeavored 

 to settle the equivalents of different articles, as, 

 for instance, how much hydrocarbon would pro- 

 duce the same effect as a certain weight of fat. 

 He did this in his usual fashion, from a purely 

 chemical standpoint, without even, as it were, 

 putting a question to the living body. When, 

 therefore, experiments were tried in accordance 

 with these theories, many things turned out other- 

 wise than he expected, just as the fields did not 

 yield the results he looked for, on his experiments 

 with mineral manures ; but in spite of all this, it 

 is surprising how much has been confirmed, and 

 no one will dispute the services rendered in this 

 sphere by him as a pioneer. 



Liebig was much engrossed at one time with 

 the formation of fat in the body. I cannot state 

 the subject better than Voit has done in his 

 lecture on theories of nutrition : " It had long 

 been agreed that the albuminous and nitrogenous 

 elements in the body are solely produced by al- 

 bumen in the food ; but great difference of opin- 

 ion prevailed as to what produced fat, and yet 

 it is very important to ascertain it. It was 

 thought at first, and the view was entertained by 

 two distinguished French chemists, Dumas and 

 and Boussingault, that fat was formed solely from 

 the fat intoduced into the body. But Liebig soon 

 perceived that this could not be, and proved to a 

 certainty that the fat in the food, especially in 

 the case of the herbivora, could not produce all 

 the fat in the body, and that the hydrocarbons had 

 the greatest influence on the production of fat." 



From these facts Liebig drew the conclusion 

 that fat is formed chiefly from the non-nitrogen- 

 ous elements in food, starch, sugar, and the hy- 

 drocarbons generally. A lively controversy sprang 

 up between Giessen and Paris, and there was 

 much sharp-shooting on both sides. Liebig came 

 off victorious, inasmuch as he was correct in 

 maintaining that the fat in the food could not, 

 in most cases, account for the amount of fat in 

 the body, and that the hydrocarbons were cer- 

 tainly concerned in its production. 



It was not until experiments were undertaken 

 with the respiratory apparatus in the Physiologi- 

 cal Institute at Munich that other views came to 

 be entertained. This apparatus enabled us to 

 ascertain the exact amount of receipts and ex- 

 penditure in the body during twenty-four hours, 



