237 



incapable of composing from their elements, or of forming from any 

 other substance excepting caseine, the substances albumen, fibrine, 

 &c, absolutely necessary to its development and support ; that the 

 animal must indeed receive substances ready prepared, in order to 

 apply them to its nutrition, or to convert them into gelatine for the 

 formation of its bony structures. Albumen, fibrine and caseine are 

 therefore rightly named by Liebig the exclusive materials for nutri- 

 tion ; they cannot be replaced by any other substance ; when they 

 are entirely withheld the body must necessarily die of starvation. 

 But the components devoid of nitrogen must also be present, as it 

 were for fuel on the hearth of organic life ; and these substances, 

 which are in common life also called food, Liebig appropriately de- 

 nominates materials for respiration.'''' — p. 136. 



After some further interesting illustrations of the different classes of 

 vegetables which produce these materials in the greatest abundance, 

 the author introduces some brief historical sketches of the introduc- 

 tion of coffee, chocolate and tea as beverages, for the purpose of call- 

 ing attention to an unsolved physiological problem. "Everywhere 

 have these beverages become necessaries of life ; everywhere is the 

 origin of their use enveloped in mystical obscurity; everywhere has 

 man, not led by rational considerations, by knowledge of their proper- 

 ties and action, or by comparison of them with already known nutri- 

 tive substances, but, as it were, instinctively, added them to the num- 

 ber of his daily wants." Chemistry has endeavoured to discover the 

 cause of this phenomenon ; and the result has shown that in all these 

 substances exists an element, distinguished from all other vegetable 

 productions by the very large proportion of nitrogen contained in it ; 

 but experiments have hitherto failed to detect any special action 

 upon the animal economy resulting from the admini stration of large 

 quantities of pure theine, the substance alluded to. 



Returning from this digression, the author goes on to show in what 

 way the recent revelations of chemistry as to the constituents of sub- 

 stances used as food, account for the varying habits of those nations 

 into whose ordinary diet enter a greater or less amount of either ani- 

 mal or vegetable products ; and continues : — 



" Our investigations have thus led us to recognize that the whole 

 animal world lives upon the vegetable kingdom, either immediately 

 by actual vegetable food, or mediately by the vegetable feeders collect- 

 ing the peculiar nutritive matters for the Carnivora, from the plants, 

 depositing the material for respiration, which contains no nitrogen, in 

 the form of fat. But we do not arrive at the conclusion of our inqui- 

 VOL. III. 2 K 



