COMPOSITION OF EXCREMENTITIOUS MATTER. 97 



coloring matters of flowers. Nitrogen is not pre- 

 sented to wild plants in any other form capable of 

 assimilation. Ammonia, by its transformation, fur- 

 nishes nitric acid to the tobacco plant, sun-flower, 

 Chenopodium, and Borago officinalis, when they grow 

 in a soil completely free from nitre. Nitrates are 

 necessary constituents of these plants, which thrive 

 only when ammonia is present in large quantity, and 

 when they are also subject to the influence of the 

 direct rays of the sun, an influence necessary to 

 effect the disengagement within their stem and 

 leaves of the oxygen, which shall unite with the 

 ammonia to form nitric acid. 



The urine of men and of carnivorous animals 

 contains a large quantity of nitrogen, partly in the 

 form of phosphates, partly as urea. Urea is con- 

 verted during putrefaction into carbonate of ammo- 

 nia, that is to say, it takes the form of the very salt 

 which occurs in rain-water. Human urine is the 

 most powerful manure for all vegetables containing 

 nitrogen ; that of horses and horned cattle contains 

 less of this element, but infinitely more than the 

 solid excrements of these animals. In addition to 

 urea, the urine of herbivorous animals contains hip- 

 puric acid,* which is decomposed during putrefaction 

 into benzoic acidf and ammonia. The latter enters 

 into the composition of the gluten, but the benzoic 

 acid often remains unchanged : for example, in the 

 Anthoxanthum odoratum. 



The solid excrements of animals contain compar- 

 atively very little nitrogen, but this could not be 

 otherwise. The food taken by animals supports 

 them only in so far as it offiers elements for assimila- 

 tion to the various organs which they may require 



* Rouelle announced the discovery of an acid in the urine of the 

 horse, which he called benzoic jhui in 1834 Liebig showed that this was 

 not benzoic acid, but one easily convertible into it, and distinguished it 

 by the name hippuric^ from tTTTro?, a horse, and ovqov, urine. 



t Benzoic acid exists in gum benzoin, &c. ; it is formed, according 

 to Liebig, by the oxidation of a supposed base called benzule. Its 

 composition is carbon 14, hydrogen 5, oxygen 2. 



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