72 ON THE ORIGIN AND ACTION OF HUMUS. 



present. Formic acid here represents the excre- 

 mentitious matters ; ammonia, the new substance, 

 assimilated by an organ of a plant or animal. 



Each organ extracts from the food presented to it 

 what it requires for its own sustenance ; while the 

 remaining elements, which are not assimilated, com- 

 bine together and are separated as excrement. The 

 excrementitious matters of one organ come in con- 

 tact with another during their passage through the 

 organism, and in consequence suffer new transfoj*- 

 mations ; the useless matters rejected by one organ 

 containing the elements for the nutrition of a second 

 and a third organ: but at last, being Capable of no 

 further transformations, they are separated from the 

 system by the organs destined for that purpose. 

 Each part of an organized being is fitted for its 

 peculiar functions. A cubic inch of sulphuretted 

 hydrogen introduced into the lungs w^ould cause 

 instant death, but it is formed, under a variety of 

 circumstances, in the intestinal canal without any 

 injurious effect.* 



In consequence of such transformations as we 

 have described, excrements are formed of various 

 composition ; some of these contain carbon in ex- 

 cess, others nitrogen, and others again hydrogen 

 and oxygen. The kidneys, liver, and lungs, are or- 

 gans of excretion ; the first separate from the body 

 all those substances in which a large proportion of 

 nitrogen is contained; the second, those with an 

 excess of carbon; and the third, such as are com- 

 posed principally of oxygen and hydrogen. Alco- 

 hol, also, and the volatile oils which are incapable of 

 being assimilated, are exhaled through the lungs, 

 and not through the skin. 



Respiration must be regarded as a slow process 

 of combustion or constant decomposition. If it be 

 subject to the laws which regulate the processes 



* The danger of breathing carbonic acid gas is well known, but 

 large quantities can be taken into the stomach with impunity and 

 even benefit. 



