D4 PHILOSOPHY OF ZOOLOGY. 
obtained, may be regarded in many cases as modifications 
of the elements of the substance, occasioned by the process- 
es employed, rather than the display of the number or na- 
ture of the ingredients as they existed previous to the ana- 
lytical operations. Errors, we know, are to be guarded 
against in the analysis of mineral waters, arising from com- 
binations taking place during the process, which did not 
exist in the compound previous to analysis*. How much 
more necessary is it, to exercise caution in drawing 
our conclusions regarding the composition of animal bo- 
dies ? 
In this section I shall enumerate the elementary bodies, 
which are considered as entering into the composition of the 
parts of animals, and then consider the combinations which 
these form, or the substances in which they have been 
detected. | 
I. ELEMENTARY SuBSTANCES. 
1. Carbon.—This substance exists in various states of 
combination, in the fluids as well as the solids of every ani- 
mal. It has never been detected in a separate state in any 
animal substance. In the lungs, however, it appears to 
occur in the form of charcoal, according to the observations 
of Dr Pearson +. The lungs, in youth, are light colour- 
ed ; but they increase in darkness with age, and in old 
persons they are nearly black. This change of colour is 
produced, by the deposition of charcoal in the cells of the 
lungs, and the bronchial glands. Dr Prarson considers 

"See Dr Mourray’s valuable paper, “ An Analysis of Sea-water.” 
Edin. Trans, vol. viii. p. 205 
+ ‘On the Colouring Matter of the Black Bronchial Glands, and of the 
Black Spots of the Lungs.” Phil. Trans. 1813, p. 159. 
