Organic Substances and Formations. 301 



The horse can digest cellulose in a more compact form 

 than man, the camel perhaps still more so, and certain wood- 

 devouring insects possess additional faculties of the same sort. 

 A clean cotton fibre is nearly pure cellulose, and so is that 

 elegant material erroneously called " rice paper," and made by 

 the Chinese from the pith of the Aralia jiapyrifera. 



The hair, hoofs, horns, and feathers of animals are very 

 similar in chemical composition, and silk is very like them, and 

 belongs to the same series. Speaking generally, we may say 

 that these substances are rather more than half carbon, about 

 one quarter oxygen, about a sixth nitrogen, and they contain 

 about 3 per cent, of sulphur. 



The solid parts of insects, such as the elytra and hard skin 

 of beetles, their jaws, ovipositors, etc., and the shells of certain 

 rotifers, etc., are composed of chitin, a substance chemically 

 resembling the preceding, but with a larger allowance of 

 oxygen. This substance is also found in the organic matter of 

 the shells of crustaceans — those of water fleas (Eutomostracea), 

 for example, contain little else. 



Professor Miller says " sugar appears to be the basis or 

 foundation of organic matter in general, and from it all the 

 varieties of organized products might be obtained by the addition 

 or subtraction of water, oxygen, and ammonia."* This is cited 

 not to convey the impression that all organic bodies do actually 

 arise out of such a mode of treating sugar, but to illustrate 

 the relation which different organic compounds bear to each 

 other. 



Organic compounds placed under appropriate conditions 

 are susceptible of endless changes and modifications, first by 

 simple addition of definite quantities of materials they already 

 possess j secondly, by removals of certain constituents, wholly 

 or partially, but always in definite proportions ; thirdly, by 

 substitution, that is, by taking away certain elements in defi- 

 nite proportions, and adding new elements in equivalent pro- 

 portions. Living beings take matter into their system in one 

 condition, and change it by processes of this kind. The plant 

 is remarkable for the extent to which it can convert inorganic 

 matter into organized combinations, and the animal with some 

 power of this sort is chiefly occupied in receiving from plants, 

 or from other animals, materials belonging to the organic 

 series, and modifying them according to its needs. 



The chemical processes carried on by organized beings 

 range from comparatively simple ones to others that are highly 

 complicated. A vegetable cell, such as a yeast cell, is able 

 through its delicate walls to allow a certain intercourse between 

 its internal contents and the matters which the fluid in which 

 * Elements of Chemistry, vol. iii. p. 723. 



