70 MOLECULAR CONSTITUTION 



and yet in many respects like them. They include various horny 

 and elastic substances found in skeletal tissues and in skin develop- 

 ments ; and also the extremely important group of ' enzymes ' or 

 soluble ferments to which reference was made in the last chapter — 

 including ptyalin, pepsin, trypsin, pancreatin, etc. 



Multitudes of other organic compounds exist as decomposition 

 products of the proteids — results of disassimilation. Many of these 

 are thrown off as waste products, the cemmonest of all being car- 

 bonic dioxide and urea; others, such as 'fats' and 'carbohy- 

 drates,' are very important non-nitrogenous compounds, the latter 

 including different kinds of sugar and starch met with principally 

 in plants and of great importance in their economy. 



Although such a radical distinction exists between crystalloids 

 and colloids, in regard to their intimate molecular constitution, as 

 to have led Graham to say, " Every physical and chemical property 

 is characteristically modified in each class," so that " they appear 

 like different worlds of matter," yet the transition from the one 

 state to the other seems to be comparatively easily brought about. 

 As Graham has shown, one and the same saline substance may 

 exist with its molecules now in the crystalloid and now in the 

 colloidal mode of aggregation — according to the different influences 

 to which it has been subjected — or under which it has been 

 produced. This, for instance, is the case with silica, with the 

 sesquioxides of chromium and iron, and with other mineral 

 substances. Nay, more, the absence of any natural barrier 

 between the crystalloid and the colloidal mode of aggregation 

 may be still further seen by the fact that even the most typical 

 colloids are capable of undergoing that kind of isomeric molecular 

 change which converts them into crystalloids. As one of the best 

 instances of this we may mention the fact of the change which 

 blood pigment undergoes. Hsematoidin is frequently met with in 

 the form of oblique rhombic crystals, and in addition there are 

 other crystalline forms of albuminoid substances obtainable from 

 blood,' and also from the cells of some plants (known as aleurone- 

 granules). Chlorophyll has likewise been observed in a crystalline 

 state by Tr6cul.= While Marcet,3 as a conclusion drawn from cer- 

 tain experimental researches expressed the opinion, " That there 



■ See an article on " Albuminous Crystallisation," in ' Brit, and For. Med. Chir. 

 Review,' Oct. 1853. 

 ' ' Comptes Rendus,' t. LXI., p. 436. 

 3 ' Proceed, of Roy. Soc.,' Vol. XIX. (1871), p. 455. 



