114 GENERAL PHYSIOLOGY 



occurs also when neutral fats are boiled with alkaline liquids, such 

 as caustic potash or soda. The fatty acids thus set free combine 

 with the alkali to form the so-called soaps, which may be distin- 

 guished as potash soaps, sodium soaps, calcium soaps, etc. 



The fats are all lighter than water and do not dissolve in water, 

 but they are easily soluble in ether. A characteristic property, 

 which is important for the microscopic recognition of the fat-drop- 

 lets in cells, is their power of reducing perosmic acid to metallic 

 osmium, the latter forming a black coating to the fat-droplet. 

 This osmic acid reaction is not to be employed alone as a sure test 

 in the diagnosis of fat ; for doubtless other reducing substances 

 exist, , which, under certain circumstances, can be blackened by 

 osmium ; hence it should be used only in conjunction with other 

 tests, solubility in ether, strong refracting power, etc. 



The fact that fats, like carbohydrates, can appear as cleavage- 

 products of proteids has already been mentioned. 



d. The Inorganic Constituents of Living Substance 



In the case of the organic compounds of the cell the general con- 

 stituents (proteids) and the special constituents (carbohydrates and 

 fats) can be contrasted ; the same distinction can be made with 

 the inorganic compounds. 



Here, also, the greater interest is associated with the general 

 inorganic constituents, among which there are distinguished water, 

 salts, and gases. 



Water is that constituent of living substance that gives to it its 

 liquid nature and thus renders possible the easy shifting of its 

 particles, which is so necessary for the occurrence of vital 

 phenomena. It is contained in the cell, in part chemically 

 combined as water of constitution, and in part free, as the 

 solvent medium of all sorts of substances. Accordingly, water is 

 present in abundant quantity, constituting upon the average more 

 than 50 per cent, by weight of living substance. If, e.g., the whole 

 water contents of the human body be investigated, which with the 

 great variety of the forms of tissue affords a good average, 

 approximately 59 per cent, of water is found; this is shown especially 

 by the detailed investigations of Bezold. The different tissues 

 vary very greatly in this respect. Thus, bones contain onlj- about 22 

 per cent, of water, the liver 69 per cent., muscles 75 per cent., and 

 the kidneys 82 per cent. Hence it is not strange that the water 

 contents of living substance varies much more in different species 

 of animals, and that all intermediate stages in percentage composi- 

 tion are met with between the slight traces of water contained in 

 a rotifer when dried but still capable of life, and the water-contents, 

 amounting to more than 99 per cent., of certain pelagic Cteno- 

 phora. 



