NITROGENOUS ORGANIC CELL-CONSTITUENTS. 89 



animal tissues. Animals, therefore, do not have the power of manu- 

 facturing albuminoids, although they may transform albuminous bodies 

 of one kind into those of another. Thus, casein may be transformed 

 into the albuminous constituents of muscle-tissue ; it may be combined 

 with other substances so as to form, for example, the haemoglobin of 

 the blood-corpuscles, or may become so modified as to form what are 

 termed the derived albuminoids of the different tissues. 



After serving the purposes of the organism such bodies are excreted, 

 not as proteids, but as products resulting from their retrograde meta- 

 morphosis. All albuminous bodies are so intimatety associated with 

 inorganic matter that their isolation in a pure state is a matter of the 

 greatest difficulty, or, it may be, impossibility ; consequently the inciner- 

 ation of albuminous bodies — a process which is accompanied with the 

 development of an odor like burning horn — always leaves an ash composed 

 of potassium and magnesium phosphates and small quantities of carbo- 

 nates. If sulphur is regarded as a constant and normal component of 

 proteids and not as an occasional accidental addition, they all possess a 

 very high molecular weight. In all forms of proteids the percentage of 

 chemical elements entering into their composition is only subject to 

 slight variation in the different classes. Thus, according to Hoppe-Seyler, 

 C. may vary from 52. 7 to 54.5 per cent.; H., 6.9 to 7.3 per cent.; 1ST., 

 15.4 to 16.5 per cent. ; 0., 20.9 to 23.5 per cent.; S., 0.8 to 2.0 per cent. 



Physical Properties. — When dry, albuminous bodies form perfectly 

 amorphous, yellowish, brittle masses without odor or taste, and closely 

 resembling gums in appearance, and, like gums, hygroscopic to a high 

 degree : they rotate the plane of polarized light to the left, and in watery 

 solutions, which are nearly always opalescent, are not, as a rule, capable 

 of osmosis, — a fact, which seems to show, as Briicke has pointed out, 

 that their condition in the form of fluid is more one of particulate 

 suspension than of true solution. When shaken with fluid oils, the latter 

 are mechanically separated into minute particles, each of which is sur- 

 rounded by a layer of the albuminous solution (emulsion). Some are 

 soluble in water, others not; nearly all are insoluble in alcohol and 

 ether; most are soluble in strong alkalies and acids, but in the process 

 - of solution undergo chemical change. Most of the albuminous bodies 

 may exist in two modifications, either in a soluble or in an insoluble 

 form. They exist usually in the soluble form in animal and vegetable 

 cells, but become insoluble by the action of heat and various chemical 

 reagents. 



When watery solutions of albuminous bodies are evaporated in a 

 vacuum, or at 40° to 50° C, a yellowish, brittle, soluble residue is left; 

 in other words, albumen may be recovered unaltered in general prop- 

 erties in the dry form from solutions when subjected to evaporation by 



