PEOTEIDS. 



and animal proteids is chemically no greater than that between 

 different kinds of vegetal or different kinds of animal proteids : 



PERCENTAGE COMPOSITION OF PROTEIDS. 



There is a corresponding likeness in the general properties 

 and reactions of proteids. They are for the most part soluble ; 

 they are colloidal or non-diffusible, i.e., they will not pass through, 

 the membrane of a dialyser, or only with great difficulty ; they 

 are rarely crystalline ; they rotate the plane of polarized light to 

 the left. Though not all soluble in water, they may be dissolved 

 by the aid of heat in strong acetic acid and in caustic alkalies, 

 but are insoluble in cold absolute alcohol and in ether. They 

 may be precipitated from solution by strong mineral acids, etc., 

 and their presence may be detected by methods more fully out- 

 lined in the practical study (p. 43). Many proteids are precipi- 

 tated by heat (a process which is called coagulation] ; and it is 

 worthy of note that temperatures which produce coagulation of 

 proteids (4:0-75 C.) produce also the death of organisms.* 



* " Amongst the organic proximate principles \vhich enter into the compo- 

 sition of the tissues and organs of living beings, those belonging to the class of 

 proteid or albuminous bodies occupy quite a peculiar place and require an ex- 

 ceptional treatment, for they alone arc never absent from the active living cells 

 which we recognize as the primordial structures of animal and vegetable or- 

 ganisms. In the plant, whilst we recognize the wide distribution of such con- 

 stituents as cellulose and chlorophyl, and acknowledge their remarkable physi- 

 ological importance, we at the same time are forced to admit that they occupy 

 altogether a different position from that of the proteids of the protoplasm out 

 of which they were evolved. We may have a plant without chlorophyl, and a 

 vegetable cell without a cellulose wall, but our very conception of a living, 

 functionally active, cell, whether vegetable or animal, is necessarily associated 

 with the integrity of its protoplasm, of which the invariable organic constitu- 

 ents are proteids. 



" In the animal, the proteids claim even more strikingly our attention than in 

 the vegetable, in that they form a very much larger proportion of the whole 

 organism, and of each of its tissues and organs. We may indeed say that the 

 mateiial substratum of the animal organism is proteid, and that it is through 



