232 Journal of Agi'iciiUiire. [8 April, 1907 



This list, it must be remembered, is by no means complete, it omits 

 many of the amino-acids (such as asparagin), but, as stated, it gives the 

 important ones. 



Now, if we imagine a large number of representatives of these amino- 

 acids united to each other and forming an enormous mo'lecule — a veritable 

 tangle of amino-acid groups — we shall have something resembling a pro- 

 tein. The building up of a protein from its groups is performed rapidly 

 and with ease bv the animal or nlant, but, so far, the chemist has not 

 succeeded in performing this task in the laboratory.^ 



Proteins differ from each other, not only in the number of the groups 

 present, but in the manner in which these components are arranged. There 

 is practically an infinite variety of proteins possible, and we according! v 

 find a wide diversity between the proteins contained in living cells, of which 

 they constitute the greater portion of the solid matter present. 



Proteins in dry form are white and tasteless ; a few can be prepared m 

 crystalline form, for instance, the albumen of the white of egg, but only 

 by very special treatment. Drv protein heated over a flame chars and 

 gives off fumes that smell of burnt feathers; this smell, indeed, tells us 

 that nitrogenous groups are present. The charring protein will readily 

 take fire, and the char, which is chiefly carbon, will, if heated stronglv 

 enough, burn slowly away, leaving a very small residue of mineral ash. 

 It is still a matter of debate how far these mineral salts enter intO' the com- 

 position of the protein ; certainly no protein appears in the animal body 

 that does not contain such salts, and the presence of the latter seems to be 

 of vital importance ; but, by suitable methods of washing, the amount of 

 mineral matter may be reduced to a. \-ery small fraction of what was 

 originally present. 



A protein in solution tends to break up into a mixture of its amino- 

 acid components. This is a very slow process, and can onlv be detected 

 after the lapse of years, yet it is noticeable in tinned meats and preserved 

 milk that have been stored a long time. This decomposition can be greatly 

 accelerated, in fact, it can be completed in a few hours, by various means, 

 one of which has been already stated, namely, boiling with an acid. A 

 very interesting property of proteins, and one which has a very important 

 practical as well as theoretical application is their inability to pass 

 through a membrane composed of parchment paper or gelatine. If, for 

 instance, we place white of egg in a bag made of parchment paper and 

 dip this bag into pure water, the salts and sugars of the egg-white can 

 diffuse into the water, but the protein remains behind as in a trap. If the 

 water be renewed continuoiusly, a time will come when nothing remains 

 inside the bag but protein and water. This procedure is termed dialvsis, 

 and can be emploved with a number of substances which, like protein, have 

 large and highly complex molecules. 



Proteins in solution can be thrown out as insoluble precipitates by 

 means of various reagents. A number of these precipitations are frequently 

 given as tests for proteins. Anything which precipitates protein destroys 

 life if it gains access to the cell. The poisonous pronerties of corrosive 

 sublimate, blue stone, sugar of lead, and most metallic salts are due to 

 this fact, whilst a number of antiseptics owe their utility to their power 

 of precipitating a part or all of the protein in bacteria. 



1 It may be safely prophesied that this problem will shortly be solved by Dr. Bmil Fischer, who 

 appears to be within measurable reach of success. 



