INTRODUCTION 7 



ternal impressions, or stimuli, by actions peculiar to each kind 

 of cell. (3) The living substance reproduces itself. All the 

 manifold activities included under these three heads have but 

 one source, the transformation of the energy of the food. It is 

 not, however, upon the whole, peculiarities in food, but in mole- 

 cular structure, that underlie the peculiarities of function of 

 different living cells. A locomotive is fed with coal ; a steam- 

 pump is fed with coal. The one carries the mail, and the other 

 keeps a mine from being flooded. Wherein lies the difference 

 of action ? Clearly in the build, the structure of the mechanism, 

 which determines the manner in which energy shall be trans- 

 formed within it, not in any difference in the source of the 

 energy. So one animal cell, when it is stimulated, shortens or 

 contracts ; another, fed perhaps with the same food, selects 

 certain constituents from the blood or lymph and passes them 

 through its substance, changing them, it may be, on the way ; 

 and a third sets up impulses which, when transmitted to the 

 other two, initiate the contraction or secretion. In the living 

 body the cell is the machine ; the transformation of the energy 

 of the food is the process which ' runs ' it. The structure and 

 arrangement of cells and the steps by which energy is trans- 

 formed within them sum up the whole of biology. 



PRACTICAL EXERCISES. 



Reactions of Proteins. 



i. General Reactions of Proteins. Egg-albumin may be taken 

 as a type. Prepare a solution of it by adding water to white of 

 egg, which consists mainly of egg-albumin with a little globulin. 

 In breaking the egg, take care that none of the yolk gets mixed 

 with the white. Snip the white up with scissors in a large capsule, 

 then add ten or fifteen times its volume of distilled water. The 

 solution becomes turbid from the precipitation of traces of globulin, 

 since globulins are insoluble in distilled water. Stir thoroughly, 

 strain through several layers of muslin, and then filter through 

 paper. 



Colour Reactions. 



(1) Add to a little of the solution in a test-tube a few drops of 

 strong nitric acid. A precipitate is thrown down, which becomes 

 yellow on boiling. Cool, and add strong ammonia ; the colour 

 changes to orange (xantho-proteic reaction). The reaction depands 

 upon the presence of aromatic groups in the protein molecule, which 

 are converted into nitro-compounds. 



(2) To a third portion add a drop or two of very dilute cupric 

 sulphate and excess of sodium or potassium hydroxide ; a violet 

 colour appears (Piotrowski* s test}. Peptones and proteoses (albu- 

 moses) give a pink (biuret reaction}.* See p. 426. 



* The reaction is also given, although more faintly, with the hydroxides of 

 lithium, strontium, and barium. It is given by all substances containing 

 at least two CONH. 2 groups attached to one another (as in oxamide), or 

 to the same nitrogen atom (as in biuret), or to the same carbon atom. 



