jo A MANUAL OF PHYSIOLOGY 



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 transformed 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 sub- 

 stance, 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 Proteids. 



i. General Reactions of Proteids. Egg-albumin may be taken as 

 a type. Prepare a solution of it. 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 



(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}. 



(2) Acidify another portion strongly with acetic acid, and add a 

 few drops of a solution of potassium ferrocyanide. A white pre- 

 cipitate is obtained. Peptones do not give this reaction. 



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

 sulphate and excess of sodium or potassium hydrate ; a violet colour 

 appears. Peptones and proteoses (albumoses) give a pink (biuret 

 reaction}* See p. 378. 



(4) To another portion add Millon's reagent ;t a precipitate comes 



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

 of lithium, strontium, and barium. 



f Millon's reagent consists of a mixture of the nitrates of mercury with 

 nitric acid in excess, and some nitrous acid. To make it, dissolve mercury 



