44 DIGESTION AND FOOD. 



and peroxide of manganese, or by sulphuric acid and bichro- 

 mate of potash, or by fusion with caustic potash, they all 

 yield similar products; namely, compounds of the series of 

 aldehyde; of that of acetic acid; .of the nytrile series, benzoic 

 acid, and oil of bitter almonds, leucine and tyrosine." 

 (GKEGOKY.) 



I have referred to the tendency to decomposition in these 

 complex organic compounds. But a very remarkable feature 

 of the group is their disposition to induce change in materials 

 with which they come in contact, and often by catalysis 

 that is to say, without suffering change themselves. 



The peculiar tendency to solidify characterizes all proteine 

 compounds, and the condition assumed is so peculiar as to 

 merit the distinctive name ''coagulation.' Casein curdles or 

 coagulates when acids are thrown in milk ; fibrine coagulates 

 spontaneously from exposure to air, and albumen hardens 

 when heated. 



The whole of the albuminoid principles are susceptible of 

 being simply dried, and thus transformed from the liquid to 

 the solid state; but in that condition they greedily absorb 

 water, and are restored by it to their natural condition. They 

 are termed hygroscopic from this peculiar property. 



The farmer supplies them in his crops by dressing the land 

 richly with ammoniacal manure; and, so important are the 

 nitrogenous principles to life, that both animals and vegetables 

 thrive most certainly and most rapidly when the elements 

 capable of forming proteine compounds are freely supplied 

 them. Excess is injurious, and probably more so than excess 

 in the supply of other principles, saline, or hydro-carbonaceous, 

 which are more easily stored or discharged from the system, 

 and do not induce a dangerous plethora. - 



I. ALBUMEN. Animal albumen is found in the shape of 

 the white of eggs, and its analysis leads to the formula 



