THE USE AND ABUSE OF FOOD. 343 



useless food, instead of being left free to choose, as it might 

 otherwise do, such forms of food as would best supply the 

 requirements of the system. Salads and uncooked fruits, 

 for instance, contain saline ingredients in large proportion, 

 and could be used advantageously after a meal of boiled 

 meat Potatoes are likewise a valuable article of food on 

 account of the mineral elements contained in them. And 

 there can be no doubt that the value of potatoes as an article 

 of food is largely increased when they are cooked in their 

 skins, after the Irish fashion. 



Lastly, we must consider those articles of food which 

 promote the natural vital changes, but do not themselves 

 come to form part of the frame, or, at least, not in any large 

 proportion of their bulk. Such are tea, coffee, and cocoa : 

 alchoholic drinks ; narcotics ; and lastly, spices and condi- 

 ments. We may compare the use of these articles of food 

 to that of oil in lubricating various parts of a steam-engine. 

 For, as the oil neither forms part of the heat-supply nor of 

 the force-supply of the steam-engine, nor is used to replace 

 the worn material of its structure, yet serves to render the 

 movements of the machine more equable and effective, so 

 the forms of food we are considering are neither heat-pro- 

 ducing nor flesh-forming, nor do they serve to replace, to 

 any great extent, the mineral constituents of the body, yet 

 they produce a sense of refreshment accompanied with re- 

 newed vigour. It is difficult to determine in what precise 

 way these effects are produced, but no doubt can exist as to 

 the fact that they are really attributable to the forms of food 

 to which we have assigned them. 



Tea, coffee, and cocoa owe their influence on the nervous 

 system to the presence of a substance which has received the 

 various names of theme, caffeine, and theobromine. It is 

 identical in composition with piperine, the most important 

 ingredient in pepper. It may be separated in the form of 

 delicate white, -silky crystals, which have a bitter taste. In 

 its concentrated form this substance is poisonous, and to 

 this circumstance must be ascribed the ill effects which follow 



