402 CHEMISTRY. 



570. Relation of Food to Labor. This topic, incidentally touched 

 upon in 569, merits a more particular notice. If a horse is not worked 

 he will retain his good condition on such food as hay and potatoes. If oats 

 or corn be added he will gain in flesh ; that is, the tissues will be more 

 fully developed by this addition of plastic food, and at the same time the 

 fat will be increased, as his heat-making food is not used up freely in pro- 

 ducing heat. If now with this mixed diet he is put to work, he will retain 

 from day to day his usual bulk both in respect to fat and muscular fibre. 

 Laboring men require a larger proportion of nitrogenous food than those 

 who are inactive. It is for this reason that when men live almost wholly 

 on such articles as potatoes or rice or plantains there is the same failure 

 both in bulk and power as in the working horse that is fed on hay alone. 

 The Israelites could not have endured their journey if their manna had 

 been like the article now called by that name. They needed food which 

 was in part nitrogenous, and such was the manna miraculously furnished 

 to them, as the change in it when it was kept for any length of time clearly 

 showed ( 4G6). It is calculated by Liebig that the proportion of nitrog- 

 enous to non-nitrogenous food most suitable to the wants of a laboring 

 man is about as one to four. If he eat too much of the former, like the sav- 

 age hunter who lives almost wholly on meat, there is deficiency of heat- 

 making food, and he is obliged to eat a larger amount of nitrogenous food 

 than is needed for nutrition, in order to get a sufficiency of that non-nitrog- 

 enous food which is combined with it, unless he pursue, like carnivorous 

 animals ( 569), so active a life that the waste of the tissues shall supply 

 the requisite amount of fuel. The use of so large a quantity of animal food 

 by no means proportionally develops the tissues, for it burdens the digest- 

 ive and other organs with too much labor, and therefore produces disease 

 in spite of the invigorating influences of an outdoor life. 



571. Mingling of Heat -Pood and Building -Food. Gen- 

 erally in articles which are eaten the two kinds of food are 

 mingled together. Thus even in the lean part of meat 

 there is always some fat in addition to that which is de- 

 posited in masses in the neighborhood of the muscles ; and 

 gluten and starch are mingled in the grains. That mixture 

 of nitrogenous and non-nitrogenous food which we have in 

 bread is so especially suited to man that this article of diet 

 has from remote antiquity been styled " the staff of life." 

 The instincts of men seem to lead them to mingle the two 



