332 PLEASANT WA YS IN SCIENCE. 



I distinguish it thus from food which serves other ends, but 

 of course it is not to be understood that any article of diet 

 serves solely the end of maintaining heat Accordingly, we 

 find that heat-maintaining substance exists in nearly all the 

 ordinary articles of food. Of these there are two sugai 

 and fat which may be looked on as special " heat-givers." 

 Starch, also, which appears in all vegetables, and thus comes 

 to form a large proportion of our daily food, is a heat-giver. 

 In fact, this substance only enters the system in the form of 

 sugar, the saliva having the power of converting starch 

 (which is insoluble in water) into sugar, and thus rendering 

 it soluble and digestible. 



Starch, as I have said, appears in all vegetables. But it 

 is found more freely in some than in others. It constitutes 

 nearly the whole substance of arrowroot, sago, and tapioca, 

 and appears more or less freely in potatoes, rice, wheat, 

 barley, and oats. In the process of vegetation it is converted 

 into sugar ; and thus it happens that vegetable diet whether 

 presenting starch in its natural form to be converted into 

 sugar by the consumer, or containing sugar which has 

 resulted from a process of change undergone by starch is 

 in general heat-maintaining. Sugar is used as a convenient 

 means of maintaining the heat-supply ; for in eating sugar 

 we are saved the trouble of converting starch into sugar. A 

 love for sweet things is the instinctive expression of the 

 necessity for heat-maintaining food. We see this liking 

 strongly developed in children, whose rapid growth is con- 

 tinually drawing upon their heat-supply. So far as adults 

 are concerned, the taste for sweet food is found to prevail 

 more in temperate than in tropical climes, as might be 

 expected; but, contrary to what we might at first expect, 

 we do not find any increase in the liking for sweet food in 

 very cold climates. Another and a more effective way of 

 securing the required heat-supply prevails in such countries. 

 As starch is converted into sugar, so by a further process 

 sugar is converted into fat It is bv the conversion of sugar 

 into fat that its heat-supplying power is made available. 



