168 GENERAL SCIENCE 



truth, since removing the cover of a vessel in which water is being 

 heated allows much heat to escape. Long before water begins to 

 boil steam is leaving the surface of the water, and each 5 drops of 

 water thus taken away as steam absorb enough heat to heat about 

 a pound of water 1. If a tight cover is kept in place the escaping 

 steam will mostly condense on the cover, and much of the heat 

 which it contains will be kept within. 



When water is kept boiling for cooking foods the object is to 

 keep the food hot enough for cooking; that is, at the temperature of 

 boiling water. But the water also prevents the food from getting 

 too hot, because no matter how fast water boils it gets no hotter, 

 and the cooking will proceed no faster. Whatever heat is used up 

 in boiling away more water than necessary is entirely wasted, not to 

 mention the disastrous effect of allowing all the water to boil entirely 

 away. Therefore, after boiling has begun, a gas or oil flame can be 

 turned down considerably without delaying the cooking, provided 

 the water keeps boiling. 



To Save Heat, Keep the Cover On. But there are other proc- 

 esses where the boiling away of the water is the main object, as in 

 the boiling down of sirups, the making of candies or jellies, etc. Here, 

 instead of saving heat by boiling slowly, heat may be saved by boil- 

 ing briskly, since by so doing it will take less time, therefore less gas 

 or coal, to boil away the required amount of water. Here no cover 

 should be used, since a cover would prevent the escape of some of 

 the steam. 



In baking operations there is not much chance for the saving of 

 heat except -in the selection of an oven. Baking in the ordinary 

 oven is a very wasteful process indeed, for more than 90 per 

 cent of the heat supplied is usually lost through the sides of the 

 oven. Such loss of heat could be made much less if the walls 

 of all ovens were made with a thick layer of heat-insulating material. 

 Electric ovens are usually well insulated, since without some means 

 of saving heat the cost of baking by electricity would be altogether 

 prohibitive because of the relatively high cost of heat supplied in 

 "this way. 



Regulation of Stoves, Ranges, and Other Heating Appliances. 

 The air which enters cold below the fire in any stove or furnace 



