86 HEAT. 



205. In cooking, this method could 



Can food be 



cooked by the not be employed. The water might, in- 



same method ? ^^ be made tQ boil ftt lgOOj bm the boi l ing 



water, owing to its less heat, would not have the effect 

 of water boiling at 212. Many vegetable juices and 

 infusions which are used for medicines, and would be 

 injured by a high temperature, are boiled down, like 

 sugar syrup, under diminished pressure. 



206. SlNQING OF THE TEA-KETTLE. 



ringing of the The singing sound which precedes boiling , 

 tea-kettle. ^ owing to the collapse of the first bubbles 

 of steam, as they rise into the colder water above. 

 The very first bubbles that form are not steam, but air 

 which the heat expels. Steam bubbles are then formed, 

 which rise a little way, and, being reconverted into water, 

 contract, and finally collapse. If the heat is continued 

 and the water made hotter, the next are able to rise 

 further. Finally, when the water becomes as hot as 

 the bubbles, they make their way through, and boiling 

 is thus commenced. 



What is a 207. STEAM BOILERS. The boiler is the 



steam boiler? vessel in which steam is formed. From 

 the boiler it passes to other parts of the apparatus to 

 move the machinery. Steam boilers 

 are of various forms, but are always 

 made of great strength, to resist the 

 internal pressure to which they are 

 subjected. 

 Explain the 208. The figure repre- 



sents an or( ji nar y ste am boiler, with 



