THE EFFECTS OF HEAT. 241 



If the temperature of the iron is not elevated sufEciently, the moisture wets 

 the surface, and is evaporated ; but at a higher degree of temperature, the 

 moisture is repelled. 



The phenomenon of the spheroidal condition of water furnishes an explana- 

 tion of the feats often pertbrnied by jugglers, of plunging the hands with im- 

 punity into molten lead, or iron. The baud is moistened, and when passed 

 into the liquid metal the moisture is vaporized, and interposes between the 

 metal and the skin a sheath of vapor. In its con\ersion into vapor, the 

 moisture absorbs heat, and thus still further protects the skin. 



What is ebui- 5^^- When a liquid is heated sufficiently to 

 htion? ^Qj.^^ steam, the production of vapor takes 



place principally at that part where the heat enters ; and 

 when the heating takes place not from above, but from 

 the bottom and sides, the steam as it is produced rises in 

 bubbles through the liquid, and produces the phenomenon 

 of boiling, or ebullition. 



wTiat is the ^56. The temperature at which vapor rises 

 bouiug point? -^{lYi sufficient freedom to cause the phenome- 

 non of ebullition, is called the boiling point, 

 la the boiling •'^57. Different liquids boil at different tem- 

 ent'liquiS; pGraturcs. The boiling^ point of a liquid is, 

 **™^''' therefore, one of its distinctive characters. 



Thus water, under ordinary circumstances, begins to boil when it is heated 

 up to 212° F. ; alcohol at 173°; ether at 96°; syrup at 221°; linseed oil 

 at 0-40°. 



„ ^ . . The gentle tremor, or undulation, on the surface of water 



WTiat IS Sim- . 



mering? which precedes boihng, and which is termed " simmering," is 



owing to the collapse of the bubbles of steam as they shoot 

 upward and are condensed by the colder water. The first bubbles which 

 form are not steam, but air which the heat expels from the water. As the 

 temperature of the whole mass of the water increases, the bubbles are no 

 longer condensed and collapsed, but rise through to the surface ; and the 

 moment that this takes place boiling commences. The singing of a tea-kettle 

 before boiling is occasioned by the irregular escape of the air and steam ex- 

 pelled from the water through the spout of the tea-kettle, which acts in tho 

 manner of a wind-instrument in producing a sound. 



^ , ,^. 558. Liquids, in general, being boiled in open vessels, are 



How does the i < o i o r 



pressure of the sub]ected to the pressure of the atmosphere. The tendency 

 f'"tt!i'^^b'^T'^*^' °^ ^^^^ pressure is to prevent and retard the particles of 

 of liquids? water from expanding to a sufficient extent to form steam. 



Hence if the pressure of the atmosphere varies, as it does at 

 different times and places, or if it bo increased or diminished by artificial 

 means, the boiling point of a Uquid will undergo a corresponding change. 



11 



