44 WATER. 



power of the steam-engine* is indebted ; and volcanic 

 eruptions and earthquakes are supposed to be often 

 caused by water meeting with subterraneous fires, and 

 being suddenly converted into steam. It is through the 

 expansive power of steam that that well-known motion 

 called boiling is caused. The theory of heating and boil- 

 ing is as follows : When a vessel of cold water, for in- 

 stance, is placed over the fire, that layer of water nearest 

 the fire will become heated first, and will consequently, 

 through its expansion, acquire a specific gravity different 

 from that of the mass : the heated portion will ascend, 

 and its place will be occupied by another portion, and 

 thus the whole mass will be brought successively into 

 contact with the heated bottom of the vessel, and con- 

 tinue to have its temperature increased until the caloric 

 or heat generates vapour so rapidly and violently that 

 it rushes up and causes the boiling. So that, in this 

 operation, there is a constant circulation of the water 

 about the vessel; that which was nearest the bottom, be- 

 ing converted into vapour, ascends, giving way to that 

 which was immediately above it. The degree of heat to 

 produce boiling depends on the weight or pressure of 

 the atmosphere. When the barometer is at 29 inches, 

 water boils at about 1 ZIO. When the barometer is at 

 31 inches, the boiling point is rather more than 215. 

 It is not through the conductive power of water that its 

 temperature so soon becomes raised ; water being so bad 

 a conductor of heat, that a portion of water, in a glass 

 tube containing ice, may be made to boil without even 

 melting the ice. 



Water is the most penetrative of all bodies, and the 

 most difficult to confine ; passing through leather, and 

 even bladders, which will confine air. It used to be sup- 

 posed that water is incompressible : from recent experi- 

 ments, it is found that water may be compressed, but 

 not to any considerable extent. 



* Some steam-engines are sufficiently powerful to raise a weight 

 equal to forty millions of pounds one foot,and that with a single bushel 

 of coals. 





