LATENT HEAT. 101 



properties of this most important element, and we must 

 proceed to learn something of the means .by which it 

 may be developed, independently of its solar source. 



This extraordinary principle exists in a latent state in 

 all bodies, and may be pressed out of them. The black- 

 smith hammers a nail until it becomes red hot, and from 

 it he lights the match with which he kindles the fire of 

 his forge. The iron has by this process become more 

 dense, and percussion will not again produce incan- 

 descence until the bar has been exposed in fire to a red 

 heat. The only inference we can draw from this result 

 is, that by hammering the particles have been driven 

 closer together, and the heat driven out ; now further 

 hammering will not force the atoms nearer, and con- 

 sequently no additional quantity of heat can be deve- 

 loped ; the iron is made hot in a fire, it absorbs heat, the 

 particles are restored to their former state, and we can 

 now again by hammering develope both heat and light. 

 The Indian produces a spark by the attrition of two 

 pieces of wood. By friction, two pieces of ice may be 

 made to melt each other ; and could we, by mechanical 

 pressure, force water into a solid state, an immense 

 quantity of heat would be set free. By the condensation 

 of hydrogen and oxygen gases, pulverulent platinum 

 will become glowing red-hot, and, with certain pre- 

 cautions, even the compact metal, platinum, itself; 

 the heat being derived from the gases, the union of 

 which it has effected. A body passing from the solid to 

 the fluid state absorbs heat from all surrounding sub- 

 stances, and hence a degree of cold is produced. The 

 heat which is thus removed is not destroyed it is held 

 combined with the fluid; it exists in a latent state. 

 Fluids, in passing into a gaseous form, also rob all sur- 

 rounding bodies of an amount of heat necessary to 

 maintain the aeriform condition. From the air or 1'roni 

 the fluid, this heat may, as we have shown above, be 

 again extracted. Locked in a pint measure of air, there 



