330 * LATENT AND SPECIFIC HEAT. 



same until the last particle of ice was melted? After the water 

 began to boil, heat was continuously supplied. Why then was 

 there not a continued increase of temperature ? 



516. Molecular Energies. Heat is a form of energy and 

 maybe kinetic or potential. There can be no doubt that when a 

 body is heated its molecules are thrown into violent motion, and 

 that as the temperature is raised the energy of this molecular motion 

 is increased, or that as this molecular motion is increased, the tem- 

 perature is raised. But some of this molecular energy that we call 

 heat, instead of being used to set the molecules of the body in motion, 

 has work of a different kind to perform. That part of the heat 

 which is spent in producing molecular vibrations, which increases 

 the temperature, is called sensible heat. Another part is employed 

 in pushing the molecules of the body asunder, producing expansion 

 and change of condition. In forcing these molecules asunder, in- 

 visible energy of motion is changed to energy of position as truly 

 and as necessarily as visible energy of motion is changed to the 

 potential variety in throwing or carrying a stone from the earth to 

 the house-top. ( 159.) 



517. Transmutation of Molecular Energy. In most 

 cases, but little of the heat communicated to a body is thus changed 

 to potential energy, the greater part remaining energy of motion 

 and increasing the temperature. But there are certain crises, or 

 " critical occasions," on which the greater part of the heat communi- 

 cated is transformed into energy of position. Thus, at the melting 

 point, a large quantity of heat maybe given to ice without affecting 

 the temperature at all ; instead of raising the temperature, it merely 

 melts the ice. The energy used has been changed from the kinetic 

 to the potential variety. In like manner, at the boiling point, a 

 large quantity of heat may be given to the water without affecting 

 the temperature at all. Instead of raising the temperature further, 

 it merely vaporizes the water, and the steam has the same tempera- 

 ture as the water from which it came. The same change of molec- 

 ular energy of motion into molecular energy of position has again 

 taken place. This heat, which is thus used to overcome cohesion 

 and change the condition of matter, does not affect the temperature 

 and therefore is not sensible, but is stored up as potential energy 

 and thus hidden or rendered latent. 



518. Definition of Latent Heat. I7ie latent 

 heat of a substance is the quantity of heat that is 



