LATENT HEAT. 273 



cold of winter, be placed over a steady fire with a ther- 

 mometer in the snow, the mercury will rise by the increas- 

 ing heat of the snow until it reaches the freezing point. 

 At this moment it will stop rising, and the snow will be- 

 gin to melt ; and although the heat is all the time passing 

 rapidly into the snow, the thermometer will remain per- 

 fectly stationary until it is all converted to water. The 

 heat that goes to melt the snow does not make it any hot- 

 ter ; in other words, it becomes latent (the Latin word for 

 hidden), so as neither to affect the sensation of the hand 

 nor to raise the thermometer. N"ow it has been found that 

 the time required to melt the snow is sufficient to heat the 

 same quantity of water, placed over the same fire, up to 

 172 degrees, or 140 degrees above freezing ; that is, 140 

 degrees have become latent, or hidden, in melting the 

 snow. 



This same amount of heat may be given out again by 

 placing the vessel of water out of doors to freeze. A 

 thermometer will show that the water is growing colder 

 by the escape of tlie heat, until freezing commences. Af- 

 ter this it still continues to pass off, but the water becomes 

 no colder until all is frozen, as it was only the latent heat 

 of the water that was escaping. 



A simple and familiar experiment exhibits the same 

 principle. Place a frozen apple, which thaws a little be- 

 low freezing, in a vessel of ice-cold water. The latent 

 heat of the water immediately passes into the apple and 

 thaws it, and in an hour or two it will be found like a 

 fresh apple and entirely free from frost ; but the latent 

 heat having escaped from the water next the apple, a thick 

 crust of ice is found to encase it. 



The amount of latent heat may be shown in still an- 

 other way. Mix a pound of snow at 32 degrees, or at 

 freezing, with a pound of water at 172 degrees. All will 

 be melted, but the two pounds of water thus formed will 

 12* 



