-1859] WRITINGS OF JOSEPH HENRY. 195 



comes heavier, and again gives place to another stratum, to 

 pass through the same process. This continues till the col- 

 umn of water originally included between the surface and 

 the bottom is reduced to a temperature of about 39° F., at 

 which point the fluid ceases to shrink, or in other words to 

 become heavier, but on the contrary, expands with every 

 diminution of heat until it becomes entirely solidified. After 

 it has assumed a solid condition, it follows the law observed 

 by other solids and shrinks with every subsequent fall of 

 temperature. After the water of a given reservoir has arrived 

 at a temperature of 39°, since it does not increase in weight, 

 it continues to float on the surface, and is rapidly cooled down 

 to 32°, or the point of congelation. Before however it can be 

 converted into a solid at this temperature, it is necessary to 

 abstract from it a large amount of latent heat. 



To render this plain, let us suppose a lump of ice, taken 

 at zero, and with the bulb of the thermometer in it, placed 

 under such conditions that it shall receive from surrounding 

 bodies one degree of heat in one minute of time. "We shall 

 find in thirty-two minutes the thermometer will come up to 

 the freezing point; but here we shall observe that the mer- 

 cury ceases to rise, although the supply of heat remains the 

 same, and it will continue stationary during one hundred 

 and forty minutes, or until all the ice is melted, after which 

 it will again begin to rise, and continue its upward march 

 until the water begins to boil, when a second stationary point 

 will be reached. The heat which continued to flow into the 

 ice during the stationary period, was necessary to convert it 

 from a solid to a liquid state, and inasmuch as it does not 

 affect the thermometer, it has been called latent or concealed 

 heat. Water at 32° therefore contains 140° of heat more 

 than ice at the same temperature. 



In the freezing of water, a reverse process takes place, and 

 140° of heat have to be abstracted before the liquid is con- 

 verted into a solid. Freezing is therefore comparatively a slow 

 process, independently of the previous cooling down of the 

 whole mass in the reservoir to 39°, and the upper film to 

 32°. For example, if on the exposure of a stratum of water 



