200 WRITINGS OF JOSEPH HENRY. [1855- 



domestic purposes it is therefore important that it should 

 be gathered in masses as thick and large as possible. The 

 lower side of the ice, as a general rule, contains more impuri- 

 ties than the upper, since the process of crystallization tends 

 to expel all the foreign ingredients downwards ; and hence 

 a storehouse filled with thin ice will contain more impuri- 

 ties, and, on account of the multitude of bubbles and amount 

 of surface exposed, will melt much sooner than if well packed 

 with thicker blocks. The temperature of ice moreover may 

 be reduced considerably by exposure for some time to the 

 weather, when below the freezing point, and thus the value 

 of its cooling effect be enhanced. This diminution of tem- 

 perature however is continued only by the slow conducting 

 power of the ice, and though it may retard considerably the 

 melting of the mass, we think the effect is scarcely percep- 

 tible in ice transmitted to warmer climates. We have never 

 found a thermometer, inserted in a hole in the centre of 

 blocks of Boston ice, in the city of Washington, to sink below 

 32°. In filling the ice-house however and in compacting 

 the mass, advantage should be taken of the coldest weather. 



In the preservation of ice the smaller the amount of sur- 

 face exposed between the several parts, and the greater the 

 amount accumulated in a given place, the longer it will resist 

 melting ; for the tendency to become liquid will be in propor- 

 tion to the surface exposed, since the heat which produces 

 this effect must pass through the surface; for example, in a 

 cubic block of ice, measuring one foot on each edge, there are 

 six surfaces exposed, each one foot square. Now if we cut 

 this same block into two parts, by a plane parallel to one of 

 the sides, we shall present two additional faces each a square 

 foot in extent, and the aggregate amount of surface exposed 

 will be increased in the ratio of six to eight. For a similar 

 reason, if we have two ice-houses of like form, the one ten and 

 the other twenty feet in diameter, the capacity will be in 

 the ratio of one to eight, while their surfaces will be as one to 

 four ; hence the tendency to resist melting will be in direct 

 proportion to the diameters of reservoirs of similar forms. 



Of all geometrical solids, a sphere is that which contains 



