414 On a new Mode of Arlijiclal Coiigdail'm. 



Mode of proceeding. 



We are tlnis enabled, in the hottest weather, to freeze n niusi 

 cf water, and to kef^p it frozen, till it gradually wastes away, hv a 

 continued but invisible prftcess of evaporation. The only thing 

 required is, that the surface of the acid should approach tolerably 

 near to that of the water, and should have a greater extent ; for 

 otherwise the moisture would exhale more copiously than it 

 could be transferred and absorbed, and, consequently, the dry- 

 ness of the rarefied medium would become reduced, and its eva- 

 porating energy essentially impaired. The acid should be poured 

 to the depth perhaps of half an inch in a broad flat dish, which 

 is covered by a receiver of a form nearly hemispherical; the 

 water exposed to congelation may be contained in a shallow cup, 

 about half the width of the dish, and having its rim supported by 

 a narrow porcelain ring upheld above the surface of the acid by 

 three slender feet. It is of consequence that the water should 

 be insulated as much as possible, or should present only a humid 

 surface to the contact of the surrounding medium ; for the dry 

 sides of the cup might receive, from communication with the 

 external air, such accessions of heat, as greatly to diminish, if 

 not to counteract the refrigerating effects of evaporation. This 

 inconvenience, however, is in a great measure obviated, by in^ 

 vesting the c\x\> with an outer case at the interval of about half 

 an inch. If both the cup and its case consist of glass, the pro- 

 cess of congelation is viewed most completely; yet when they are 

 formed of a bright metal, the effect appears on the whole more 

 striking. But the preferable mode, and that which prevents any 

 waste of the powers of refrigeration, is to expose the water in a, 

 pan of porous earthen-ware. If common water be used, it will 

 evolve air bubbles very copiously as the exhaustion proceeds; in 

 ^ few minutes, and long before the limit of rarefaction has been 

 attained, the icy spiculai will shoot beautifully through the liquid 

 mass, and entwine it with a reticulated contexture. As the pro- 

 cess of congelation goes forward, a new discharge of air from 

 the substance of the water takes place, and marks the regular 

 advances of consolidation. But after the water has all become 

 solid ice, which, unless it exceed the depth of an inch, may ge- 

 nerally be effected in less than half an hour, the circle of evapo- 

 ration and subsequent absorption is still maintained. A minute 

 f.hn of ice, abstracting from the internal mass a redoubled share 

 of heat, passes, by invisible transitions, successively into the state 

 of water and of steam, which, dissolving in the thin ambient air, 

 is conveyed to the acid, where it again assumes the liquid form, 

 and, in the act of combination, likewise surrenders its heat. 



Moderate 



