416 On a new Mode of Arli/irial Corgdalion. 



broad circular lid of plate-glass, which is suspended honzorttally 

 from a rod passing through a collar of leather. This cup is nearly 

 lilied with fresh distilled watfer, and supported by a slender me- 

 tallic rir.g, with glass feet, aboilt ad inch above the suiTace of X 

 body of sulphuric acid, perhaps three quarters of ah inch in thick- 

 ness, and occupying the bottom of a deep glass bason that has 

 •A diameter of nearly seven inches. In this state, the receiver 

 being adapted, and the lid pressed down to cover the mouth of 

 the cup, the transferrer is screwed to the air-pump, and the rare- 

 faction, under those circumstances, pushed so far as to leave only 

 about the hinulrcd and fiftieth part of a rtsidiunn; and the cock 

 being turned to secure that exhaustion, the compound apparatus 

 is then detached from the pump, and removed to some conve- 

 nient apartment. As long as the cup is covered, the water will 

 remain quite unaltered ; but, on drawing up the rod half an inch 

 or more, to admit the play of the rare medium, a bundle of spi- 

 cular ice will, after the lapse perhaps of five minutes, dart sud- 

 denly through the whole of the liquid mass ; and the consolida- 

 tion will afterwards descend regularly, thickenitig the horizontal 

 stratum by insensible gradations, and forming in its progress a 

 beautiful transparent cake. On letting down the cover again, 

 the process of evaporation being now checked or almost entirely 

 stopped, the ice returns slowly into its former liquid conditioil. 

 In this vvay, the same portion of water may, even at distant in- 

 tervals of time, be repeatedly congealed and thawed successively 

 twenty or thirty times. During the first operations of freezing, 

 some air is liberated ; but this extrication diminishes at each 

 subsequent act, and the ice, free from the smallest specks, re- 

 sembles a piece of the purest crystal. 



Progress of Congelation. 



This artificial freezing of water in a cup of gla&s or m^tal, af- 

 fords the best opportunity of examining the progress of crystalli- 

 zation. The appearance presented, however^ is extremely va- 

 rious. When the frigorific action is most intense, the congela- 

 tion sweeps at once over the whole surface cf the water, obscur- 

 ing it like a cloud. But, in general, the process advances more 

 slowly; bundles of spiculae, from differeiit points^ sometimes from 

 the centre, though commonly from the sides of the cup, stretch- 

 ing out and spreading by degrees with a sort of feathered texture. 

 By this combined operation, the surface of the water soon be- 

 comes an uniform sheet of ice. Yet the effect is at times singu- 

 larly varied; the spicular shoots, advancing in different direction", 

 come to inclose, near the middle of the cup, a rectilineal »paci% 

 which, by unequal though continued encroachment, is reduced 

 to a triangle J and the mass below, being partly frozen, and 



therefore 



