On a new Mode of Artificial Congelation. 415 



Moderate Rarefaction sufficient to maintain the Ice. 



It) performing this experiment, the object is generally to seek 

 at first to push the rarefaction as far as the circumstances will 

 admit. But the disposition of the water to fill the receiver with 

 vapour, heing only in part subdued by the action of the suljdiuric 

 acid, a limit is soon opposed to the progress of exhaustion, and the 

 included air can seldom be rarefied above a hundred times, or till 

 its elasticity can support no more than a column of mercury about 

 three-tenths of an inch in height. A smaller rarefaction, per- 

 haps from ten times to twenty times, will be found sufficient to 

 support congelation after it has once taken place. The ice then 

 becomes rounded by degrees at the edges, and wastes away in- 

 sensibly, its surface being incessantly corroded by the play of the 

 ambient air, and the minute exhalations conveyed by an invisi- 

 ble process to the sulphuric acid, which, from its absorbing the 

 vapour, is all the time maintained above the temperature of the 

 apartment. The ice, kept in this way, suffers a very slow con- 

 sumption ; for a lump of it, about a pound in weight and two 

 inches thick, is sometimes not entirely gone in the space of 

 eight or ten days. During the whole progress of its wasting, 

 the ice still commonly retains an uniform transparent consistence; 

 but, in a more advanced stage, it occasionally betrays a sort of 

 honey-combed appearance, owing to the minute cavities formed 

 by globules of air, set loose in the act of freezing, yet entangled 

 in the mass, and which are afterwards enlarged by the erosion 

 of the solvent medium. 



But almost every practical object is attained, through far in- 

 ferior powers of refrigeration. Water is the most easily frozen, 

 by leaving it, perhaps for the space of an hour, to the sIoav ac- 

 tion of air that has been rarefied only in a very moderate degree. 

 This process meets with less impediment, and the ice formed 

 by it appears likewise more compact, when the water has been 

 already purged of the greater part of its combined air, either hy 

 distillation or by long continued boiling. The water which has 

 undergone such operation, should be introduced as quickly as 

 possible into a decanter, and filled up close to the stopper, else 

 it will attract air most greedily, and return nearly to its former 

 state in the course of a few hours. 



Elegant Mode of Freezing. 



The most elegant and instructive mode of effecting artificial 

 congelation, is to perform the process under the transferrer of 

 an air-pump. A tiiick but clear glass cup being selected, of 

 about two or three inches in diameter, has its lips ground flat, 

 ;*nd covered occasionally,, though not absolutely sliut, with a 



broad 



