f>1 VAPORIZATION. 



such an inclination as to cause the gas to circulate round the 

 circumference of the cavity of the box, instead of traversing it 

 in a straight line. The box is speedily filled with a light pow- 

 der, having the appearance of snow, which is solid carbonic acid, 

 one portion of the liquid carbonic acid being frozen by the eva- 

 poration of the other, The solid carbonic acid is an imperfect 

 conductor of heat, and is on that account not immediately dis- 

 sipated by evaporation. It is most conveniently applied as a 

 frigorific agent when mixed with ether, with which it forms a 

 soft mass, like half-melted snow. Mercury may be frozen in 

 large quantity, by throwing a portion of this compound upon 

 the surface of the fluid metal. The ether evaporates as well as 

 the carbonic acid, and contributes to produce the cold. To form 

 the liquid carbonic acid itself in large quantity, M. Thilorier 

 makes use of two strong cylindrical vessels of wrought iron, 

 like mercury bottles in size and form, one of which is called 

 the generator, and the other the receiver. The generator is 

 lined with lead, and is intended for the reception of strong 

 muriatic acid and marble, the materials for the production of the 

 gas. It is connected with the receiver by a short iron pipe, 

 which is provided with a stop-cock, so that the receiver can be 

 separated from the generator, without loss of gas from the 

 former. The generator is charged with materials several times 

 in succession, and the product accumulated in the receiver till 

 it may amount to a pound or two. The stop-cock and screws 

 in this apparatus must be of the most accurate workmanship, 

 and be screwed down upon leaden washers. 



The question arises, do those bodies which evaporate at a 

 moderate temperature continue to evaporate at all temperatures, 

 however low ? The opinion has prevailed, that bodies which 

 are decidedly vaporous at high temperatures, such as sulphuric 

 acid and mercury, never cease to evolve vapour, however far 

 thei/ temperature may be depressed, although the quantity 

 emitted becomes less and less, till it ceases to be appreciable 

 by our senses. Even fixed bodies, such as metals, rocks, 

 &c , have been supposed to allow an escape of their sub- 

 stance into air at the ordinary temperature ; and hence the 

 atmosphere has been supposed to contain traces of the va- 

 pours of all the bodies with which it is in contact. Certain 

 researches of Dr. Faraday, published in the Philosophical 

 Transactions for J 826, on the existence of a limit to vaporiza- 



