ON ARTIFICIAL REFRIGERATION. 909 



my giving more tlian a bare statement of tlie imi^ortaut results lie lias 

 arrived at by accurate experiment and admirable reasoning. Under 

 the name "cryohydrates" he includes the bodies resulting from the union 

 of water with another body, and which solidify below the freezing-point 

 of water. These compounds have a constant composition and deiinito 

 freezing and melting points, and can only exist in the solid form below 

 0° C. As Mr. Guthrie says, i)erhaps one of the most interesting aspects 

 of the experimental results is the establishment of fixed temperatures 

 below zero. With the exception of the melting-points of a few organic 

 bodies, such as benzol, and the boiling-points of a few liquids, such 

 as liquid ammonia, sulphurous acid, and carbonic acid, and the defined 

 temperatures to be got from freezing-mixtures, there are no means in 

 the hands of the physicist for obtaining and maintaining with certainty 

 and ease a fixed temperature below 0° C. Kow, if we surround a body 

 with one of the solid cryohydrates, the body is kept at a corresponding 

 temperature as long as any of the cryohydi'ate remains solid, and this 

 with as much certainty as the temperature, 0° C. can be maintained by 

 melting ice. We thus command temperatures between —23° and 0° C. 

 with the greatest precision. Mr. Guthrie has applied the term " cryo- 

 gen" to an api^liance for obtaining a temperature below 0° 0. 



Looking upon ice as the cryohydrate of water, this is seen to shrink as 

 it loses heat till it reaches 4P C. At this point ice is formed, which, how- 

 ever, is dissolved in the water. A solution is obtained having a temper- 

 ature of soUdification below 4° C, namely, at 0° C. or 32° Fahr. At this 

 the ice and the water solidify together, j)roducing the compound body or 

 cryohydrate called ice, which is thus a cryohydrate of water. The ex- 

 pansion from 4° to 0° (from 39°.2 to 32° Fahr.) is due to the greater and 

 greater amount of ice which the water holds in solution, and whose ex- 

 I)ansion is greater than the contraction of the water due to the dimin- 

 ished temperature. 



Common salt and ice sohdify immediately below the temperature —21^ 

 to —22° C, which is the lowest temj)erature to be got by an ice salt- 

 freezing mixture. This minimum temj^erature seems to be attained 

 between the somewhat wide margins of 3 of salt to 1 of ice, and 1 of 

 salt to 2 of ice. This shows that " freezing-mixtures may be bodies of 

 precise temi^eratures under widely- varying cu-cumstances." 



" It is clear," says Dr. Guthrie, that the liquid portion of a freezing- 

 mixture is a brine of such a composition as to resist solidification at the 

 temperature of the freezing-mixture. 



" The enormous latent heat of water, the ftict that the specific heat of 

 ice is only about half that of water, while the specific heats of all salts 

 are far less than that of ice, and, therefore, a fortiori, less than that of 

 water, together with the good thermal conductivity of water, all argue 

 that, if constantly stirred, all parts of a freezmg-mixture will have the 

 same temperatiu?e. The fact that the liquid portion of a freezing-mix- 

 Ture of ice and a solid salt is the cryohydrate of that salt, insures the 



