Vol. LXXVII.] PHILOSOPHICAL TRANSACTIONS. 281 



diminishing, so that one might be placed within the other, were procured. 

 The largest of these pans was placed in another vessel still larger, in which the 

 materials for the 2d frigorific mixture were thinly spread, in order to be cooled. 

 The 2d pan, containing the liquor (viz. vitriolic acid properly diluted) was placed 

 in the largest pan. The 3d pan, containing the salts for the 3d mixture, was 

 immersed in the liquor of the 2d pan; and the liquor for the 3d mixture was 

 put into wide-mouthed phials, which were immersed in the 2d pan likewise, and 

 floated round the 3d pan. The 4th pan, which was the smallest of all, con- 

 taining its cooling materials, was placed in the midst of the salts of the 3d pan. 

 Of the materials for the mixtures to be made in these 4 pans, the 1st and 2d 

 consisted of diluted vitriolic acid and Glauber's salt, the 3d and 4th of diluted 

 nitrous acid, Glauber's salt and sal ammoniac, in the proportions assigned. ' 



The pans being adjusted in the manner above described, the materials of the 

 first and largest pan were mixed: this mixture reduced the thermometer to 

 + 10, and cooled the liquor in the 2d pan to 20; and the salts for the 2d mix- 

 ture, which were placed underneath in the large vessel, nearly as much. The 2d 

 mixture was then made with the materials thus cooled, and it reduced the ther- 

 mometer to 3*^. The ingredients of the 3d mixture, by imrhersion in this, 

 were cooled to-}- 10°, and when mixed reduced the thermometer to— 15". 

 The materials for the 4th mixture were cooled by immersion in this 3d mixture 

 to about — 1 2°. On mixing they made the mercury in the thermometer sink 

 rapidly, and as it appeared to Mr. Walker, below — 40°. Its thread seemed to 

 be divided below that point; but the froth occasioned by the ebullition of the 

 materials prevented his making so accurate an observation as he could have 

 wished. 



The reason why this last mixture reduced the thermometer more than the 3d, 

 though both were of the same materials, and the last at a lower temperature, 

 Mr. Walker imagines to have been partly because the 4th pan had not another 

 immersed in it to give it heat, and partly because the materials were reduced to a 

 finer powder. I should imagine, that mercury reduced to its freezing point will 

 freeze more quickly than water reduced to its freezing point, because it appears, 

 from experiments on their capacity for heat, that the latter of these bodies has 

 so much more latent heat in its liquid state; which greater quantity of latent 

 heat must, as it becomes sensible, more retard the congelation. 



I forbear to enumerate many variations of these experiments which Mr. 

 Walker has among his notes; but there is one mixture which, though its power 

 is not equal to that last described, may prove very serviceable in experiments of 

 this nature, on account of its cheapness. It consists of oil of vitriol diluted 

 with an equal weight of water: added to Glauber's salt, it produces about 46 

 degrees of cold. The addition of sal ammoniac renders it more intense by a few 



VOL. XVI. Oo 



