134 Mr. Walker's Experiments on 
The tube and cup should be very smooth within-side, and 
perfectly central ; the tube having as little seam as possible, 
that the grinding instrument be not obstructed. 
The grinding instrument acts best when the edge, instead of 
being quite horizontal, is a little inclined from each shoulder, 
towards the centre. 
In order to keep this Paper within tolerable limits, I have 
carefully avoided a repetition of all matters mentioned in my 
former Papers on this subject ; I must therefore refer to those, 
especially that “ On the best Methods of producing artificial 
Cold,”* for the particular mode of conducting experiments on 
cold ; this being essentially the same in principle, whatever be 
the materials made use of to effect it. Hence, the apparatus just 
described is applicable to the use of the mineral acids, f as well as 
to that of muriate of lime ; recollecting that it is necessary to 
substitute glass for tin, when the former are used ; or to give 
the inside of the cup, or vessel containing it, a coating of wax, 
to defend the tin from their action. 
Having given an account of Pr. Lowitz’s experiments on 
the power of muriate of lime for producing artificial cold, and 
added such observations of my own as resulted from them, I 
to it : and, when nitrous acid is used, (instead of muriate of lime,) four times its 
weight ; and about four times the diameter of the vessels to that of the tube. 
The preparatory mixtures, that is, such as are used for cooling the materials 
previously to mixing, are best made of ice and salts ; these retaining their tem- 
perature longer than those consisting of solutions of salts in water or acid ; but, 
in either case, if necessary, they may be occasionally renewed, after the water ie 
become solid. 
# Phil. Trans, for 1795. p. 270. 
f Nitrous acid, and vitriolic acid, may at any time be immediately procured from % 
chemist’s shop ; whereas the muriate of lime, not being used for any other purpose. 
