564 PHILOSOPHICAL TRANSACTIONS. [aNNO 1795. 



I give the inside such a coating, by pouring melted white wax into the vessel, 

 previously clean and dry, and turning it about by hand, so as to leave no point 

 of the metal uncovered for the acid to act on, pouring the surplus away. 



In the experiment above described, I used a single vessel tor cooling the 

 nitrous acid ; a cupping-glass (represented by the dotted line at b, fig. U) being 

 cemented into the tin, and so forming that part in which the nitrous acid was 

 first cooled, and the mixture afterwards made in which the quicksilver was frozen: 

 but from the trouble and impediments arising from letting out the mixture, and 

 clearing the bottom from the lumps of ice, &c. adhering to it, I was led to the 

 addition of the other part (fig. 1 2) by which all these difficulties are got rid of, 

 and it is besides a much more comfortable and neat way of conducting it; the 

 upper part which contains the nitrous acid being lifted off and placed on the 

 table, immediately before the powdered ice is added. The whole of this ap- 

 paratus may be of tin, that part only (when the cooling mixtures are made with- 

 out using any corrosive acid) in which the acid mixture is to be made, being pre- 

 viously coated in the manner above-mentioned ; or a thin glass tumbler of a pro- 

 per size may be cemented in. I have occasionally used a thin glass tumbler for 

 the mixture in which the quicksilver is to be frozen, immersing it with the acid 

 in a frigorific mixture till the acid is sufficiently cooled, then adding the ground 

 ice to it, previously removing the tumbler out of the frigorific mixture, as in the 

 experiment above-mentioned; this simplifies the apparatus, but is less convenient 

 on many accounts. 



The scale of this apparatus may be diminished or increased at the will of the 

 operator ; for there is no doubt that a small quantity of quicksilver may be 

 frozen at any time with \ of this quantity, with an apparatus of this kind, by any 

 one conversant in such experiments. 



I have frequently frozen quicksilver, by mixing together, at 0°, 3 drs. of 

 ground ice with 2 drs. of nitrous acid. Whenever the intention is, as in these 

 experiments, to cool the materials to nearly the same temperature with the frigo- 

 rific mixture in which they are immersed, the proportion of the frigorific mix- 

 ture to the intended mixture (or materials to be cooled) should not be less than 

 12 to 1 ; a greater disproportion is still better. By attending to the directions 

 particularly mentioned in the experiment made on Dec. 28th, a thermometer 

 may be always dispensed with ; the proportions of the materials to be cooled be- 

 ing exactly adjusted ; and when they are to be mixed precisely determined, by 

 the time employed in grinding the ice to powder. The proportions of snow, or 

 pounded ice, and salt, or salts, may be guessed sufficiently near without weigh- 

 ing, miless in very nice experiments. Imagining that a recapitulation of the 

 different mixtures, described in my former paper, for producing artificial cold, 

 brought into one view might not be unuseful, I have subjoined a table of the salts. 



