Mr. Walker on the artificial Production of Cold. 405 



ble any accidental accession of heat. A freezing mixture com- 

 posed of sal ammoniac and nitre with water, all at the tempe- 

 rature of 50°, to which temperature, or nearly so, they may 

 all be reduced by water from a pump by drawing off a suffi- 

 cient quantity first, will from 50° produce a cold of 22° below 

 the freezing point, and with the addition of Glauber's salt to 

 28°. The confectioners find a degree of cold at 12° or 15° be- 

 low the freezing point sufficient for their purpose ; but it must 

 be recollected that the cold produced by salts dissolved in water 

 is not so durable as with ice and salt ; the duration of the refri- 

 gerating power in the above mixtures will of course be in pro- 

 portion to the quantity and thickness of the apparatus. In the 

 way the confectioner managed, the mixture in the apparatus 

 retained its freezing property till the morning : my usual way 

 is, in extreme hot weather, to place the vessel containing the 

 powdered salts in the coldest water drawn from the pump pre- 

 viously ; but in the ordinary way it will suffice to add the cold 

 water without the above precaution : it may be advisable to be 

 provided with a second quantity of the ingredients to preserve 

 the cold by a renewal of the mixture. The drawings are taken 

 from an apparatus of each kind of my own, — they are made 

 of tin, for want here of a fitter material, and are painted out- 

 side of a grass-green colour. The confectioner abovemen- 

 tioned laid in a stock of a hundred weight of each of the arti- 

 cles ; viz. sal ammoniac and nitre ; the former at the rate of 

 one shilling per pound, and the nitre at fourpence — which of 

 course when mixed, was at the moderate price of only eight- 

 pence per pound. Glauber's salts may be procured in the large 

 way at the rate of about twopence per pound, and by the single 

 pound at fourpence. The apparatus abovementioned may be 

 only half or three parts filled for use ; care must be taken in 

 every instance that the surface of the subject to be acted upon 

 be rather below the surface of the freezing mixture. 



For cooling wine, the coldest water drawn from a pump 

 will be quite sufficient ; however, if required, a small portion 

 of the cooling powder may be added to the water. 



The addition of Glauber's salt, it may be observed, increases 

 the density of the mixture, which then becomes a better con- 

 ductor of the cold, if I may so express myself, and more- 

 over retains the same temperature longer: of course it will be 

 better of the two to overcharge than undercharge the propor- 

 tion of the salts to the water. It will be apparent, for obvious 

 reasons, that the part containing the subject to be cooled should 

 be as thin as may be, and the whole of the external part in 

 every apparatus thick. 



This detail may probably appear prolix to any person in- 

 duced 



