402 Mr. Walker on the artificial Production of Cold. 



out, in my original communications, the complete efficiency of 

 them for such intention to their utmost extent, and the best 

 mode, as it appeared to me, of applying them in hot climates. 



Understanding, a few summers ago, that a manufactory had 

 been established for preparing ice-creams, as well without the 

 use of ice, as with it; and likewise for making for sale an ap- 

 paratus for the purpose, — I was induced to visit it. I examined 

 the apparatus, — a very appropriate one for the purpose, and 

 likewise the freezing powder, which I instantly recognized to 

 be the weakest in power of my various compositions for the 

 purpose, but possessing the advantage of being readily re- 

 covered repeatedly for the same purpose with undiminished 

 effect. This powder, by its taste and appearance, I found 

 to be a mixture of sal ammoniac and nitre, which I was in- 

 formed was repeatedly recoverable in a fit state for refrigera- 

 tion. I originally exerted every effort, in vain, to increase its 

 power by the addition of a third ingredient, possessing like- 

 wise the advantage merely by evaporation to dryness, of being 

 repeatedly recovered for the same use. This powder, as re- 

 lated in my original communications, consists of equal parts 

 by weight of sal ammoniac and nitre. By way of test, I reco- 

 vered it by evaporation twelve times, without any abatement 

 of its efficacy, as originally stated*. 



It is unnecessary to enter into a description of the apparatus 

 just mentioned, or the principle and mode of its application, 

 especially as the whole is embraced in the following statement. 



A circumstance occurred here (at Oxford) which occasioned 

 the method to be put to the test of useful application. A con- 

 fectioner, happening in a scarce season to be unprovided with 

 natural ice, applied to me for assistance. I assured him that 

 in the large way (as I have stated in my original communica- 

 tions) the best method was to freeze water first, and then to 

 use the ice in the usual way for freezing creams. Accordingly an 

 apparatus of large dimensions, of rather an oblong form, was 

 made of tin (fitter for the purpose if cased with wood) con- 

 sisting of channels so constructed that the water to be frozen 

 should be subjected to the freezing mixture on both sides. This, 

 properly prepared, was placed in a cool cellar during the night, 

 and early in the morning (the temperature in the open air in 

 the shade in the day-time being above 80°) the ice was col- 

 lected, which amounted to several pounds in weight. This ice, 

 which was as limpid as the finest flint-glass, was applied in the 

 usual way, and with the apparatus ordinarily used by confec- 

 tioners for the purpose of freezing creams, and the mixed 



* The manufactory alluded-to is at * No. 41, New Bridge- street, Black- 

 friars, London (late Patterson's), now Armstrong's." 



powder, 



