4o8 O'ffcfvathns and Experiments on 



In this climate a mixture much lefs expenfive will be fufficient j viz. that compofcd of 

 ililutcd nitrous ?cid, Glauber's fait, fal-ammoniac anil nitre ; a mixture of this kiiul finking 

 a thermometer in the warmelt weather to near o°. At the temperature of 70 degrees, or u 

 little higlier, the quantity of diluted nitrous acid may be about one-fourth lefs than is men- 

 tioned in the table for 50 degrees. 



Thcfc methods are the mofl expeditious, and attended with the lead trouble ; but as ice. 

 may be ufcd with equal certainty, and with much lefs expence, I fiiall give a particular de- 

 tail of an experiment made with the ufe of it, firft mentioning a preparatory experiment, 

 to which I was immediately led by the recolledion that Sir Charles Blagden, in his paper 

 " on the Point of Congelation," (Phil. Tranf. vol. Ixxviii.) had found that common fal- 

 ammoniac and common fait mixed with fnow produced a cold of — 12 degrees, whereas 

 the latter ufed alone with fnow produces only — 5 degrees. I ufcd a mixed powder of 

 equal parts of common fal-ammoniac and nitre with the common fait, by which the ther- 

 mometer funk to — 18 degrees ; and when I ufed nitrous ammoniac with common fait, to 



25 degrees. This cold I could not incrcafe by the addition of any other falts, nor could 



1 equal it by any other combination of falts. Thofe I tried were, Glauber's fait, fait of tartar, 

 foda, and fal catharticus amarus. By feveral trials, I found the bed proportions to be, fnow. 

 or pounded ice twelve parts, common fait five parts, and of nitrous ammoniac, or a powder 

 of equal parts, fal-ammoniac and nitre mixed, five parts ; or one-third of common fait, 

 when I ufed that alone with fnow or pounded ice. 



My apparatus then (Dec. 28. laft) confifted of two velTels (fig. 3 and 4.) ; an inftrument 

 (fig. 6.) to grind or rather fcrape the ice to powder; a kind of fpatula (I ufe a marrow 

 fpoon) to (lir the powder occafionally ; a thermometer (fig. 8.) and a fmall thermo- 

 meter glafs, with the bulb three-fourths full of quickfilvcr (fig. 7.) I filled the veflel fig. 3, 

 holding when inverted two ^ints, Jlratiim/uperJIratum, -with pounded ice, common fait, 

 and a powder confiding of equal parts fal-ammoniac and nitre mixed together ; by firft 

 putting in fix ounces of pounded ice, then two ounces and a half of common fait, and, after 

 ilirring thefe well together, two ounces and a half of the mixed falts, mixing the whole 

 well together : this was repeated in the fame manner until the veflel was quite full. It 

 was then tied over fecurely with a wet bladder, turned upright, and one ounce and a half 

 cf rain water poured into the tube through a funnel, the tube covered with a cork, and the 

 veflel left undillurbed till the water was froaen perfe£lly folid. The inftiument foY 

 grinding it was then put in to acquire cold, whilR the veflel fig. 4, holding a pint, was 

 filled in the fame manner, with the fame proportions of materials ; a bladder tied over 

 it, fet upright, and one ounce of fuming nitrous acid poured in to be cooled. The ice was 

 then ground to powder ; and when finiflied, the nitrous acid being found to liave ac- 

 quired a fuflicient degree of cold, viz. — 13 degrees, the frigorific mixture of ice and 

 falts was let out of the veflel which contained the nitrous acid, and the powdered ice (0,111 

 furrounded by its frigorific mixture) added to the acid as quick as pofliblc ; when the ther- 

 mometer funk to near — 50 degrees, and the mixture foon froze the quickfilvcr in the glafs 

 fiulb. In this experiment 18 minutes were required to freeze the water perfetHy folid, 

 and 15 to reduce the ice by moderate labour to very fine powder. The experiment was 

 over in 55 minutes, and the temperature of the preparatory cooling mixture then found to 

 be — 10 degrees. . 



Iliad 



