the icjl Methods of producing Artificial Cold. ^q^- 



i.s'wei"i/7'°f"'"t ^'°^^!'P^"«'= g-^^-'y'^ '.^4S, required to be diluted with half 

 ns we.ghtof water, and produced with fnow at the temperature of + 30 decrees about 

 e.ght degrees lefs than with nitrous acid, finking the the„non,eter toJ^,l'Zs ^T. 

 parts of the diluted vitriohc acid required at that temperature fix parts of fnow 



It perhaps will be remarked, that I have taken no notice before of the vitriolic acid T^ir 

 reafon.s, becaufe, the freezing point of quickfilver being 39, it may be frozen toler.bl. 

 hard by a mixture of nitrous acid with fnow or ground ice, though, the utmoft decree of 

 cold this acid can produce with fnow is _ 46", which degree of cold may be produ^ced by 

 mixing the fnow or ground ice and nitrous acid at o". 



If it be required to make it perfedly folid and hard, a mixture of equal parts of the di- 

 luted ntnolic acid and nitrous acid Ihould be ufed with the powdered ice ; but then the 

 materials fhould not be lefs than — 10" before mixing. 



If a ftiU greater could be required than a mixture of this kind can give, which is about 

 - 56 degrees, the diluted vitriolic acid alone Ihould be ufed with fnow or powdered ice 

 and the temperature at which the materials are to be mixed not lefs than - 20 degrees ' 

 Seleft, according to the intention, either of the three following mixtures • 

 Firft, fnow or pounded ice two parts, and common fait one part, which produces a cold 

 of — 5 degrees. 



Second, fnow or pounded ice twelve parts, common fait five parts, and a powder con- 

 fiftmg of equal parts of common fal-ammoniac and nitre mixed, five parts, which produce* 

 a cold of — ip -f-""-" *^ 



_ Third, fnow or pounded ice twelve parts, common fait five parts, and nitrous ammoniac 

 m powder five parts, which produces a cold of — 25 degrees. 



The proportions which I have found to be the beft for mixing the fnow or powdered ice 

 with the different acids at different temperatures are thefe : viz. at + 30 degrees feven of 

 of the former to four of the nitrous acid ; at + 5 degrees, (with a trifling allowance, if any 

 for a few degrees above or below) three to two; at _ 12 degrees, four to three, 4h the 

 mixed acids, and at — 20 degrees, with the diluted vitriolic acid, equal parts 



If It be required to prepare the materials in a frigorific mixture without the ufe of ice a 

 mixture of the proper llrength may be chofen from the table. ' 



It is immaterial, when the exad proportions of each arc known, whether the powdered ice 

 be added to the acid, or the acid poured upon that, provided the powdered ice be kept 

 ftnred to prevent lumps forming, and the materials be mixed as quick as poffible But 

 when the proportion is not known, it is better to be provided with more powdered ice than 

 IS expcaed to be wanted, and add it to the acid by degrees, until the greateft efled is pro- 

 duced, as (hewn by a thermometer. ^ 



The confidence is a pretty fure guide to thofe accuftomed to mixtures of this kind viz 



when frcfli additions of fnow or ice do not readily dilTolve in the acid, though well llirred' 



and the mixture acquires a thickilh llocculent appearance. ' 



Snow or powdered ice that have ever been fubjefted to a cold lefs than freezing are 



Ipoiled, or rendered much lefs fit for experiments of this kind. 



I prefer the method of adding the powdered ice or fnow to the acid in a feparate venil 

 principally becaufe the fize of that vcfll-1 may be exadly adjufted to the quantity of mixture 

 It IS fb contain. 



A mix- 



