the hji Methods of producing Arii/tciat CM. ■ 49^' 



I had a fpirit thermometer by me } but a mercurial thermometer being much more fen- 

 iible, and confcquently defcending much quicker, I prefer i: in experiments made merely to 

 freeze quickfilver— knowing from experience how the congelation is going on, from the 

 irregular defcent of the mercury when a few degrees below its freezing point, and from 

 having ufually found that the quickfilver in the thermometer glafs begins to freeae as fuoii 

 as tlie mercurial thermometer reaches — 40°. 



Whenever I have occafion to ufe ice in fummer for tliis purpofe, I ufually pound to- 

 gether firft fome ice and fait in a ftone mortar, about two parts of the former to one of 

 the latter ; throw this away, and wipe the peQle and mortar perfedly dry : the mortar 

 being thus cooled, the ice may afterwards be pounded fmall without melting. 



And as a mixture made of fnow or ice in powder and falts does not give out its greateft 

 cold till it is become partially liquid by the aftion of the ice and falts on each other, it is 

 neceflary that the whole be ftirred well together, till it is become of an uniformly moiit 

 pulpy confiftence; efpecially fince, in becoming liquid, the mixture Ihrinks fo much, that, 

 if this be not attended to, the vefi'el will not be near full, and confequently the upper part 

 of the tube not furrounded, as it ought to be, by the frigoritic mixture. The diflbiution of 

 the ice and falts may, if required, be haftened, by adding occafionally a little water ; but 

 then the cold produced will be lefs intenfe, and not fo durable. 



That particularform of the veflel in which the ice is made and reduced to powder is chofen, 

 becaufe it fubjeds the powdered ice in the tube to the conftant aftion of the freezing mix- 

 ture, without which it would be lefs fit, particularly in warm weather, for the intended ufe ; 

 and becaufe in it the ice is not liable to be impregnated with the falts of the mixture, by 

 which it would be utterly fpoiled ; and that for cooling the nitrous acid, and making the 

 fecond mixture in, becaufe it is fteady, and is befides infulated, as it were, from the ex- 

 ternal warm air, and furrounded in its ftead by an atmofphere much colder. 



It is fcarcely neceflary to add, that when fpow which has never thawed can be procured, 

 it may be cooled in this apparatus by a mixture of fnow (inftead of the pounded ice) and 

 the falts, and the trouble of reducing the ice into powder faved. 



I prefer the red fuming nitrous acid, becaufe, as I have obferved in a former paper, it 

 requires no dilution. Being under the necelTity at one time of ufing the pale nitrous acid^ 

 I found it required to be diluted with one-fifth its weight of water. The belt and only 

 way of trying or reducing any acid to the proper ftrength, is by adding fnow, as Mr. Ca- 

 vendifli dirc£ls, or the powdered ice to it, until the thermometer ceafcs to rife ; then cool 

 the acid to the fame temperature of the fnow again ; add mote fnow, which will make the 

 thermometer rife again, though lefs; cool it again, and repeat this until the addition of 

 fnow or powdered ice will not make the thermometer rife : to be very accurate, it fliould 

 be reduced in this manner to the proper (Irengih, at the temperature, whatever it be, at 

 which the nitrous acid and fnow, or powdered ice, are to be mixed together when cooled. 



In the courfc of my experiments I have endeavoured to afccrtain the comparative powers 

 of ice to produce cold with nitrous acid in the different forms I have had occafion to ufe 

 it. The rcfult is, that frefli fnow funk a thermometer to — 32 degrees, ground ice to 

 — 34. degrees, and the moR rare frozen vapour to below — 35 degrees \ the vcfill and ma- 

 icrials each time being -f- 30 degrees. 



3 S '■ The 



