Freezing Prtcejit. 497 



III. 



Ohfervatians on the tefl Methods of proSuctng Artificial Cold. By Mr. Rl CHARD WaLKER *. 



H, 



.AVING already invefligated the means of proLlucing artificial cold, and at the con- 

 clufion of my laft paper (on the Congelation of Quickfilver) difmifTed that part of the fub- 

 je£t, the belt method of ranking ufe of thofe means naturally becomes a defideratum; to 

 that therefore I have lately given my attention, and flatter myfelf that the following obfer- 

 vations may be conGdered as an ufeful Appendix to my former papers. The freezing 

 point of quickfilver being novi' as determined a point on the fcale of a thermometer, viz. 39°, 

 as the freezing point of water ; and as this metal, exhibited in its folid ftate, affords an in- 

 terefting as well as curious phenomenon, 1 fhall apply vi'hat I have to fay principally to that 

 objeft. 



Frequent occafions having occurred to me of obferving the fuperiority of fnow in experi- 

 ments of this kind to falts, even in their fittefl ftate, that is, frefli cryftallifed, and reduced 

 to very line powder, I refolved upon adopting a kind of artificial fnow. 



The firft method which naturally prefented itfelf %vas by condenfing fteam into hoar froft. 

 This anfwered the purpofe, as might be expedled, exceedingly well ; but the difficulty and 

 expence of materials in colleding a fufficient quantity determined me to relinquifh this mode 

 for another, by which I can eafily and expeditioufly procure ice in the fitteft form for ex- 

 periments of this kind ; the method I mean is, by firft freezing water in a tube, and after- 

 wards grinding it into very fine powder. Thus pofTefTed of the power of making ice, and 

 afterwards reducing it to a kind of fnow, the congelation of quickfilver becomes a very eafy 

 and certain procefs; for, by the ufe of a very fimple apparatus, (Plate XX. fig. i.) quickfilver 

 may be frozen perfeftly folid in a few minutes, wherever the temperature of the air does 

 not exceed 85° : thus, one ounce of nitrous acid is to be poured into the tube b of the vefTel, 

 obferving not to wet the fide of the tube above with it ; a circular piece of writing-paper, 

 of a proper fize, is to be placed over the acid, refting upon the fhoulder of the tube, and the 

 paper brufhed over with fome melted white wax. Thus prepared, the vefTel is to be in- 

 verted, and filled with a mixture of tliluted nitrous acid, phofphorated foda, and nitrous 

 ammoniac, in proper proportions for this temperature *, and tied over fecurely, firft with 

 waxed paper, and upon that a wet bhidder. 



The vefiel being then turnetl upright, and placed in a fliallow vefTel, viz. a faucer or 

 plate, an ounce and a half of rain or diftilled water is to be poured into the tube, which it 

 to be covered with a ftopper or cork, arnl, as foon as frozen folid, ground to very fine pow- 

 der, an afTiftant holding it firmly and fleadily the wliile ; obferving occafionally to work the 

 inftrument in different diredionp up and down, that no lumps may be formed. When the 

 whole of the ice is thus reduced to powder, and the lumps, if any, broken, the frigorific 

 mixture is to be let out quickly, by cutting or untying the ftring, and removing the bladder, 

 &c. which confines it ; a communication made by forcing a rod of glafs or wood through 

 the partition ; and the whole mixed expeditioufly together. 



* Ph'ilofophical Tranfaaions, 1795. 



f I hive, by a very accurate preparation of this niixturr, funk a ihcrmorocici from 85 drj^rcct (tempera- 

 tore of the vcffcl and mat«rialO to + 2 i)ej;rcei. 



Vol.. 1.— FtBKUART 179S. 3S In 



