500 ObJervatiofU and Experiments tn 



The veflelsfor thefe mixtures, particularly that in which the qulckfilver is to be froret>, 

 (hould be thin, and made of the beft conduftors of heat ; firft, bccaufe thin veflels rob the 

 mixture of lefs cold at mixing, that is, if two mixtures of the fame kind are made, one in 

 a thin, the other in a thick veffel, the former will be coldcft ; fecondly, becaufe the air ii 

 a fufficientiy bad conductor ; and thirdly, for the very obvious reafon that the cold U 

 iraiifmitted through them quicker. 



For thefe reafons, and from the difficulty I have found in procuring vefTels of glafs, 

 which are undoubtedly fitteft for experiments of this kind, 1 have ufed tin, which is 

 readily had in any form, and, if coated with wax, is fufficientiy fecured againll the a£lion 

 of the acids. 



I give the infide fuch a coating by pouring melted white wax mto. the veflel, previoufl^ 

 clean and dry, and turning it about by hand, fo as to leave no point of the metal uncovered 

 for the acid to acl on, pouring the furplus away. 



In the experiments above defcribed, I ufed a fingle veflel for cooling the nitrous acid ; 

 a cupping-glafs (reprelented by the dotted line at b fig. 4.) being cemented into the tin, 

 and thereby forming that part in which the nitrous acid was firft cooled, and the mixture 

 afterwards made in which tlie quickfilver was frozen ; but from the trouble and impedi- 

 ments arifing from letting out the mixture, and clearing the bottom from the lumps of ice, 

 &c. adhering to it, I was led to the addition of the other part (fig. 5.) by which all 

 thefe difficulties are got rid of: and it is befides a much more comfortable and neat 

 way of conducing it ; the upper part, which contains the nitrous acid, being lifted off, 

 and placed on the table immediately before the powdered ice is added. 



The whole of this apparatus may be of tin ; that part only (when the cooling mix- 

 lures are made without ufing any corrofive acid) in which the acid mixture is to bo 

 made being previoufly coated in the oiannei above mentioned ; or a thin glafi tumbler, 

 of a proper fize, may be cemented in. 



1 have occaGonally ufed a thin glafs tumbler for the mixture in which the quickfilver 

 ii to be frozen, immerfing it, with the acitl, in a frigorific mixture, till the acid ij 

 fufficientiy cooled ; then adding the ground ice to it, previoufly removing the tumbler 

 out of the frigorific mixture, as in the experiment above mentioned : this fimplifics the 

 apparatus, but is lefs convenient on many accounts. 



The fcale of this apparatus may be diminiffied or Increafed at the will of the opera- 

 tor ; for there is no doubt that a fmall quantity of quickfilver may be frozen at any time 

 with one-fourth of this quantity, with an apparatus of this kind, by any one convcrfant 

 in fuch experiments. 



I have frequently frozen quickfilver by mixing together at 0% three drams of ground 

 ice with two drams of nitrous acid. 



Whenever the intention is, as in thefe experiments, to cool the materials to nearly the 

 fame temperature with the frigorific mixture in which they are immerfed, the proportion 

 of the frigorific mixture to the intended mixture (or materials to be cooled) (hould not 

 be lefs than twelve to one 5 a greater difproportion is ftill better. 



By attending to the direftions particularly mentioned in the experiment made on 

 December 28, a thermometer may be always difpenfed with ; the proportions of the ma- 

 terials 



