VOL. LXXXV.3 PHILOSOPHICAL TRANSACTIONS. 507 



salt 5 parts, and a powder, consisting of equal parts of common sal ammoniac 

 and nitre mixed, 5 parts, which produces a cold of — 18° : 3d, snow or pounded 

 ice Imparts, common salt 5 parts, and nitrous ammoniac in powder 5 parts, 

 which produces a cold of — 25°. 



The proportions which I have found to be the best for mixing the snow or 

 powdered ice with the different acids, at different temperatures, are these ; viz. 

 at + 30°, 7 of the former to 4 of the nitrous acid ; at + 3° (with a trifling al- 

 lowance, if any, for a few degrees above or below), 3 to 2 ; at — 12°, 4 to 3, 

 with the mixed acids ; and at — 20°, with the diluted vitriolic acid, equal parts. 



If it be required to prepare the materials in a frigorific mixture, without the 

 use of ice, a mixture of the proper strength may be chosen from the table. It 

 is immaterial, when the exact proportions of each are known, whether the pow- 

 dered ice be added to the acid, or the acid poured upon that, provided the pow 

 dered ice be kept stirred to prevent lumps forming, and the materials be mixed 

 as quick as possible. But when the proportion is not known, it is better to be 

 provided with more powdered ice than is expected to be wanted ; and add it to 

 the acid by degrees, till the greatest effect is produced, as shown by a thermo- 

 meter. The consistence is a pretty sure guide to those accustomed to mixtures 

 of this kind ; viz. when fresh additions of snow or ice do not readily dissolve in 

 the acid, though well stirred, and the mixture acquires a thickish flocculent ap- 

 pearance. Snow, and powdered ice, that have ever been subjected to a cold less 

 than freezing are spoiled, or rendered much less fit for experiments of this kind. 

 I prefer the method of adding the powdered ice or snow to the acid in a separate 

 vessel, principally because the size of that vessel may be exactly adjusted to the 

 quantity of mixture it is to contain. A mixture made of diluted nitrous acid, 

 phosphorated soda, and nitrous ammoniac (by much the most powerful of any 

 compounded of salts with acids), prepared with the greatest accuracy, is not 

 quite equal to a mixture of snow and nitrous acid, each mixed at -|- 30°, though 

 very nearly so. Though quicksilver may be frozen by salts dissolved in acids, it 

 is necessary that the materials be cooled, previously to mixing, much lower than 

 when snow or ground ice are used. 



If it be required to mix the powdered salts and acids at a low temperature, the 

 best method is this : put first the nitrous ammoniac into the tube of such an 

 apparatus as fig. 8, shaking it down level, gently pressing the upper surface 

 smooth ; then the phosphorated soda or Glauber's salt ; cover this with a circular 

 piece of writing-paper, and pour a little melted white wax on it, and when cold, 

 pour on this the diluted nitrous acid ; immerse this in a frigorific mixture till it 

 is sufficiently cold, as found by dipping the thermometer into the liquor occa- 

 sionally ; force a communication through, and stir the whole thoroughly toge- 

 ther, contriving that the upper stratum of salt, that is, the phosphorated soda or 



