VOL. LXXXV.] PHILOSOPHICAL TRANSACTIONS. 563 



ing occasionally a little water : but then the cold produced will be less intense, 

 and not so durable. 



That particular form of the vessel, in which the ice is made and reduced to 

 powder, is chosen, because it subjects the powdered ice in the tube to the con- 

 stant action of the freezing mixture, without which it would be less fit, parti- 

 cularly in warm weather, for the intended use, and because in it the ice is not 

 liable to be impregnated with the salts of the mixture, by which it would be 

 utterly spoiled : and that for cooling the nitrous acid, and making the 2d mix- 

 ture in, because it is steady, and is besides insulated as it were from the external 

 warm air, and surrounded in its stead by an atmosphere much colder. It is 

 scarcely necessary to add, that when snow which has never thawed can be pro- 

 cured, it may be cooled in this apparatus by a mixture of snow, instead of the 

 pounded ice, and the salts, and the trouble of reducing the ice into powder 

 saved. 



I prefer the red fuming nitrous acid, because, as I have observed in a former 

 paper, it requires no dilution. Being under the necessity at 1 time of using the 

 pale nitrous acid, I found it required to be diluted with ^ its weight of water. The 

 best and only way of trying or reducing any acid to the proper strength, is by 

 adding snow, as Mr. Cavendish directs, or the powdered ice to it, till the ther- 

 mometer cease to rise ; then cool the acid to the same temperature of the snow 

 again, add more snow, which will make the thermometer rise again, though 

 less ; cool it again, and repeat this, till the addition of snow or powdered ice 

 will not make the thermometer rise : to be very accurate, it should be reduced in 

 this manner to the proper strength, at the temperature, whatever it be, at which 

 the nitrous acid and snow, or powdered ice, are to be mixed together when 

 cooled. 



In the course of my experiments I have endeavoured to ascertain the com- 

 parative powers of ice to produce cold with nitrous acid, in the different forms I 

 have had occasion to use it. The result is, that fresh snow sunk a thermometer 

 to — 32° ; ground ice to — 34° ; and the most rare frozen vapour to below — 

 35" ; the vessel and materials each time being -j- 30°. The vessels for these 

 mixtures, particularly that in which the quicksilver is to be frozen, should be 

 thin, and made of the best conductors of hent ; first, because thin vessels rob the 

 mixture of less cold at mixing, i. e. if 2 mixtures of the same kind are made, 1 in 

 a thin, the other in a thick vessel, the former will be coldest ; 2dly, because the 

 air is a sufficiently bad conductor ; and 3dly, for the very obvious reason, that 

 the cold is transmitted through them quicker. For these reasons, and from the 

 difficulty I have found in procuring vessels of glass, which are undoubtedly fittest 

 for experiments of this kind, I have used tin ; which is readily had in any form, 

 and if coated with wax, is sufficiently secured against the action of the acids* 



4 c 2 



