VOL. LXXIX.] PHILOSOPHICAL TRANSACTIONS. 585 



first induced him, while pursuing the subject of cold, to try the effect of dis- 

 solving the vitriolated natron in the mineral acids. Equal quantities, by weight, 

 of phosphorated natron and vitriolated natron, u^ere evaporated separately over a 

 gentle fire, till each was reduced to a perfectly dry powder. He then weighed 

 them, and found the residuum of the phosphorated natron somewhat lighter 

 than that of the vitriolated natron ; whence it is probable the former contains 

 the greater quantity of water of crystallization. He has found, that each of the 

 neutral salts which produce any remarkable degree of cold by solution in the 

 mineral acids, viz. phosphorated natron, vitriolated natron, and vitriolated mag- 

 nesia, lose this property entirely, when deprived by any means of their water of 

 crystallization. 



A short time after he had first succeeded in freezing water in summer, by one 

 mixture composed of 3 different salts in water (having been induced to try the 

 effect of such a method, from the consideration that water, already saturated 

 with one kind of salt, will dissolve a portion of another, and after that a 3d, or 

 even more,) he met with the account of an experiment made by M. Homberg, 

 related in one of the earlier volumes of the Philos. Trans, in which it is said he 

 produced an extraordinary degree of cold, by pouring a pint and a half of dis- 

 tilled vinegar on 2 lb. of a powder composed of equal parts of crude sal ammo- 

 niac and corrosive sublimate, and shaking them well together. Mr. W. imme- 

 diately (July 30, 1786) prepared a mixture of this kind in smaller quantity, but 

 found it produced only 32° of cold, the temperature of the air and materials be- 

 fore mixing being 63° ; which is no more than he had found may be effected by 

 a solution in water of crude sal ammoniac alone, previously dried and powdered. 



By a trial made with great accuracy, he found, that even the mixture com- 

 posed of diluted vitriolic acid and vitriolated natron is adequate to any useful 

 purpose that may be required in the hottest country ; for, by adding 1 1 parts of 

 the salt in fine powder to 8 parts of the vitriolic acid diluted with an equal weight 

 of water, the thermometer sunk from 80°, the mean temperature of the hottest 

 climate, and to which these materials were purposely heated before mixing, to 

 rather below 20°. Vitriolated natron, added to the marine acid undiluted, pro- 

 duces very nearly as great a degree of cold as when mixed with the diluted ni- 

 trous acid. At the temperature of 50°, 2 parts of the acid require 3 parts of 

 the salt in fine powder, which will sink the thermometer to O''; and if 3 parts of 

 a mixed powder, containing equal parts of muriated ammonia and nitrated kali, 

 be added afterwards, the cold of the mixture will be increased a few degrees more. 



The frigorific mixture above described, composed of phosphorated natron and 

 nitrated ammonia dissolved in the diluted nitrous acid, being the most powerful, 

 it will probably be found most convenient for freezing mercury, when snow is 

 not to be procured. The materials for this purpose may be previously cooled in 



VOL. XVI. 4 F 



