VOL. LXXXV.] PHILOSOPHICAL TRANSACTIONS. 501 



paper brushed over with some melted white wax; thus prepared, the vessel is to 

 be inverted, and filled with a mixture of diluted nitrous acid, phosphorated soda, 

 and nitrous ammoniac, in proper proportions for this* temperature, and tied 

 over securely, first with waxed paper, and on that a wet bladder. 



The vessel being then turned upright, and placed in a shallow vessel, viz. a 

 saucer or plate, 14- oz. of rain or distilled water is to be poured into the tube, 

 which is to be covered with a stopper or cork, and, as soon as frozen solid, 

 ground to very fine powder, an assistant holding it firmly and steadily the while; 

 observing occasionally to work the instrument in difi^erent directions 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 string, and removing the bladder, 

 &c. which confines it; a communication made, by forcing a rod of glass or 

 wood through the partition ; and the whole mixed expeditiously together. In 

 this climate, a mixture much less expensive will be sufficient, viz. that composed 

 of diluted nitrous acid, Glauber's salt, sal ammoniac, and nitre; a mixture of 

 this kind sinking a thermometer in the warmest weather to near 0°. At the 

 temperature of 70°, or a little higher, the quantity of diluted nitrous acid may 

 be about -^ less than is mentioned in the table, for 50°. 



These methods are the most expeditious, and attended with the least trouble; 

 but as ice may be used with equal certainty, and with much less expence, I shall 

 give a particular detail lof an experiment made with the use of it, first mention- 

 ing a preparatory experiment, to which I was immediately led by the recollection 

 that Sir Charles Blagden, in his paper " on the point of congelation," (Phil. 

 Trans, vol. 78,) had found that common sal ammoniac and common salt, mixed 

 with snow, produced a cold of — 12°, whereas the latter used alone with snow 

 produces only — 5°. I used a mixed powder of equal parts of common sal am- 

 moniac and nitre with the common salt, by which the thermometer sunk to — 

 18°; and when I used nitrous ammoniac with common salt, to — 25°; this cold 

 I could not increase by the addition of any other salts, nor could I equal it by any 

 other combination of salts: those I tried were Glauber's salt, salt of tartar, soda, 

 and sal catharticus amarus ; by several trials, I found the best proportions to be, 

 snow or pounded ice 12 parts, common salt 5 parts, and of nitrous ammoniac, 

 or a powder of equal parts sal ammoniac and nitre mixed, 5 parts; or 4- of com- 

 mon salt, when I used that alone, with snow or pounded ice. 



My apparatus then (Dec. 28th last) consisted of 2 vessels (fig. 10 and 11); an 

 instrument, (fig. 13) to grind or rather scrape the ice to powder; a kind of 

 spatula, I use a marrow-spoon, to stir the powder occasionally ; a thermometer 



* 1 have, by a very accurate preparation of this mixture, sunk a thermometer from 85", tempe- 

 rature of the vessel and materials, to + 2°. — Orig. 

 VOL. XVII. 4 C 



