56? PHILOs6'pHICAL TRANSACTIONS. [anNO 1795. 



(fig. 15); and a small thermometer glass with the bulb -f- full of quicksilver (fig. 

 14). I filled the vessel, fig. 10, holding when inverted 2 pints, stratum super 

 stratum, with pounded ice, common salt, and a powder consisting of equal 

 parts sal ammoniac and nitre mixed together; by first putting in 6 oz. of pounded 

 ice, then 2-i- oz. of common salt, and, after stirring these well together, 2-i- oz. 

 of the mixed salts, mixing the whole well together; this was repeated in the 

 same manner till the vessel was quite full ; it was then tied over securely with a 

 wet bladder, turned upright, and l-i oz. of rain water poured into the tube 

 through a funnel, the tube covered with a cork, and the vessel left undisturbed 

 till the water was frozen perfectly solid. The instrument for grinding it was then 

 put in to acquire cold, while the vessel, fig. 11, holding a pint, was filled in the 

 same manner, with the same proportions of materials, a bladder tied over it, set 

 upright, and 1 oz. of fuming nitrous acid poured in to be cooled. The ice was 

 then ground to powder, and when finished, the nitrous acid being found to have 

 acquired a sufficient degree of cold, viz. — 13°, the frigorific mixture of ice and 

 salts was let out of the vessel which contained the nitrous acid; and the powdered 

 ice, still surrounded by its frigorific mixture, added to the acid as quick as pos- 

 sible; when the thermometer sunk to near — 50°, and the mixture soon froze 

 the quicksilver in the glass bulb. In this experiment, 18 minutes were required 

 to freeze the water perfectly solid; and 15 to reduce the ice, by moderate labour, 

 to very fine powder. The experiment was over in 55 minutes; and the tempe- 

 rature of the preparatory cooling mixture then found to be — 10°. 



I had a spirit thermometer by me, but a mercurial thermometer being much 

 more sensible, and consequently descending much quicker, I prefer it in experi- 

 ments made merely to freeze quicksilver; knowing from experience how the con- 

 gelation is going on, from the irregular descent of the mercury when a few de- 

 grees below its freezing point; and from having usually found that the quicksilver 

 in the thermometer glass begins to freeze, as soon as the mercurial thermometer 

 reaches — 40". Whenever I have occasion to use ice in summer for this pur- 

 pose, I usually pound together first some ice and salt in a stone mortar, about 2 

 parts of the former to 1 of the latter; throw this away, and wipe the pestle and 

 mortar perfectly dry; the moitar being thus cooled, the ice may afterwards be 

 pounded small without melting. And as a mixture made of snow, or ice in 

 powder, and salts, does not give out its greatest cold till it is become partially 

 liquid, by the action of the ice and salts on each other ; it is necessary that the 

 whole be stirred well together, till it is become of a uniformly moist pulpy con- 

 sistence, especially since in becoming liquid the mixture shrinks so much, that 

 if this be not attended to the vessel will not be near full, and consequently the 

 upper part of the tube not surrounded, as it ought to be, by the frigorific mix- 

 ture. The dissolution of the ice and salts may, if required, be hastened by add- 



