504 PHILOSOPHICAL TRANSACTIONS. [aNNO 1788. 



added to the same quantity of water, under the same circumstances as before, 

 when it again sunk the thermometer to 14°. Since that time he has repeatedly 

 used a composition of this kind for the purpose of producing cold, without ob- 

 serving any diminution in its effect after many evaporations. The cold may be 

 economically kept up and regulated any length of time, by occasionally pouring 

 off the clear saturated liquor, and adding fresh water, observing to supply it 

 constantly with as much of the powder as it will dissolve. 



The degree of cold at which water begins to freeze has been observed to vary 

 much; but that it might be cooled 22° below its freezing point was perfectly un- 

 known to him till lately. He filled the bulb of 2 thermometers, one with the 

 purest rain iwater he could procure, the other with pump water; the water was 

 then made to boil in each, till -J- only remained: these were kept in a frigorific 

 mixture, at the temperature of + 10°? foi" a much longer time than he thought 

 necessary to cool the water to the same temperature; and by repeated trials he 

 found it was necessary to lower the temperature of the mixture to near + 5°, to 

 make the water in either of them freeze. These were likewise suspended out of 

 doors, close to a thermometer, during the late frost, and the water never ob- 

 served frozen. On March the 22d, at 6 in the morning, the water in each re- 

 mained unfrozen, though the tubes were gently shaken, the thermometer 

 standing at that time at 23°, There appeared to be little difference with respect 

 to the degree of cold necessary to freeze the water, whether the tube of the ther- 

 mometers were open or closed in vacuo (which was very nearly effected by 

 suff^ering the water to boil up to the orifice of the tube, and then suddenly 

 sealing it) or not, but unboiled water in the same situation froze in a higher 

 temperature. 



It is commonly supposed that gentle agitation of any kind will dispose water, 

 cooled below its freezing point, to become ice; but Mr. W. repeatedly cooled 

 rain water and pump water, boiled a long time, and unboiled, in open vessels to 

 30° or lower, and constantly succeeded, after trying other kinds of agitation in 

 vain, by stirring, or rather scraping gently, the bottom and sides of the vessel 

 containing the water to be frozen, when after some short time small filaments 

 of ice appeared, and by continuing this motion about every part of the vessel 

 beneath the surface of the water, about |- of the water 'commonly froze. A 

 slender, pointed glass rod he used for this purpose. 



Extract oj a second Letter from Mr. Walker, — A more intense cold may be 

 produced by a solution of salts in uater in summer, than can be produced by a 

 mixture of snow and salt in winter. To rain water 6 drs. by weight, I added 

 6 drs. of nitrated ammonia reduced to a very fine powder, which made the ther- 

 mometer sink from -\- 50°, temperature of the materials, to 4*^; then adding 

 6 drs. of mineral alkali very finely powdered, the thermometer sunk to — 'J^f 



