VOL. I.] PHILOSOPHICAL TRANSACTIONS. 87 



salt, upon the quickness of which depends very much the intensity of the cold 

 that will ensue. 



I. That a considerable degree of cold is really produced by this is evident, 

 1 St. To the touch ; 2dly, By the dew which collects on the sides of the vessel 

 containing the solution ; 3dly, By plunging into it (which is the best and surest 

 test) a good sealed weather-glass furnished with tinctured spirit of wine.* For 

 the ball [bulb] of this being put into our frigorific mixture, the crimson liquor 

 will nimbly enough descend much lower than when it was kept either in the 

 open air or in common water of the same temperature with that wherein the 

 sal ammoniac was put to dissolve. 



II. The duration of the cold produced by this experiment depends upon 

 several circumstances; as, 1st, Upon the season of the year and present tempera- 

 ture of the air ; 2dly, Upon the quantity of the salt and water ; 3dly, Upon the 

 goodness and fitness of the particular parcel of salt that is employed ; 4thly, 

 Upon the way of putting the salt into the water; for if you cast it in all at once, 

 the water will sooner acquire an intense degree of coldness, but it will also the 

 sooner return to its former temperature : whereas, if you desire but an inferior 

 degree of that quality, but that may last longer (which will usually be the most 

 convenient for the cooling of drinks) then you may put in the salt little by little. 

 For keeping a long weather-glass (thermometer) for a good while in our im- 

 pregnated mixture, I often purposely tried that when the tinctured liquor sub- 

 sided but slowly or was at a stand, by putting in, from time to time, two or 

 three spoonfuls of fresh salt, and stirring the water to quicken the solution, the 

 spirit of wine would begin again to descend. The refrigerating process may be 

 lengthened by having part of the sal ammoniac but grossly pounded, so as to be 

 longer in dissolving. In spring I have found by a good weather-glass (thermo- 

 meter) a sensible artificial cold, made by a pound of sal ammoniac at the utmost, 

 to last about two or three hours. 



III. To cool drinks with this mixture, you may put them in thin glasses, the 

 thinner the better ; which (their orifices being stopped and kept above the mix- 

 ture) may be moved to and fro in it, the liquor being then immediately poured 

 out and drank ; or, if the glass be conveniently shaped, it may be drank out of 

 that, without pouring it into another, which lessens the coolness. 



IV. Whether sal ammoniac mixed with sand or earth, and not dissolved, but 

 only moistened with a little water sprinkled upon it, will answer for cooling 



* In the subsequent part of the paper this weather-glass is more fitly termed a thermoscope or 

 thermometer. 



