430 PHILOSOPHICAL TRANSACTIONS. [ANNO 1783. 



that the addition of more snow began to produce cold ; the quantity of snow re- 

 quired for this purpose being pretty exactly J- of the weight of the spirit of nitre, 

 and the heat of the snow and air of the room, as well as of the acid, being 28°. 

 The reason of this is, that a great deal of heat is produced by mixing water with 

 spirit of nitre, and the stronger the spirit is, the greater is the heat produced. 

 Now it appears from this experiment, that before the acid was diluted, the heat 

 produced by its union with the water formed from the melted snow was greater 

 than the cold produced by the melting of the snow; and it was not till it was 

 diluted by the addition of -A- of its weight of that substance, that the cold gene- 

 rated by the latter cause began to exceed the heat generated by the former. 

 From what has been said it is evident, that the cold of a freezing mixture, made 

 with the undiluted acid, cannot be quite so great as that of one made with the 

 same acid, diluted with a quarter of its weight of water, supposing the acid and 

 snow to be both at 28° of heat, and there is no reason to think, that the event 

 will be different if they are colder ; for the undiluted acid will not begin to gene- 

 rate cold until so much snow is dissolved as to increase its heat from 28° to 51°, 

 so that no greater cold will be produced than would be obtained by mixing the 

 diluted acid heated to 5 1° with snow of the heat of 28°. This method of adding 

 snow gradually to an acid is much the best way of finding what strength it ought 

 to be of, in order to produce the greatest effect possible. 



By means of this acid, diluted in the above-mentioned proportion, Mr. C. 

 froze the quicksilver in the thermometer called g by Mr. Hutchins, on the 26th 

 of February. He did not, indeed, break the thermometer to examine the state 

 of the quicksilver in it ; for as it sunk to — 1 10° it must certainly have been in 

 part frozen ; but immediately took it out, and put the spirit thermometer in its 

 room, in order to find the cold of the mixture. It sank only to — 30° ; but, by 

 making allowance for the spirit in the tube being not so cold as that in the ball, 

 it appears, that if it had not been for this cause it would have sunk to — 35°, 

 which is 5° below the point of freezing, and is as great a degree of cold, within 

 1°, as was produced in any of Mr. Hutchins's experiments. 



In this experiment the thermometer g sank very rapidly, and, as far as Mr. 

 C. could perceive, without stopping at any intermediate point, till it came to the 

 above-mentioned degree of — 1 10°, where it stuck. The materials used in 

 making the mixture were previously cooled, by means of salt and snow, to near 

 O ; the temper of the air was between 20° and 25°; the quantity of acid used 

 was 44 oz. ; and the glass in which the mixture was made was surrounded with 

 wool, and placed in a wooden box, to prevent its losing its cold so fast as it 

 would otherwise have done. Some weeks before this, he made a freezing mix- 

 ture with some spirit of nitre, much stronger than that used in the foregoing 

 experiment, though not quite so strong as the undiluted acid, in which the cold 



