462 PHILOSOPHICAL TRANSACTIONS. [aNNO I788. 



degree or more. Hence was ascertained the unexpected fact, that the lower such 

 solutions are cooled, the higher is their freezing point. 



The nitre deposited by the solution as it cooled, formed, if the vessel remained 

 at rest, small but very white and compact prismatic or i>eedle-lik.e crystals, of 

 considerable length, pointing different ways, and at last curiously interwoven with 

 each other. But if these were broken down, or the solution was stirred with 

 any force, the remaining nitre deposited itself in such minute crystals as to have 

 much the appearance of a powder; probably from the destruction of the regular 

 surfaces on whith it would otherwise have continued to form. Frequently, in 

 the stronger solutions, there appeared near the bottom and side of the tumbler 

 many elegant stellated crystals, perhaps a quarter of an inch in diameter, all 

 separate, but sometimes crouding very close on each other, so as to exhibit a 

 spectacle of much beauty. The ice of solutions of nitre, especially when it 

 began to thaw, was very different from common ice, having a soft woolly appear- 

 ance, as if of a more tender and loose texture. Something of the same kind 

 was observable in the ice of all the other solutions, sufficiently distinguishing it 

 from any that can be formed of pure water. All the solutions of nitre were 

 remarkably limpid, having no tendency to an opaque or turbid cast; and accord- 

 ingly they were very easily cooled below the freezing point, and could not but 

 with difficulty be made to shoot till they had passed it many degrees. In 2 in- 

 stances they cooled more than 10°; namely, a solution of 1 part of nitre in 24 

 of water cooled slowly to 1 g-^^, and then shooting, the thermometer came up to 

 30°; and another solution, in which the nitre was to the water as 1 : 10, cooled 

 rather below l6°, and having produced some stellated crystals, rose, when the 

 perfect congelation took place, up to 27°. 



As, when pure water is cooled below its freezing point, the least particle of 

 ice or snow brought into contact with it causes an instant congelation. Dr. B. 

 was curious to know whether the same effect would be produced when salts were 

 dissolved in the water. Therefore, having one of these nitrous solutions, whose 

 proportions were 8 : 1, he cooled it to 24°, about 2° below its freezing point, and 

 then, no salt being deposited, he put into it a small bit of ice. The effect of 

 this was not instantaneous, as in pure water, though ultimately the same; the 

 bit of ice gradually enlarged, and when it was stirred about in the liquor, a 

 number of star-like crystals formed, which being scattered through it soon 

 brought it to a uniform temperature of 20°. This same solution, when cooled 

 in a preceding experiment to 18°, had its freezing point at 27°, from the quan- 

 tity of nitre that had been deposited. In all solutions therefore, of such salts as 

 are much more soluble in hot than in cold water, if it be desired to find their 

 freezing point when they are loaded with as much of the salt as the water can 

 contain at that temperature, the most effectual method is to oblige them to 



