6l2 PHILOSOPHICAL TRANSACTIONS; f'l [anNO 1775. 



when they had been an hour exposed, and finding ice on the boiled water, and 

 none on the other, I gently stirred the unboiled water with my tooth-pick, and 

 saw immediately fine feathers of ice formed on its surface, which quickly 

 increased in size and number, till there was as much ice in this cup as in the 

 other, and all of it formed in one minute of time, or 2 at most. And in the 

 rest of the trials, though the congelation began in general later in the unboiled 

 water than in the other; when it did begin in the former, the ice quickly 

 increased so as, in a very short time, to equal, or nearly equal in quantity, that 

 which had been formed more gradually in the boiled water. The opinion, there- 

 fore, which I have formed from what I have hitherto seen is, that the boiled 

 and common water differ from one another in this respect ; that whereas the 

 common water, when exposed in a state of tranquillity to air that is a few 

 degrees colder than the freezing point, may easily be cooled to the degree of such 

 air, and still continue perfectly fluid, provided it still remain undisturbed : the 

 boiled water, on the contrary, cannot be preserved fluid in these circumstances ; 

 but when cooled down to the freezing point, if 'we attempt to make it in the 

 least colder, a part of it is immediately changed into ice ; after which, by the 

 continued action of the cold air on it, more ice is formed in it every moment, 

 till the whole of it be gradually congealed before it can become as cold as the 

 air that surrounds it. From this discovery it is easy to understand, why they 

 find it necessary to boil the water in India, in order to obtain ice. The utmost 

 intensity of the cold which they can obtain by all the means they employ, is 

 probably not greater than 31° or 30° of Fahrenheit's thermometer. Common 

 water, left undisturbed, will easily descend to this degree without fieezing ; 

 and, if they have not the means of making it colder, may continue fluid 

 for any time, provided it be not disturbed : the refrigerating causes of that 

 part of the world when they have done so much, have done their utmost, and 

 can act no further on the water. But this cannot happen to the boiled water ; 

 when the refrigerating causes have cooled it to 32°, the next effect they produce, 

 is to occasion in it the beginning of congelation, while the water is afterwards 

 gradually assuming the form of ice, we know, by experience, that its tempera- 

 ture must remain at 32°; it cannot be made colder, so long as any considerable 

 part of it remains unfrozen.* The refrigerating causes continue therefore to 

 have power over it, and to act upon it, and will gradually change the whole into 

 ice, if their action be continued sufficiently long. 



The next object of investigation may be the cause of this difference between 

 the boiled and the common water. In considering this point, the following idea 



* Conunon water, when cooled in a state of tranquillity to several degrees below the fieezing 

 point, will suddenly rise up to it again, if disturbed in such a manner as to occasion in it a beginning 

 of congelation. — Orig. 



