VOL. LXXVIII.] PHILOSOPHICAL TRANSACTIONS. 4ll 



frigorific mixture. In general it congealed sooner by 1 or 2° than unboiled dis- 

 tilled water ; that is, at 25° or 24° of the thermometer ; and as there was some 

 variation in this respect, I was led to remark, that the greatest cooling usually 

 took place when the water was most clear and transparent. With a view to this 

 circumstance I took some New-River water, which happened at that time to be 

 considerably turbid, and tried it in the frigorific mixture ; when I found, very 

 unexpectedly, that it was not in my power to cool any of it below the freezing 

 point ; a crust of ice always forming round the sides and at the bottom of the 

 vessel, while the thermometer, suspended about the middle of the water, was 

 2 or 3° above 32°. To try how far this depended on the foulness of the water, 

 I collected some of the muddy sediment which had been deposited from the 

 New-River water, and added it to the pump water, which had before borne to 

 be cooled to 24° or 25°, so as to render it turbid ; when it congealed, in the 

 same manner as the New-River water had done, before the thermometer in the 

 middle of it came to the freezing point. It must not however be imagined, that 

 water thus made turbid is incapable of being cooled below 32", without freezing: 

 I have since repeated the experiments, with more caution in conducting them, 

 and reduced it 2 or 3° below the point of congelation. But still they have all 

 confirmed the general fact, that substances which lessen the transparency of 

 water, render it at the same time much more difficult to be cooled below the 

 freezing point, and dispose it to shoot into ice more readily, after it has passed 

 that point, than pure water would do. It seems to be of little consequence 

 what the substance is that renders the water turbid ; small particles of any kind 

 floating through it, I believe, have this effect, which does not take place, or at 

 least to the same degree, when the extraneous substance has subsided to the 

 bottom. It is this circumstance, I suppose, which gave rise to the opinion, that 

 boiled water freezes sooner than unboiled : for if the water contain calcareous 

 earth, held in solution by means of fix§d air, as is the case with most kinds of 

 spring water, this will be precipitated by the boiling, and will sensibly trouble 

 the transparency of the water ; which, if exposed to the cold in that state, will 

 be liable to freeze sooner than the same kind of water unboiled and transparent. 

 The effect of this want of transparency was very different from that of che- 

 mical mixture, as appeared by subsequent experiments. Though the property 

 of being cooled below the freezing point appeared to belong essentially to water 

 in its pure state, it was probable that it would be in some measure altered or 

 modified by the various substances which are capable of being dissolved in, or 

 chemically combining with, the water. But here a further circumstance came 

 to be considered. It is well known, that such substances, uniting with water, 

 have a power of lowering its point of congelation a greater or less number of 

 degrees^ according to the nature and quantity of the substance employed. The 



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