410 PHILOSOPHICAL TRANSACTIONS. [aNNO 1788, 



the quicksilver, but also because this very quality in water was itself a curious 

 subject of research. 



I began with endeavouring to determine, whether this property belongs to it 

 as pure water, or depends on extraneous admixtures. For that purpose I poured 

 some clean distilled water into a common tumbler glass, till it reached 2 or 3 

 inches above the bottom, and then set the glass in a frigorific mixture, made 

 with snow and common salt. This was the method I used in most of the fol- 

 lowing experiments, sometimes employing ice instead of snow, substituting a 

 glass jar or cylinder instead of a tumbler, and filling the vessel to a greater or 

 less height above the bottom. 1 found that, in the frigorific mixture, the dis- 

 tilled water readily sunk many degrees below 32°, still continuing fluid ; and by 

 repeating the experiment with care, I several times cooled it to 24°, 23°J , and 

 even almost to 23°. From these experiments therefore it seemed evident, that 

 the property of being cooled below the freezing point did not depend on extra- 

 neous admixture, especially as I found, by comparative trials, that common 

 pump-water would scarcely ever bear to be cooled so much. An ambiguity 

 however still remained, on account of the air which is always mixed with water 

 that has lain exposed to the atmosphere. In order to determine what might be 

 ascribed to this circumstance, 1 put some of the same distilled water over the 

 fire to boil, in a clean silver vessel, and kept it in violent ebullition for a con- 

 siderable time. In a few minutes after it had been taken off the fire, and before 

 it was nearly cold, I set it in the frigorific mixture after the usual manner ; 

 when, instead of freezing more readily, it bore to be cooled 2° lower than I had 

 ever been able to reduce the unboiled water, not congealing till the thermometer 

 in it had sunk to 21°. Subsequent experiments were attended with a similar re- 

 sult, and have sufficiently convinced me, that, other things equal, boiled v/ater 

 may be cooled a greater number of degrees below the freezing point, without 

 congealing, than water which, not having undergone that operation, retains the 

 air it naturally imbibes. 



As a further proof that the presence of an aerial fluid in water rather lessens 

 than increases its quality of being cooled below the freezing point, I found that 

 distilled water, which had been for that purpose impregnated with fixed air, ge- 

 nerally shot into ice at a less degree of cold than the same water in its ordinary 

 state. I suspect however, that it is usually by the admixture of other aerial 

 substances, such as dephlogisticated air, phlogisticated air, or perhaps both, and 

 not of fixed air, that water is inclined to congeal soon after it has passed the 

 freezing point ; for, as will be seen hereafter, acids rather improve than diminish 

 the quality in water of resisting congelation. 



To determine the effect of other extraneous substances, I took some very hard 

 pump water, such as is found in the northern parts of London, and set it in the 



