VOL. LXXIII.] PHILOSOPHICAL TRANSACTIONS. 43y 



be cooled some degrees below its freezing point, before it begins to form solid 

 crystals, the phenomenon in question may depend on that circumstance : for if, 

 from whatever cause, the mercury in the thermometer should begin to congeal as 

 soon as it was cooled down to — 39° or — 40°, while that which surrounded it 

 would sustain a cold of — 43° or — 44° without becoming solid ; it is evident 

 that the whole of the former might be congealed, and yet no part of the latter, 

 though the real freezing point of both were the same, that is, though the sur- 

 rounding quicksilver as soon as it came to shoot its crystals would rise immediate- 

 ly to — 39°, the point at which that in the thermometer froze. 



As this is undoubtedly the most obscure part of our knowledge relative to the 

 congelation of quicksilver, Dr. B. endeavoured to illustrate it by some experi- 

 ments on the freezing of water. The purest water he could obtain bore to be 

 cooled to -|- 21°, no less than 11° below the temperature to which it instantly 

 rose as soon as the crystals of ice shot through it. This was distilled water very 

 recently boiled : it is a mistake, therefore, that boiling necessarily renders water 

 not so capable of being cooled below the freezing point. In proportion as the 

 water was less pure, it seemed to congeal the sooner ; and the kind of impurity 

 which had the most effect appeared rather to be extraneous matter diffused 

 through the water, so as to trouble its transparency, than such as was chemically 

 dissolved in it.* The smallest particle of ice also, whenever the water was below 

 the freezing point, either added from without, or by any means formed in it, 

 would instantly cause a crystallization, by which the whole came immediately up 

 to -f- 32°. Likewise a crack in the bottom of the containing glass vessel effec- 

 tually prevented the water from being cooled below the freezing point, as ice con- 

 stantly formed on the bottom, perhaps in consequence of the early generation of 

 some minute portions of it in the crack. But independently of these circum- 

 stances, neither stirring, agitation, a current of fresh air on the surface, nor the 

 contact of any extraneous body not colder, would cause the water to shoot into 

 ice, even after it was cooled many degrees below the freezing point, notwith- 

 standing the repeated assertions of authors to the contrary. 



This account of mercurial congelation by artificial means would remain incom- 

 plete, Dr. B. says, were he not to mention that at Hampstead, on the 26th of 

 February last (1783), the temperature of the air being then above -+- 20°, Mr. 

 Cavendish, by an ingenious artifice of diluting the nitrous acid to a proper degree, 

 sunk the quicksilver in his thermometer to 1 10 , and consequently froze it in part. 

 He then interrupted the experiment to try the cold of his frigorific mixture by a 



* This appears to be the reason that boiling has been thought to render water incapable of being 

 cooled below the freezing point. In most kinds of water, the application of heat occasions the pre- 

 cipitation of earthy substances which were before held in solution : hence the water comes to be in the 

 state of having extraneous matter diffused through it, and therefore readily congeals. — Orig. 



