VOL. LXXIII.] PHILOSOPHICAL TRANSACTIONS. 42<J 



mixture was not more than 5° or 6° below the point of freezing quicksilver. In 

 the first experiment also, it sunk to — 448°, at a time when the cold of the 

 mixture was only 1% below that point ; so that it appears, that the contraction 

 of quicksilver, by freezing, must be at least equal to its expansion by 404° of 

 heat. This, however, is not the whole contraction which it suffers ; for it ap- 

 pears, by an extract from Mr. Hutchins's meteorological journal, kept by him 

 at Albany Fort, that his thermometer once sunk to 4gO° below O, though it ap- 

 peared, by a spirit thermometer, that the cold scarcely exceeded the point of 

 freezing quicksilver. There are two experiments also of Professor Braun, in 

 which the thermometer sunk to 544° and 556° below 0, which is the greatest 

 descent he ever observed without the ball being cracked. It is not indeed known 

 how cold his mixtures were ; but from Mr. Hutchins's there is great reason to 

 think that they could not be many degrees below — 40°. If so, the contraction 

 which quicksilver suffers in freezing is sometimes not much less than its expan- 

 sion by 500° or 510° of heat, that is almost -^-V of its whole bulk, and in all 

 probability is never much more than that, being probably no very determinate 

 quantity. 



On the cold of the freezing mixtures. — The cold produced by mixing spirit of 

 nitre with snow is owing, as was before said, to the melting of the snow. Now, 

 in all probability, there is a certain degree of cold in which the spirit of nitre, so 

 far from dissolving snow, will yield out part of its own water, and suffer that to 

 freeze, as is the case with solutions of common salt ; so that if the cold of the 

 materials before mixing be equal to this, no additional cold can be produced. If 

 the cold of the materials be less, some increase of cold will be produced ; but 

 the total cold will be less than in the former case, since the additional cold cannot 

 be generated without some of the snow being dissolved, and thus weakening the 

 acid, make it less able to dissolve more snow ; but yet the less the cold of the 

 materials is, the greater will be the additional cold produced. This is conform- 

 able to Mr. Hutchins's experiments; for in the 5th experiment, in which the 

 cold of the materials was — 40°, the additional cold produced was only 5°. In 

 the first experiment, in which the cold of the materials was only — 25°, an 

 addition of at least 1 9° of cold was obtained ; and by mixing some of the same 

 spirit of nitre with snow in this climate, when the heat of the materials was 

 + 26°, I have sunk the thermometer to — 2Q°; so that an addition of 55° of 

 cold was produced. 



However extraordinary it may at first appear, there is the utmost reason to 

 think, that a rather greater degree of cold would have been obtained if the spirit 

 of nitre had been weaker ; for Mr. C. found, by adding snow gradually to some 

 of this acid, that the addition of a small quantity produced heat instead of cold ; 

 and it was not until so much was added as to increase the heat from 28° to 51°, 



