VOL. LXXIII.] PHILOSOPHICAL TRANSACTIONS. 431 



was less intense by 4%, as the thermometer g sank to — 40%. It is true, that 

 the temper of the air was much less cold, namely, 35° ; but the spirit of nitre 

 was at least as cold, and the snow not much less so. The experiment was tried 

 in the same vessel and with the same precautions as the former. The cold pro- 

 duced by mixing oil of vitriol, properly diluted with snow, is not so great as that 

 procured by spirit of nitre, though it seems not to differ from it by so much as 

 8° ; for a freezing mixture, prepared with diluted oil of vitriol, whose specific 

 gravity, at 6o° of heat, was 1.5642, sunk the thermometer g to — 37°, the 

 experiment being tried at the same time, and with the same precautions, as the 

 foregoing. It was previously found, by adding snow gradually to some of this 

 acid, as was done by the spirit of nitre, that it was a little, but not much 

 stronger than it ought to be, in order to produce the greatest effect. 



XXI. History of the Congelation of Quicksilver. By Chas. Blagden, M. D., 



F. R. S. p. 32Q. 



The late experiments at Hudson's Bay have determined a point, on which 

 philosophers not only were much divided in their opinion, but also entertained, 

 in general, very erroneous sentiments. Though many obvious circumstances 

 rendered it improbable, that the term of mercurial congelation should be 5 or 

 600 degrees below of Fahrenheit's scale, as had been at first supposed ; yet 

 scarcely any one ventured to imagine that it was short of 100°. Mr. Hutchins, 

 however, has clearly proved, that even this number is far beyond the truth ; 

 and that quicksilver freezes in a degree of cold not exceeding that which some- 

 times occurs in the northern parts of Europe, and frequently in the more 

 rigorous climates of Asia and America. 



It was undoubtedly M. Braun, professor of philosophy in the Imperial 

 Academy at Petersburg, who first, on decisive evidence, established the fact, 

 that quicksilver can be made solid by a diminution of its heat. M. Braun under- 

 took the experiments at the suggestion of Dr. Zeiher, professor of mechanics in 

 the same academy, who having repeated Fahrenheit's experiments with frigorific 

 mixtures in Germany, before he came to settle at Petersburg, wished to try whe- 

 ther they might not be prosecuted further in the great natural cold which some- 

 times prevails in that city. Illness prevented Dr. Zeiher from carrying his ideas 

 into execution ; he therefore communicated them to Professor Braun, who was 

 already much conversant in thermometrical experiments, and engaged him to 

 take up the subject of artificial cold whenever the weather should be favourable 

 for this purpose. A proper opportunity occurred on the 14th of December, 

 175g, o. s. the thermometer sinking in the open air so low as — 34° of Fahren- 

 heit's scale, which we now know to be within a few degrees of the point at 

 which mercury freezes. M. Braun accordingly prepared a frigorific mixture with 



