We are disposed to think, that this experiment might also be repeated very conveniently, by means of a Tor ricellian vacuum obtained by water.
As it was well known that ice suffered a gradual dimi nution by evaporation, even when the temperature was considerably below the freezing point, Mr Leslie con cluded, that this evaporation would go on still more rapidly in vacuo, and that a great reduction of tempera ture would be obtained, by placing a quantity of ice under a receiver, in circumstances similar to those we have already described, and then exhausting the air. He therefore attempted to freeze mercury, by incrusting the bulb of a thermometer with a thin coating of ice, and then exposing it in that state over a cup of sulphuric acid, under an exhausted receiver ; but the experiment was unsuccessful ; for, though the reduction of tempera ture was considerable, the mercury, owing to the imper fection of the air-pump, was not frozen. Soon after, however, he had an opportunity of trying the experiment a second time, with the very excellent air-pump con structed by Miller and Adie already mentioned, when he bad the high gratification of realizing his conjecture, and seeing the mercury in the bulb of the thermometer par tially congeal. For the sake of those who may wish to attempt this very beautiful experiment, we may mention, that the air-pump should be sufficiently powerful to bring the gage within of an inch at least of the barometri cal column, at the time of the experiment ; and that the sulphuric acid employed for absorbing the vapour of the ice, should be in the most concentrated state in which it can be procured. The crust of ice which enveloped the bulb of the thermometer in this experiment of Mr Leslie, was attached to it, by suspending the instrument from a rod, which passed through a collar of leather on the top of a receiver, and then alternately immersing the bulb in a cup of water, and exposing it, in circumstances already mentioned, to the cooling influence of evaporation in a partial vacuum. The cup of water was placed im mediately under the bulb for the conveniency of the im mersions, and removed from the receiver when the crust of ice on the bulb was sufficiently thick. The bulb with its coat of ice was then suspended about an inch abovo Cie cup containing the sulphuric acid, and the receiver was exhausted, until the gage was brought to within of an inch of the height of the barometrical column, at the time of the experiment. After the experiment had
been allowed to proceed during half an hour, a sudden contraction was observed in the mercury ; the air was re-admitted, and the thermometer being withdrawn as quickly as possible, it was found, on breaking the bulb, that a small portion of the mercury was actually in a solid state.
'We have performed this experiment with the same air-pump, by arranging the apparatus a little differently, and with a more decisive result, as the congelation of the mercury was more rapid and complete. The principal difficulty in Mr Leslie's mode of performing the experi ment, is experienced in giving the coating of ice to the thermometrical bulb. We avoided this, by taking a lump of solid ice, and giving it, by means of a piece of hot iron, the shape of a cup about ath of an inch thick. This cup was suspended in net-work, about an inch above the sulphuric acid. The mercury to be frozen, which was previously reduced to 32°, to prevent it from melting the bottom of the cup, was poured into it, and then covered with a shallow stratum of water, about the same depth as the thickness of the cup. The whole was covered with a receiver, and the air was exhausted as quickly as possible. The water above the mercury was immediately frozen, so that the mercury was completely enveloped in ice, and in contact with it. The air of the room was about 60°, and the gage was brought within Atli of an inch of the height of the barometrical column. In about 3 minutes, the ice began to exhibit an evident appearance of a rapid waste by evaporation, its surface becoming rugged and uneven. It then cracked in several places and discovered part of the mercury, which seemed to be in the solid state. At the end of 6 minutes, the air was re-admitted, and the cup of ice being taken out of the receiver and broken, the mercury was found to be completely solid. It sustained repeated blows of a ham mer, and appeared to possess an imperfect degree of malleability. A part of it being thrown into water, was immediately surrounded with a shell of ice.