mercury continued fluid in other barometers and thermometers, the immobility and hardnefs obferved in fome of these inftruments was attributed more probably to the lead or the bifmuth, with which the mercury had been adulterated, and was not confidered as a real freezing of the mercury: but this has fince been put out of all doubt; fince it it certain that pure mercury would not freeze under fuch fmall degrees of cold, great as they were for natural cold. The experiments, which the profeffor made, in order to congeal mercury, demonftrate this moft evidently; befides which, they exhibit new phænomena.

There happened at Petersburg, on the 14th of December, 1759, a very great froft, equal, if not more intenfe, that any which had been obferved there for, betwen nine and ten o'clock in the morning, Delifle's thermometer ftood at 205; at feven o'clock, at 201; which laft was the greatest degree of cold, that had been obferved at Peterfburg, either by himself or others. At one o'clock at noon, the thermometer stood at 197. Mr. Braun had been employed feveral days before this, in obferving the feveral degrees of cold, which different fluids would bear, before they were converted into ice: partly to confirm thofe things which he had already laid before the academy; and partly to make experiments upon liquors, which had not yet been examined; as on the days between the 7th and 14th the cold was intenfe enough to be between the degrees of 181 and 191.

When the natual cold was fo intense as to be at 205, profeffor Braun conjectured, that it was of all others the most proper occafion

to try the effects of artificial cold; not doubting, but that artificial cold would be increased in proportion as the natural was more intenfe. Aquafortis, which was found by the thermometer to be 204 degrees cold, was the greatest part of it frozen, the ice having the appearance of crystals of nitre; which, however, immediately diffolved in a fmall degree of heat. This aquafortis, which though frozen at the fides, was liquid in the middle, was poured upon pounded ice, in that proportion which was directed by Fahrenheit, the firft perfon who made artificial cold with fpirit of nitre. But before the profeffor made this experiment, he, by examination, found, that both the ice and aquafortis were of the temperature with the air, which was then 204. Upon the first pouring, the mercury fell 20 degrees; this fpirit was poured off, and fresh put on, feveral times; but it was poffible, by these means, to introduce no more than 30 degrees of cold; fo that the mercury in the the mometer fell no lower than 234, Since therefore Fahrenheit coud not produce cold greater that that of 40 below the cypher of his thermometer, which corresponds with 210 of that employed by profeffor Braun; nor Reaumur, nor Mufchenbroek, who often repeated the fame experiment, our author was upon the point of giving up this purfuit; as confidering this as the greateft degree to which artificial cold could be carried; thinking it fufficient honour to himself, to have added 20 degrees to the cold formerly known.

But reflecting that this was not all the fruit he expected from these experiments, he determined to pur

fue

fue them but at the fame time, however, to vary the manner of them. By good fortune, his ice was all gone, and he was compelled. to ufe fnow in its ftead, after having first tried, and found the fnow of the fame degree of cold with the air, at this time 203. The foow, the thermometer, and the aquafortis, being of the fame temperature, he immerfed the thermometer in fnow, contained in a glafs; and, at firft, only poured a few drops of the aquafortis upon that part of the fnow, in which the thermometer was immerfed; upon which he obferved the mercury to fubfide to 260. Elated by this remarkable fuccefs, he immediately conceived hopes, that thefe experiments might be carried further nor was he deceived in his expectations; for repeating the experiment in the fame fimple manner, he poured on only fome more aquafortis, and immediately the mercury fell to 380. Upon which he immerfed the thermometer in another glafs filled with fnow, before it had loft any of this acquired cold; and at length, by this third experiment, the mercury fubfided to 470 degrees. When he obferved this enormous degree of cold, he could fcarce give credit to his eyes, and believed his thermometer broke. But, to his infinite fatisfaction, upon taking out his thermometer, he found it whole; though the mercury was immoveable, and continued fo in the open air twelve minutes. He carried his thermometer into a chamber, where the temperature of the air was 125 degrees: and, after fome minutes, the mercury being reftored to its fluidity, began to rife. But to be certain, whether this thermometer had re

ceived any injury, and whether it would yet correfpond with his thermometer, which he keeps as a ftandard, he fufpended them together, and in twenty minutes the thermometers correfponded one with the other.

The thermometers, which our author ufually employs, have a fpherical bulb, and their scale is divided into 1200 parts, of which 600 are above the cypher, which denotes the heat of boiling water, and 600 below that heat. A thermometer of this conftruction was ufed in inveftigating the heat of boiling mercury and oils. He had another thermometer, of which the fcale went no lower than 360 degrees below the cypher, denoting the heat of boiling water. He repeated the former experiment with this, and the mercury very foon defcended fo, that the whole was contained in the bulb, which, however, it did not quite fill. The mercury in this bulb was immoveable, even though he shook the thermometer; until about a quarter of an hour, it began to afcend in the open air ; and it continued to afcend, till it became higher than the circumambient air feemed to indicate. He was ftruck with this extraordinary phænomenon, and very attentively looked at the mercury in this thermometer, and found certain airbubbles interspersed with the mercury, which were not in that of the other thermometer. From these, and other experiments (it would be unneceffary to recite them all) he was fatisfied, that the mercury in these thermometers had been fixed and congealed by the cold.

Hitherto our profeffor had only feen the mercury fixed within the bulb of his thermometers. Thefe

he

a

When these were procured, the natural cold had fomewhat relented. In the former experiment, the thermometer stood at 204; it was now at 199. In making the experiment, he varied the manner little. He firft put the bulb of the thermometer into a glass of fnow, gently preffed down, before he poured on the aquafortis; he then, in another glafs, poured the aquafortis upon the fnow, before he immerfed his thermometer therein; he then, in like manner, put the Inow to the aquafortis, before he put his thermometer therein. Which ever of these ways he proceeded, he found the event exactly the fame; as the whole depended upon the aquafortis diffolving the fnow. When he had proceeded fo far, as to find the mercury immoveable, he broke the bulb of the thermometer, which had already been cracked in the experiment, but the parts were not feparated. He found the mercury folid, but not wholly fo, as the middle part of the fphere was not yet fixed. The external convex furface of the mercury was perfectly fmooth; but the internal concave one, after the fmall portion of mercury which remained fluid, was poured out, appeared rough and uneven, as though compofed of fmall globules. He gave the mercury feveral strokes with the petle of a mortar, which stood near him. It had folidity enough to bear ex

tenfion with these strokes; its hardnefs was like that of lead, though fomewhat fofter; and, upon ftriking, it founded like lead. When the mercury was extended by thefe ftrokes, he cut it eafily with a penknife. This mercury then becoming fofter by degrees, in about twelve minutes it recovered its former fluidity, the air being then 197. The colour of the congealed mercury did fcarce differ from that of the fluid; it looked like the most polished filver, as well in its convex part, as where it was cut.

The next day, the cold had increafed to 212 degrees, which was 7 degrees beyond what it had ever before been obferved at Petersburg. The feafon fo much favouring, he thought it right to continue his purfuit, not only in further confirmation of what he had already obferved, but to investigate new pha nomena. In two thermometers, he observed the fame facts in relation to the congealing mercury, as he did the preceding day. In the bulbs which he broke, the whole of the mercury was not fixed, as a very fmall portion, much leis than that of the preceding day, con. tinued fluid. He treated the mercury as he did the former; he beat it with a peftle; he cut it, and every thing was thus far the fame, But he faw a very great difference in relation to the defcending of the mercury in the thermometer, the like of which did not occur to him, neither in the former, nor any of the fubfequent experiments. From the former ones it appeared, that the mercury in the firft experiment had only defcended to 470, when it became immoveable, though the glafs bulb was not cracked. the experiment of the 25th, it de

In

fcended

fcended to 530; and in two thermometers on the 26th, to 650. But as well in the thermometer, which he used on the 25th, as in two of the 26th, the bulbs were cracked in the experiment: they cohered however; nor was the leaft part of the bulb feparated, but the congealed mercury feemed to adhere to all parts of the bulb. In the following experiments, he invariably found, that the mercury funk lower, if the whole of it was congealed, than if any part ot it remained fluid. It then generally defcended to 680 and 700, but the bulbs were never without cracks; moreover, it defcended to 800, and beyond even to 1500; but in this laft experiment the bulb was quite broke, so that the globe of mercury, thoroughly frozen, fell out, and by its fall, of about 3 feet, the globe of mercury became a little compreffed; but in the former, only fome parts of the bulb fell off.

Mr. Braun always found, that, cæteris paribus, the more intense the natural cold was, the more eafy and more expeditiously these experiments did fucceed.

In continuing thefe experiments he obferved, that double aquafortis was more effectual than fimple ipitit of nitre; but that if both the aquafortis and Glauber's fpirit of nitre, which he fometimes alfo ufed, were well prepared, the difference was not very confiderable. When his aquafortis was frozen, which often happened, he found the fame effects from the frozen parts, when thawed, as from that part of it, which remained fluid in the middle of the bottle. Simple fpirit of nitre, though it feldom brought the mercury lower than 300 degrees, by the following method he even VOL, V.

froze mercury with it. He filled fix glaffes with fnow, as ufual, and put the thermometer in one of them, pouring thereupon the fpirit of nitre. When the mercury would fall no lower in this, he, in the fame manner, put it in a fecond, then in a third, and so in a fourth ; in which fourth immerfion, the mercury was congealed.

Another very confiderable difference prefented itself in purfuing thefe inquiries, with regard to the mode of defcent of the mercury. He conftantly and invariably obferved, that the mercury descended at firft gently, but afterwards very rapidly. But the point, at which the impetus begins, is not so easy to ascertain ; as in different experiments it begins very differently, and fometimes at about 300, at other times about 350, and even further. In the experiment before-mentioned, in which the mercury fell to 800, it proceeded very regularly to 600; about which point it began to defcend with very great fwiftnefs, and the bulb of the thermometer was broke. The mercury, however, was perfectly congealed.

He frequently obferved another remarkable phænomenon ; which was, that although the spirit of nitre, the fnow, and the mercury in the thermometer, were previously reduced to the fame temperature, upon pouring the fpirit of nitre upon the fnow, the mercury in the thermometer rofe. But as this did not always happen, he carefully attended to every circumftance; from which it appeared, and this effect arofe from his pouring the aquafortis immediately upon the bulb of the thermometer, not previously well immerfed in the fnow. He likewife obferved another effect