This power of generating heat seems to attend life very universally. Not to mention other well known experiments, Mr. Hunter found a carp preserve a coat of fluid water round him, long after all the rest of the water in the vessel had been congealed by a very strong freezing mixture. And as for insects, Dr. Martine* observed, that his thermometer, buried in the midst of a swarm of bees, rose to 97°. It seems extremely probable, that vegetables, together with the many other vital powers which they possess in common with animals, have something of this property of generating heat. Dr. B. doubts if the sudden melting of snow which falls upon grass, while that on the adjoining gravel walk continues so many hours unthawed, can be adequately explained on any other supposition. Moist dead sticks are often found frozen quite hard, when in the same garden the tender growing twigs are not at all affected. And many herbaceous vegetables, of no great size, resist every winter degrees of cold which are found sufficient to freeze large bodies of water. It may be proper to add, that after each of the abovementioned experiments of bearing high degrees of heat, they went out immediately into the open air, without any precaution, and experienced from it no bad effect. The languor and shaking of their hands soon went off, and they did not afterwards suffer the least inconvenience. XIII. The supposed Effect of Boiling on Water, in disposing it to Freeze more readily, ascertained by Experiments. By Joseph Black, M. D., Professor of Chemistry at Edinburgh. p. 124. "We had lately, says Dr. Black, one day of a calm and clear frost; and I immediately seized the opportunity, which I missed before, to make some * Essays Medical and Philosophical, p. 331.-Orig. + This celebrated chemical philosopher was born in 1728, at Bourdeaux, in France, of British parents. He was sent for education first to Belfast, and afterwards to Glasgow, where he studied physic and took the degree of м. D. He was afterwards appointed to read lectures on chemistry and medicine in that university; and in 1766 Dr. Cullen having exchanged the professorship of chemistry for that of the practice of physic in the university of Edinburgh; Dr. Black was appointed to succeed him; and the duties of this office he continued to discharge with increasing reputation for upwards of 30 years. In this situation, says Professor Robison, he soon became one of the principal ornaments of the university of Edinburgh, and his lectures were attended by a crowded audience. It could not be otherwise. His personal appearance and manners were those of a gentleman, and peculiarly pleasing. His voice in lecturing was low but fine; and his articulation so distinct, that he was perfectly well heard by an audience consisting of several hundreds. His discourse was so plain and perspicuous, his illustration by experiment so apposite, that his sentiments on any subject never could be mistaken; and his instructions were so clear of all hypothesis or conjecture, that the hearer rested on his conclusions with a confidence scarcely exceeded in matters of his own experience. Dr. B.'s constitution was never strong, and for many years preceding his death, he had been subject to a spitting of blocd, which he had prevented from proceeding to an alarming length by a very abstemious diet and remarkable serenity of mind. His bodily strength, however, declined very visibly experiments relative to the freezing of boiled water, in comparison with that of water not boiled. I ordered some water to be boiled in the tea kettle 4 hours. I then filled with it a Florentine flask, and immediately applied snow to the flask, till I cooled it to 48° of Fahrenheit, the temperature of some unboiled water which stood in my study in a bottle; then putting 4 oz. of boiled, and 4 of the unboiled water, separately, into 2 equal tea cups, I exposed them on the outside of a north window, where a thermometer pointed to 29°. The consequence was, that ice appeared first on the boiled water; and this, in several repetitions of the experiment, with the same boiled water, some of which were made 9 hours after it was poured out of the tea kettle. The length of time which intervened between the first appearance of ice on the 2 waters, was different in the different experiments. One cause of this variety was plainly a variation of the temperature of the air, which became colder in the afternoon, and made the thermometer descend gradually to 25°. Another cause was the disturbance of the water; when the unboiled water was disturbed now and then by stirring it gently with a quill tooth-pick, the ice was formed on it as soon, or very nearly as soon, as on the other; and from what I saw, I have reason to think, that were it to be stirred incessantly, provided at the same time the experiment were made with quantities of water, not much larger or deeper than these, it would begin to freeze full as soon. In one of these trials, having inspected my tea cups during 1798 and 1799; and on the 26th of Nov. of the last-mentioned year he expired suddenly, while at table, with his usual fare, some bread, a few prunes, and a measured quantity of milk diluted with water. He had the cup in his hand when the last stroke of his pulse was to be given, and had set it down on his knees, which were joined together, and had kept it steady with his hand, in the manner of a person perfectly at his ease: and in this attitude he expired, without spilling a drop, and without a writhe on his countenance. His servant thought he had been asleep. This euthanasia happened when he was in his 71st year. When we take a view of Dr. B.'s experiments on magnesia and quicklime proving that the causticity in burnt lime and alkalies, is owing to their being deprived of fixed air (carbonic acid) with which they are combined in their mild state; and of the experiments which he made on the conversion of water into steam, showing the difference between sensible and latent heat, (in which originated the great improvements made by his pupil Mr. Watt in that admirable and most useful mechanical apparatus, the steam engine) when we take a view of these experiments, from whence as, from a centre, have radiated the brilliant discoveries in pneumatic chemistry of several contemporary philosophers, we shall be fully satisfied that they who have pronounced Dr. B. to have been one of the greatest chemists of the 18th century, have by no means over-rated his scientific character. Besides his inaugural dissertation De Ácido a Cibis Orto, and his Experiments on Quicklime abovementioned, and the present paper in the Phil. Trans., Dr. B. published an Analysis of the Waters of some Boiling Springs in Iceland, (see the Trans. of the R. s. of Edinburgh). And after his death the world was favoured with the publication of his Lectures on Chemistry, in 2 vols. 4to., 1803, by his intimate friend Mr. Robison, Professor of Natural and Experimental Philosophy in Edinburgh; from whose and Dr. Ferguson's account of the author, the above particulars have been taken. when they had been an hour exposed, and finding ice on the boiled water, and none on the other, I gently stirred the unboiled water with my tooth-pick, and saw immediately fine feathers of ice formed on its surface, which quickly increased in size and number, till there was as much ice in this cup as in the other, and all of it formed in one minute of time, or 2 at most. And in the rest of the trials, though the congelation began in general later in the unboiled water than in the other; when it did begin in the former, the ice quickly increased so as, in a very short time, to equal, or nearly equal in quantity, that which had been formed more gradually in the boiled water. The opinion, therefore, which I have formed from what I have hitherto seen is, that the boiled and common water differ from one another in this respect; that whereas the common water, when exposed in a state of tranquillity to air that is a few degrees colder than the freezing point, may easily be cooled to the degree of such air, and still continue perfectly fluid, provided it still remain undisturbed: the boiled water, on the contrary, cannot be preserved fluid in these circumstances; but when cooled down to the freezing point, if we attempt to make it in the least colder, a part of it is immediately changed into ice; after which, by the continued action of the cold air on it, more ice is formed in it every moment, till the whole of it be gradually congealed before it can become as cold as the air that surrounds it. From this discovery it is easy to understand, why they .find it necessary to boil the water in India, in order to obtain ice. The utmost intensity of the cold which they can obtain by all the means they employ, is probably not greater than 31° or 30° of Fahrenheit's thermometer. Common water, left undisturbed, will easily descend to this degree without freezing; and, if they have not the means of making it colder, may continue fluid for any time, provided it be not disturbed: the refrigerating causes of that part of the world when they have done so much, have done their utmost, and can act no further on the water. But this cannot happen to the boiled water; when the refrigerating causes have cooled it to 32°, the next effect they produce, is to occasion in it the beginning of congelation, while the water is afterwards gradually assuming the form of ice, we know, by experience, that its temperature must remain at 32°; it cannot be made colder, so long as any considerable part of it remains unfrozen.* The refrigerating causes continue therefore to have power over it, and to act upon it, and will gradually change the whole into ice, if their action be continued sufficiently long. The next object of investigation may be the cause of this difference between the boiled and the common water. In considering this point, the following idea * Common water, when cooled in a state of tranquillity to several degrees below the freezing point, will suddenly rise up to it again, if disturbed in such a manner as to occasion in it a beginning of congelation.-Orig. was suggested. As we know from experience, that by disturbing common water, we hasten the beginning of its congelation, or render it incapable of being cooled below 32°, without being congealed; may not the only difference between it and boiling water, when they are exposed together to a calm frosty air, consist in this circumstance; that the boiled water is necessarily subjected to the action of a disturbing cause, during the whole time of its exposure, which the other is not? One effect of boiling water long, is to expel the air which it naturally contains; as soon as it cools, it begins to attract and absorb air again, till it has recovered its former quantity; but this probably requires a considerable time. During the whole of this time, the air entering into it must occasion an agitation or disturbance in the water, which, though not sensible to the eye, may, be very effectual in preventing it to become, in the least, colder than the freezing point, without beginning to freeze, in consequence of which, its congelation must begin immediately after it is cooled to that point. When I reflect on this idea, I remember a fact which appears to me to support it strongly. Fahrenheit was the first person who discovered that water, when preserved in tranquillity, may be cooled some degrees below the freezing point without freezing. He made the discovery while he was endeavouring to obtain ice from water that had been purged of its air: with this intention he had put some water into little glass globes, and having purged it of air, by boiling and the air-pump, he suddenly sealed up the globes, and then exposed them to the frosty air. He was surprized to find the water remain unfrozen much longer than he expected," when at last he opened some of his globes, in order to apply a thermometer to the water, or otherwise examine what state it was in. The immediate consequence of the admission of the air was a sudden congelation, which happened in the water; and in the rest of his globes, a similar production of ice was occasioned by shaking them. The inference that may be drawn from these experiments of Fahrenheit's, is sufficiently obvious; it appears to remove all doubt with regard to the above supposition. Before these experiments of Fahrenheit occurred to my memory, I had planned a few, suggested by the above supe position, that might have led to the same conclusion; but the short duration of the frost, for one day only, did not give me time to put them in execution. XIV. Experiments on the Dipping-Needle, made by Desire of the R. S. By Thomas Hutchins. p. 129. In these experiments the instrument was placed in 4 several positions, viz.. with the index placed east, and then placed west, with the poles of the needleplaced one way, and then the same with the poles changed or reversed. In each. of these 4 positions, the dip was taken and noted down 3, 4, or 5 times. And the mediums of all these, for the several places, are as follow. 1. At Stromness in the isles of Orkney, lat. 58° 59′ N., long. 3° 30' w. from London, June 9, 1774. The mean dip was 75° 51′. In these observations the needle was placed horizontal, and the vibration continued between 9 and 10 minutes. The instrument was set in the middle of a room up one pair of stairs; but being apprehensive that the iron grate, fender, poker, and tongs, might, in some measure, affect the needle, trial was made in the open air, and in a place free from such obstacles. 2. On the Holms in the entrance of Stromness Harbour, June 23, 1774. Variation per azimuth 24° westerly. Long. from London 3° 30' w. lat. 58° 59′N. Dip. 75° 40'. The needle in all these observations was left to vibrate from an horizontal position. The instrument was set on the top of the case in which it was packed, and stood in the open air, in a fine sunny day. 3. In Hudson's Straits, July London, variation 43° westerly. 23, 1774, lat 62° 3′ N., long. 69° w. from Mean dip, 82° 42′. The needle vibrated from an horizontal situation. These observations were made on a large piece of ice, to which the 3 ships were grappled. Dip 83° 11'. lat. 62° 25 N, long. 71° 30′ w. Dip 82° 46'. 4. In Hudson's Straits, July 27, 1774, lat 62° 23′ N., long. 71° 30′ w. from London, variation 42° 50′ westerly per azimuth. 5. In Hudson's Straits, July 28, 1774, from London, variation per azimuth 44° w. 6. In Hudson's Bay, August 14, 1774, lat. 50° 53′ N., long. 85° 22′ w. from London, variation per azimuth 24° w. Dip. 82° 41'. These experiments were made in the cabin of the Prince Rupert, while she lay among ice. The ship frequently varied the position of her head a point of the compass; but by replacing the instrument as often as was found necessary, there was the greatest reason to think these observations, which took up above 3 hours, are pretty accurate. 7. At Moose Fort in Hudson's Bay, September 8, 1774, lat. 51° 20′ N., long. 82° 30' w. from London, variation 17° w. Dip 80° 13'. The observations were made on shore. So remarkable a difference between them, when Mr. H. was expecting quite the reverse, surprized him as much as the increased inclination of the needle from observations made nearly in the same parallel of latitude in London. He endeavoured, by drawing a magnetical meridional line with chalk, and paying the greatest attention to keeping the instrument perfectly steady and horizontal, to render these experiments accurate, and fulfil the intention of the R. s. 4 |