Графични страници
PDF файл
ePub

botanical drawings of new plants, that had been commenced in the Royal Gardens at Kew, by the excellent artist Mr Bauer.

Sir Joseph Banks died on the 19th May 1820, leaving no issue. The Royal Society elected for their President Sir Humphrey Davy, who will equal him in all his good qualities, and who will not give rise to the same objections ; for, young as he still is, his discoveries are among the most admirable of the age. Sir Humphrey Davy was already before this a foreign member of the Institute; and the Academy of Science has named, in the place of Sir Joseph Banks, M. Gauss, Professor of Gottingen, to whom his excellent labours in the mathematics long gave a title to that honour.

Remarks and Experiments relating to Hygrometers and Evaporation. By Mr HENRY MEIKLE. Communicated by the Author.

IT is now pretty generally admitted, that hygrometers, formed of absorbent substances, being necessarily of a changing or perishable nature, are extremely liable to have their sensibility impaired through length of time; so that little confidence can be placed in them, however accurately they may have been at first constructed. Nor is there much reason to expect that two such hygrometers will agree, unless the one have been made from the other, or both have been graduated from some less vague instrument; but even admitting that they did agree, what security have we that such accordance shall continue ? Professor Leslie's hygrometer is entirely free from this objection, as likewise Mr Daniell's, and some modifications of it proposed by Mr Jones and others. The principle of the latter sort is to cool down an even or polished surface exposed to the air, till a deposition of moisture begin to adhere to it; and if we could easily and accurately ascertain this reduced temperature, we should be enabled to determine the state of the air with regard to moisture. The cooling principle here employed, as the most convenient, is the evaporation of ether ; and for that purpose, a supply of this costly liquid, of rather a superior quality, must be constantly carried along with the instrument,

dered so.

By means of his ingenious researches, and particularly his valuable “ Essays,” Mr Daniell has contributed greatly to improve and extend the science of hygrometry. But without meaning at all to detract from the undoubted merit of these labours, I am not convinced that his hygrometer is either the most convenient and distinct, or even susceptible of being ren

Whoever has attended to such instruments, must have remarked, that the instant of incipient deposition is not well defined. This defect, to be sure, is not so conspicuous when the instrument is small, and the temperature changing rapidly ; but if a cold liquid be put into a pretty large vessel, along with a sensible thermometer, it will be found, that even then the temperature of deposition cannot be ascertained with much nicety; and, of course, the uncertainty must be greater with a small fickle instrument moving by starts. Besides, good light and acute eyes are quite indispensable, simultaneously to observe the fleeting temperature, and the corresponding but ill defined commencement of the formation of dew.

On the other hand, when we use a thermometer depressed by the evaporation of water, as is the case with Professor Leslie's hygrometer, this may be observed with the greatest nicety and deliberation. A more legible indication is unnecessary; and its certainty and sensibility are placed beyond a doubt, by the exact agreement of several such instruments. Nothing, indeed, is wanting to remove prejudices, and give perfect confidence to this simplest of all hygrometers deserving the name, but a greater number of experiments by different observers. Even supposing that some imperfection did attach to its theory, yet more extensive observations could soon settle or correct this; but an infinite number of observations could not make the instant of deposition well defined, whilst in the nature of things it is otherwise.

It is much to be wished, that accurate experiments were made by different individuals, to ascertain the quantity of moisture which can exist in the air at low temperatures. Authors of great celebrity differ so widely on this head, that it is doubtful if any of them be quite correct. To attempt to ascertain the force of vapour at low temperatures by means of a column of mercury, is out of the question ; because such a method is liable to

so many errors, that no confidence can be put in it. Mr Dalton, to whom this department of science owes so much, has made many experiments in this way to ascertain the force of aqueous vapour at the freezing point; and these seem to have been very inconsistent, as Mr Dalton only concludes from them, that this force is not greater than three, nor less than two tenths of an inch of mercury.* The latter is Dr Ure's estimate; and Mr Southern makes it 0.16 inch. At lower temperatures, I suspect our tables are little else than a guess.

But it is not less remarkable, that as great a diversity of opinion should exist regarding the density of steam at the boiling point, or still higher temperatures. Thus M. Gay Lussac makes it to that of air, of the same temperature and pressure, as 5 to 8. Sir Humphrey Davy again alleges, that steam just occupies the same volume as a mixture of its constituent gases does under like circumstances. Its specific gravity would thus be to that of air as 5 to 12, if not rather lower f.

I shall now proceed to give an account of some experiments of a different sort from those just mentioned, but connected with this subject; and which I should be glad to find carefully repeated by others. In order to determine how far the depression of a wet thermometer inclosed with some drying substance, is affected by the capacity of the vessel, I made the following experiments. Into a wide mouthed bottle capable of holding an imperial gallon, I put a quantity of sulphuric acid sufficient to cover its bottom to a small depth ; and from the middle of the cork I suspended two thermometers mounted upon one broad scale. Their balls were about an inch separate, and on the same level ; the one was covered with wet linen, and the other naked. At the time of putting in the acid, the whole interior surfaee of the bottle had been wetted with it; and after the moist thermometer became considerably depressed, I wetted the interior surface afresh with acid, and then moved the bottle gently, so as to agitate the thermometers considerably. This I had also done from the beginning. After fully half an hour, the dry thermo

Manchester Memoirs, Second Series, iii. 473. + Annales de Chimie, lxxx. 218; Supp. Encyc. Britan. Art. Steam Engine,

P. 535.

meter indicated 530.5 Fahr. and the moist 40°.4, giving a depression of 13°.1. The height of the barometer was nearly 30 inches; but exactness in this, as we shall shortly see, was of no moment.

There is, however, reason to think, that a quantity of air, confined in a vessel along with a drying substance, such as sulphuric acid, can never be rendered perfectly dry, if it, at same time, contain the bulb of a thermometer covered with wet linen"; because this, to a certain extent, will continually supply it with moisture, which must require some time to pass to the acid and be there absorbed, and the more so as the vessel is larger *.

To try the effects of a smaller vessel, I put a little sulphuric acid into a small spheroidal flask having about the 27th of the capacity of the former; and introducing a single thermometer with its ball moistened as before, I fastened its stem in the neck of the flask. To note the temperature of the included air, I kept the flask and another thermometer immersed in a jar of water, which was frequently stirred. The flask was often turned on its side, rolling it round to keep the interior surface wetted with acid. At the end of about ths of an hour, the full effect seemed to have been attained. The thermometer, in the water, stood, as from the beginning, at 53° Fahr. and that within the flask at 39o.9, giving a depression of 13°.1 as before. This and the first experiment were repeated some days after, with the same result +

From these experiments I was almost led to the conclusion, that if the interior surface of a bottle be kept wet with acid, its size should be of no consequence. But reflecting, that the balls of the thermometers, in the larger bottle, had been kept in motion, and that within an inch of the acid in the bottom, I was induced to try what effect it would have to fasten the moist ball as nearly as possible in the centre of the larger bottle. Upon doing so,

• Various liquids are known to produce greater cold in the surface from which they evaporate than water does. Yet it is curious, that so volatile a fluid as oil of turpentine should have no effect in this way; and a covered thermometer, first dipped in oil of turpentine, and then in water, undergoes the same depression as if no turpentine were present.

+ The like coincidence I find, obtains at 80° Fahr., the depression then amounts to 24.03.

the effect was considerably less, especially when the sides of the bottle were dry: as the depression, in that case, was only 53° 41°.2–11.98; and by repeatedly wetting the sides with acid, as was done with the small flask, the utmost effect was only 53o.1 -40°.4=120.7. But by suspending the thermometers, and making them vibrate near the bottom as before, the depression reached 53-399.9=13.1, as in the former experiments; though such perfect coincidence may have been a little accidental.

In the small flask again, with acid only in its bottom, and its sides dry, the result was 52.6—42.4=10°.2. To do justice to such experiments, a considerable time must be spent on them ; for though, in the open air, the wet thermometer soon attain its utmost depression, yet, in a close vessel, particularly a small one, it proceeds with extreme slowness, and at a retarded rate. Those, who are not aware of this circumstance, are apt to obtain deficient results. Motion, in the free air, hastens the depression, but unless it be rapid, it scarcely affects its amount. On the other hand, a dry thermometer rises a little, by being moved swiftly through the air.

Wishing to try the effects of different atmospheric pressures on the depression, I placed the double thermometer over a broad saucer of sulphuric acid on the plate of an air-pump, and covered the whole with a receiver. The following are the results at dif

ferent pressures :

Inches.
29.7 48.2 36.6 11.6
19.4 47.3 33.2 14.1
17.2 47.2 32.5 14.7
13.3 47.0 31.2 15.8

8.8 46.4 27.2 19.2 The first column is the barometric pressure in inches ; the second the Fahrenheit temperature of the dry thermometer ; the third that of the moist, and the fourth their difference or the depression *

Experiments of this sort take such a length of time, that I only, at first, intended to have tried three different pressures, the 1st, 2d, and 5th, differing about ten inches, but before concluding, I added the other two, to come close upon the freezing point; though both, as we shall shortly see, should, when corrected, be above 32°.

« ПредишнаНапред »