Imágenes de páginas
PDF
EPUB

upon by dilute sulphuric acid, and then washed free from acid and left in water, continued to evolve pure hydrogen for the space of two months, at the ordinary temperature of the air ; in considerable quantities, at the temperature of 60° or 70° Fahr.; and in lesser quantity, but without interruption, at 320. Professor Cooke conjectured that this was owing to the zinc being thrown from the passive to an active state by the action of the acid and of the antimony; but Dr. W. F. Channing attributed it to the galvanic action developed by the acid, in partly detaching the crystals or particles of the antimony from the zinc, so as to form galvanic circuits.

Three hundred and ninety-sixth meeting. March 14, 1854. — Semi-Monthly MEETING. The PRESIDENT in the chair.

The Academy met at the house of George B. Emerson, Esq.

The Corresponding Secretary laid before the Academy a letter from Professor Peters, of Königsberg, acknowledging his election as Foreign Honorary Member of the Academy; a letter from the Museum of Practical Geology, London, acknowledging the reception of the New Series of the Academy's Memoirs to Vol. V. Part I., and Vols. I. and II. of the Proceedings; letters from the Royal Institution, the British Museum, the Linnæan Society, the Society of Antiquaries, and Chevalier Bunsen, acknowledging the reception of Vol. V. Part. I. of the Academy's Memoirs, and pp. 233 to 359 of Vol. II. of the Proceedings; and a letter from the Academy of Natural Sciences, Philadelphia, acknowledging the reception of Vol. V. Part I. of the Academy's Memoirs.

Professor Treadwell made a communication “On the Measure of Force.” In the Newtonian theory, the measure of force is the mass multiplied by the velocity, or as the momentum; according to the theory of Leibnitz and his followers, it is the mass multiplied by the square of the velocity. The philosophers who have maintained the first theory are chiefly English ; those who favor the latter theory are chiefly Continental. The object of the communication was to render intelligible, by a simple illustration, the truth of the second theory.

Dr. A. A. Hayes exhibited and described a modification of the Photometer invented by Ritchie, by which the illuminating power of two flames can be directly compared.

He alluded to the different methods by which the attainment of an accurate measurement had been sought for, by intercepting light and comparing shadows, and pointed out the objections to them ; concluding by expressing his opinion that Bunsen's mode, in its adaptation by Mr. King, with the improvements of Mr. Lewis Thompson, gives the nearest approximation to correctness which has been attained.

The instrument exhibited, in the arrangement of the two mirrors and admission of light from the two flames, was essentially Ritchie's. But the modification which renders it a more accurate indicator, and more generally useful, consists in reversing the position of the mirror-plates, and removing the oiled paper, so that the two beams of reflected light are projected downward in a small darkened chamber upon a printed page. Two rectangles of light, side by side, are thus made to illuminate a page, the printed lines on which pass across the chamber and have the same words occurring within the lighted space from each flame. The page is viewed through a partly opened leaf in front, and being at a convenient distance from the eye, a slight inequality in the light on either side is readily seen.

In making the experimental comparisons, the centre of one mirror in the instrument was placed at one hundred and twenty inches from a gas flame by moving the light pedestal supporting it, and on which it slides. A spermaceti candle burning

[ocr errors]

128 grains per hour, contained in a spring stand, was placed in line on an adjusting support. Motion of either the instrument or candle allowed the line of direction to be found and maintained constant during the experiment. The candle was allowed to burn until the projecting wick dropped its light ash away from the candle, when its burning was constant.

Dr. Hayes alluded to the fact, that the color afforded by the two beams of light was different; and this was apparent on the page, that from the gas light being nearly white, while a brown tint was given to the page by the flame of the candle. He stated that, as the light of flames is due to the ignition of solid matter, the illuminating power of any combustible cannot be inferred from its chemical composition, and although, as a general rule, those gases or vapors which deposit solid, finely divided matter by heat are found to be the best for illumination, yet the introduction of finely divided solid matter into flames composed of hydrogen or atmospheric air will produce luminous effects with the same variations in color.

In observing the page, as illuminated by the two rectangles of light, the eye soon accustoms itself to judging of the sharpness of the outline of the letters, irrespective of the color of the paper, and by retiring backward slightly, the vanishing of the letters on either side is distinctly marked, and the candle can be adjusted to produce equality. The distance of the gas light from the centre of one mirror being constant, the ratio of the light is learned by dividing the square of the gas distance by the square of the candle distance.

The illuminating power of the gas burned in this city had been the subject of his experiments, from which he obtained the result, that (for the last nine months) the light from one burner is equal to that of about twenty candles. The best solid material for illumination is the sperm candle ; the illuminating powers of wax and sperm candles are as twelve to sixteen.

Professor Gray read the following communication on the age of a large California Coniferous tree.

“ The age attained by the largest known trees is a matter of consid. erable interest; but it is seldom that an opportunity occurs of testing it by an actual counting of the annual layers of the trunk. This is said to have been done in the case of the gigantic tree recently felled near the head of the Stanislaus River, on the Sierra Nevada, California, a section of the trunk of which, at twenty-five feet from the ground and hollowed out to a shell, is now on exhibition at Philadelphia. The trunk of this tree was sound from the sap-wood to the centre’; and its annual layers are very distinct to the naked eye in pieces of the wood in my possession. The size of this tree is such as to give it a presumptive claim to rank among the oldest of the present inhabitants of the earth ; its length being 322 feet; the diameter of the trunk, at 5 feet from the ground, 29 feet 2 inches,

at 18 " " 14 " 6 6

at 200 " " 5 6 5 6 including the bark. These measurements are copied from Mr. Lobb's account of the tree, published in England, except the height (by Lobb said to be about three hundred feet), which I have given on the authority of the proprietor of the section now at Philadelphia. This section was taken at the height of twenty-five feet from the ground, and, according to the measurement of my friend, Thomas P. James, Esq. of Philadelphia, it is about twelve feet and a half in diameter, including the bark. Mr. James, at my request, has taken careful measurements of the wood itself, excluding the bark. The three diameters taken by him respectively measure 9 feet 6 inches, 10 feet 4 inches, and 10 feet 104 inches : the average diameter of the trunk at the height of twenty-five feet from the ground is a little over 10 feet 3 inches. From the statements which have appeared, it would seem as if the layers had actually been counted, and ascertained to be 3,000 in number. This surely ought to have been done ; but an examination of the statements does not prove that it was. Mr. Lobb's statement, as definite and reliable as any, is, that the trunk of the tree in question was perfectly solid, from the sap-wood to the centre ; and, judging from the number of concentric rings, its age has been estimated at 3,000 years.

“ The number of layers, therefore, has only been estimated ; and we are not in possession of the exact data on which the estimate was founded. The data wanting are the average thickness of the layers towards the centre, giving the rate of the tree's growth as a young and middle-aged tree, when it must undoubtedly, like other trees, have increased more rapidly than in later years.

“Dr. Lindley, I find, (in the Gardener's Chronicle,) has accredited the estimate which assigns to this tree an age of above 3,000 years ; stating that it may very well be true, if it does not grow above two inches in diameter in twenty years, which I believe to be the fact.' That rate would indeed give 3,500 layers at the height of five feet from the ground, where it is 29 feet 2 inches in diameter. But this measurement appears to include the bark, - to allow for which Dr. Lindley would perhaps give up the odd 500 years. There is a further consideration. At twenty-five feet from the ground the diameter of the wood is nearly 10 feet 4 inches. Here the rate of two inches in diameter in twenty years would give the trunk an age of only 1,230 years, so that, on these data, the tree in its youth would have been 1,770 years in adding twenty feet to its stature! Evidently the base of the trunk is enlarged somewhat in the manner of Taxodium and other allied trees, when old.

“ The section of the trunk at Philadelphia has been hollowed out, by fire and other means, to a shell of 3 or 44 inches in thickness. Of this I have, through the kindness of the proprietor and of Mr. James, a piece of the wood, including nearly three inches of this section. What is now wanted, and what unfortunately I do not possess, is a foot or two of the wood from the central parts of the tree, – a desideratum which may doubtless be supplied hereafter. The data at hand, however, will suffice for determining an age which the tree cannot exceed, unless it be supposed to have grown more slowly during the earlier nine tenths of its existence than during its later years, - which is directly contrary to the ascertained fact in respect 10 trees in general. Now the piece of wood in my hands exhibits an average of 48 layers in an inch. The semidiameter of the trunk at the place where it was taken is 5 feet 2 inches. If the tree increased in diameter at the same rate throughout, there would have been 2,976 annual layers; which, allowing 24 years for the tree to have attained the height of 25 feet, would give it an age of 3,000 years from the seed. This corresponds so closely with Dr. Lindley's estimate, that we may suppose him to have employed equivalent data in a similar manner. How great a deduction must we make from this estimate,

« AnteriorContinuar »