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When the gas is one which is very soluble in the liquid, we can, by operating in tubes prepared as I have indicated, bring the solutions to a pressure sufficiently feeble, or to a temperature high enough, for the quantity of gas held by the liquid to be very considerably more than the normal quantity. If a gaseous atmosphere be then introduced to the interior of the liquid, a sort of ebullition will be determined. The experiment can very readily be realized with ammonia solution. The ordinary solution is put into a prepared tube, surrounded with a freezing-mixture, and is saturated by passing for a long time a current of gaseous ammonia. The tube containing the solution is then taken out and allowed to return to the surrounding temperature, say, of 20° C.; no gas is liberated within the liquid; but if the end of a glass tube, formed at the lamp into a small bell, containing air be put into it, ammonia gas escapes in that atmosphere, and seems to issue from the bell, in bubbles the more frequent the more pronounced the supersaturation. In this case the experiment resembles the ebullition excited in a liquid by the same process. Besides, when after some time it slackens, the liberation of the gas is made more active by raising the temperature a little.

In the Note above mentioned, I had already compared the phenomenon in question to the decomposition, under the same influence, of substances such as oxygenated water. The preparation of very concentrated oxygenated water being a delicate thing to execute, I will indicate how a known reaction (studied formerly by Schönbein*) may easily be made use of for the same demonstration.

Into a glass tube from 6 to 20 millims. in diameter, closed at one end, and recently prepared as I have said above, from 5 to 10 centims. depth of distilled water is introduced, having been filtered to rid it of the solid particles held in suspension. The tube is cooled to zero; and then some liquid hyponitric acid, previously cooled, is dropped in. This liquid, gliding along the side of the tube, passes through the water without liberating any gas, and collects at the bottom of the tube in the form of a blue liquid which is regarded as containing nitrous acid; at the same time nitric acid remains in solution in the water. The tube can then be taken out of the freezing-mixture and allowed to return to the surrounding temperature, of 15° for example, without a single bubble of gas escaping from the interior of the liquid. Tubes thus prepared I have kept 15 days in a medium the temperature of which has varied from 7° to 15°: the blue liquid had been gradually diffused, without liberation of gas, in the superposed layer of water, a certain thickness of which remained colourless. If a body without chemical action upon nitric acid, and disaerated, such as a platinum wire which has been used for some minutes to maintain the ebullition of some water, be introduced to the surface of the lower layer of the liquid, it produces no effect upon it; while the other end of the wire, which has not been cleared of the adhering layer of air, is hardly brought into contact with the nitrous acid before it excites an abun* Pogg. Ann. vol. xl. p. 382.

dant liberation of binoxide of nitrogen, which suddenly ceases if the wire be immediately withdrawn, without leaving a gas-bubble, and which recommences as soon as the wire is again immersed. At the same time the water is charged with a fresh quantity of nitric acid. This decomposition can be determined with more activity by the introduction of a little bell containing air, its outer surface having been freshly disaerated in the flame of a gas-burner. The bubbles of binoxide then seem to issue from the bell, as in the solution of ammonia. This effect, of a gaseous atmosphere decomposing nitrous acid, can be observed even at the temperature of zero (Centigrade); in this case the liberation of the binoxide of nitrogen is less rapid.

There is, then, the closest analogy between the emission of a dissolved gas, effected at the surface of the solution, into a gaseous medium as into a rarefied atmosphere, and that decomposition of explosive bodies which, as I have pointed out in the case of oxyge nated water, there is no reason to attribute to a peculiar catalytic force. Moreover the evolution of heat which accompanies the decomposition of these bodies, though slight in the case of nitrous acid, explains the rapidity with which the phenomenon proceeds as soon as it has been induced at one point of the body, unless the reaction be arrested at its starting, as I have here shown.-Comptes Rendus de l'Acad. des Sciences, Jan. 4, 1875, pp. 44-47.


In this Journal*, eight years ago, a brief notice was published of some observations made by the writer on Venus when near her inferior conjunction in 1866. The planet was then (for the first time, so far as appears) seen as a very delicate luminous ring. The cusps of the crescent, as the planet approached the sun, had extended gradually beyond a semicircle, until they at length coalesced and formed a perfect ring of light.

No opportunity has since occurred of repeating these observations, until the day of the recent transit. On Tuesday, December 8th, Venus was again in close proximity to the sun; and the writer had the satisfaction of watching the delicate silvery ring enclosing her disk, even when the planet was only the sun's semidiameter from his limb. This was at 4 P.M., or less than five hours before the beginning of the transit. The ring was brightest on the side towards the sun-the crescent proper. On the opposite side the thread of light was duller and of a slightly yellowish tinge. On the northern limb of the planet, some 60 or 80 degrees from the point opposite the sun, the ring for a small space was fainter and apparently narrower than elsewhere. A similar appearance, more marked, was observed on the same limb in 1866.


These observations were made with a five-foot Clark telescope of 4 inches aperture, by so placing the instrument as to have the sun cut off by a distant building while the planet was still visible. * Silliman's American Journal, vol. xliii. p. 129.

The ring was distinctly seen when the aperture was reduced to one and a half inch. The 9-inch equatorial could not be used, as there were no means of excluding the direct sunlight.

The morning after the transit the sky was slightly hazy, and the planet could not be found, though probably it might have been if the small telescope had been mounted equatorially.

On the day following (the 10th), the crescent, extending to more than three fourths of a circle, was seen with beautiful distinctness in the equatorial; and on this and two subsequent days, measurements were taken with the filar micrometer for the purpose of determining the extent of the cusps, and consequently the horizontal refraction of the atmosphere of the planet, on the assumption that the extension of the crescent and formation of the ring are due to this refraction.

The results of these observations are given below, each result being the mean of the number of separate measurements indicated in the last column. On the 10th, the chord of the arc between the cusps was measured ; on the other days the distance between lines tangent to the cusps and to the opposite limb.

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These observations give a mean of 44'-5 as the horizontal refraction of Venus's atmosphere, or about one quarter greater than that of the earth's. The writer's observations in 1866 gave 45'3. Mädler, from observations of the cusps in 1849, when the nearest approach of the planet to the sun was 3° 26', made the refraction 43'.7.

The formula for the refraction is this:

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in which d=distance of centres of Sun and Venus.

c arc of crescent.

r=Sun's semidiameter.

p=radius vector of Venus.

x=horizontal refraction of Venus's atmosphere.

Six measurements of the diameter of the planet on the 10th give 63"-1. Twenty-four on the 11th give 63"-75. The English and American Almanacs give 62"-4 and 64" 5 respectively.Silliman's American Journal, January 1875.






MARCH 1875.

XXI. The Specific Heat of the Elements Carbon, Boron, and Silicon.-Part I. The Relation between the Specific Heat of these Elements in the free state and the Temperature. By Dr. H. FRIEDRICH WEBER, Professor of Physics and Mathematics*.

[With a Plate.]

ULONG and Petit in 1819 measured the specific heats of thirteen solid elements. In each case there appeared a very simple relation between the specific heat and the atomic weight: the product of the specific heat into the atomic weight was a constant quantity. The atoms of all the elements examined have therefore the same capacity for heat. If the specific heat of water be taken as the unit, and 16 as the atomic weight of oxygen, this constant (the so-called atomic heat) averages 6.0. By numerous researches, extending from 1840 to 1862, M. Regnault has shown the general applicability of this law of Dulong and Petit-the general result being that this law seems to hold good for the greater number of the solid elements, provided the specific heat of any element be determined at a temperature sufficiently under the melting-point of that element. The average atomic heat for thirty-two solid elements was 6-3, the extremes being 6.7 for sodium and 5.5 for phosphorus and sulphur. For three solid elements, however (viz. silicon, boron, and carbon), considerably smaller atomic heats were obtained-for crystallized silicon 4.8, for crystallized boron 2·7, and for crystallized car

An Experimental Research presented at the fifty-sixth Anniversary of the Royal Württemberg Land- and Forest-Management Academy at Hobenheim. Translated by M. M. Pattison Muir, F.R.S.E.

Phil. Mag. S. 4. Vol. 49. No. 324. March 1875.


bon, as diamond, so low a number as 1.8 (the atomic weights of these three elements being taken, in accordance with the results of vapour-density determinations, as 28, 11, and 12 respectively).

Silicon accordingly stands considerably without the sphere to which the law of Dulong and Petit applies; boron and carbon form unmistakable exceptions to this notably simple natural law. The exceptional position of these three elements induced Regnault to subject their various allotropic modifications to a searching inquiry, in order to determine their specific heats. In his second communication on the specific heats of solid bodies he showed that the different allotropic forms of carbon are possessed of different specific heats, and that no one of these fulfils the conditions of the law of Dulong and Petit. The numbers which he gave (specific heats) are as follows:

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In a research published in 1861, Regnault obtained analogous results for boron and silicon: the following are the specific heats

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Contemporaneously with Regnault, De la Rive and Marcet examined the specific heats of two modifications of carbon by the method of cooling t. These physicists also found that the specific heat of the diamond is notably less than that of porous amorphous carbon; the specific heat of the former being given by them as 0.119, while that of the latter is 0.165. These results were an evident witness to the justice of the opinion that the physical state played as great a part as the chemical nature of the elements, as regards their specific heats, and that the law of Dulong and Petit could not, therefore, be regarded as the universal expression of the law of specific heat. De la Rive and Marcet believed that the great differences between their numbers and the numbers of Regnault could be accounted for by the fact

* Ann. de Chim. et de Phys. Ser. 3. vol. i. p. 202.

Ibid. Ser. ?. vol. lxxv. p. 242, and Ser. 3. vol. ii. p. 121.

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