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served. I expect it will enable me to determine the magnetic equivalent of heat.-Comptes Rendus de l'Acad. des Sciences, March 23, 1874, vol. lxxviii. pp. 845-847.

EXPERIMENTAL RESEARCHES LEADING TO A DETERMINATION OF THE TEMPERATURE OF THE SUN. BY FATHER A. SECCHI.

During the last summer I made some experiments for the purpose of determining the ratio between the solar radiation and that of the electric light, in order to solve, if possible, the question of the temperature of the sun. I chose this source of light as that which is the least remote in intensity from that of the sun, so as to lessen the divergence of opinion to which we have been conducted concerning the law of radiation, according as the theory of Newton or that of Dulong and Petit has been adopted.

To estimate the two radiations I have employed the same apparatus, the thermoheliometer described in my work Le Soleil. This instrument, notwithstanding the objections which have been made to it, appears to me especially suitable for determining, as in the present case, simple differences. If I, and I, denote the absolute intensities of the radiations of the sun and the carbon points, 0, and O, the excesses of temperature of the black-bulb thermometer above the temperature of the vicinity in the cases of the solar and the electric radiation respectively, and a and d the apparent diameters of the radiant surfaces viewed from the centre of the blackened thermometer, we have

0, I, tan2 d

whence

=

θε

Le tan' a'

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In practice it is very difficult to determine the radiating surface of the carbons: they are in general very brilliant at the point; but thence their incandescence diminishes rapidly, and, besides, the arc which separates them has a very different radiation. We have sought to determine the surface of the radiating part of the carbons by comparing their dimensions with those of glass tubes placed very near, and valuing the distance at which a fine platinum wire entered fusion without touching them. We have thus obtained a surface almost rectangular, equal to that of a circle of 1 centim. diameter. The radiation from the parts outside this limit was intercepted by diaphragms. The pile consisted of 50 Bunsen elements supplied with fresh acids (nitric acid of 40 degrees, sulphuric acid diluted with nine times its weight of water). The elements had a diameter of 12 centims., and a depth of 20 centims. The conductors, of copper, were short and very stout; the intensity of the current was such that the insulating disks of a Foucault apparatus were almost instantly melted, and an iron wire 1 millim. in diameter and 2·5

metres long could constantly be brought to a white heat. Doubtless these are very vague indications for estimating the temperature and the intensity of the current; but they suffice to give an idea of the conditions of the experiment.

The thermoheliometer being placed on a level with the carbons, and the blackened thermometer at a distance of 395 millims., there was found, at the end of half an hour, a difference of 3°.63 between the temperature of the other thermometers and that of the blackbulb; this difference remained constant for an hour, the fluctuations being insignificant.

The temperature produced by solar radiation was determined about noon on several days in July, and with the same instrument. For the difference there was found 17°.16, a quantity which agrees very well with that which was obtained several years since. To this value, however, must be applied the correction due to the absorption produced by our atmosphere; taking into account the height of the sun at the time of the observation, we are led to the value 17°.37.

Substituting these values in the preceding formula and calculating the diameters a and & from the dimensions and distances of the radiant areas, we get

I, IX 36-468;

so that the solar radiation would be thirty-six and a half times that of the carbons. Nevertheless this valuation is below the truth; for we know that the correction made for the absorption by the atmosphere is too small. M. Soret found directly, on Mont Blanc, 21°13; probably the value at the upper limit of our atmosphere would be about 27°. These two values would give respectively

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These ratios differ considerably from those which have been given by other observers. In the fear that there might be some cause of enormous error in my electric light, I compared it with the light of a stearine taper; I found that it was equal to 1450 tapers of commerce; it possessed therefore quite the intensity ordinarily exhibited by a good pile. In another series of experiments, made after the pile had been worked for some time, I found I, I, × 47.5, a result not very far from that at which I arrived above with the temperature 21° 13 obtained directly by M. Soret.

Adhering, then, to this temperature of 21°13, which is incontestable and certainly less than the reality, and supposing that the temperature of the radiating surface of the carbons is 3000° (a number which is not exaggerated, since the whole extent of the platinum submitted to the experiment was fused), and supposing the radiation proportional to the temperature, we obtain for the potential temperature of the sun 133780°. This value may even

be raised to 169980° by adopting the number of 27° as produced by the solar radiation.

I have already remarked, in my work on the Sun (p. 270), and M. Hirn has lately repeated it, that the temperature of the radiation may depend either solely on the superficial stratum of the sun, or on a considerable thickness of its substance, according as this is opaque or transparent. M. Hirn concludes that, if the transparence were nearly perfect, the temperature might well be only a few thousand degrees; but divers phenomena prove that, on the contrary, the transparence is very imperfect. In one of my preceding communications I reported the singular observation that the currents of the penumbra cross and pass one above another; in that case the upper currents completely conceal the lower; so that the mass of the photosphere has no sensible transparency. This observation of the crossing of the currents has lately been confirmed by Mr. Langley. The defect of transparency can also be established by observation of the thick jets of the metallic protuberances, in which one branch does not permit the other to be seen through its thickness. Without admitting absolute opacity (for the very strong light of the photosphere may well prevent the lower strata from being distinguished), it is certain that the photosphere is not completely transparent; for otherwise the margin of the sun's disk would not appear sharply defined, but diffuse.

The temperature above indicated is therefore not inadmissible, It is doubtless very far from the number which would be given by the direct application of Newton's law; but it is also very far from that which would result from the law of Dulong and Petit. It seems to me that these comparative experiments almost completely eliminate the considerations drawn from the theoretic law, and that they give a lower limit of the temperature of the sun.

Besides, if the solar temperature reached only a few thousand degrees, its cooling would be sensible in a relatively brief interval of time; and this diminution of temperature would betray itself by a notable acceleration of the rotation of the sun. Doubtless the excess of velocity observed at the solar equator, which it has uselessly been attempted to account for by currents analogous to our trade-winds, is due to this cooling; for, though very slight, it is by no means nil. M. Faye's law, deduced from Carrington's, indeed results from the simple comparison of the diminution which the areas of the equatorial circles and the parallels must undergo by the cooling of the mass estimated in their respective planes of rotation. This progressive action must maintain constantly this difference of velocity; so that there is no need to have recourse, with M. Roche, to a remote crisis; for in that case the mass would soon have arrived at uniformity in consequence of friction.

The activity of the sun not being constant, it follows that neither are so its losses by radiation, and consequently that the apparent

* American Journal of Science, 3rd Series, vol. vii., February 1874.

rotation of the sun, or rather of its photosphere, is variable-which accords with the results obtained by Spörer.-Comptes Rendus de l'Académie des Sciences, March 16, 1874.

ON HYDROGENIZED PALLADIUM.

BY MM. L. TROOST AND P. HAUTEFEUILLE.

The remarkable property possessed by palladium, of absorbing as much as 982 times its volume of hydrogen gas, discovered by Graham, was at first presented by him as a phenomenon comparable to solution or condensation; and for it he invented the term occlusion.

Graham subsequently thought that palladium forms with hydrogen an alloy with "equal equivalents." This opinion is stated in the memoir in which he proves that the density of palladium charged with from 800 to 900 volumes of hydrogen is sensibly inferior to that of the pure metal, that the tenacity and electric conductivity are diminished as in the case of alloys in general, and the magnetism augmented as when palladium is combined with a very magnetic metal.

His conclusions have been generally accepted, although (as the author himself remarks) the maximum of 982 volumes of hydrogen fixed corresponds to only 0.772 of an equivalent of hydrogen for 1 equivalent of palladium (H=1; Pa =106·5).

In a recent work M. Favre, like Graham, thinks that "the hydrogen fixes itself on its equivalent of palladium," on the ground that, within the limits of his experiments, the metal disengages sensibly equal quantities of heat in absorbing equal weights of hydrogen.

We shall prove that the phenomenon is more complex than has hitherto been supposed.

We will examine successively the two following points:-(1) Does the hydrogen form a true combination with the palladium, or is it merely dissolved in that metal? (2) If there is a combination, what is the formula of the compound produced?

The study of the tensions taken by the hydrogen disengaged at various temperatures by hydrogenized palladium will furnish us with the elements necessary to solve both these questions apart from any hypothesis. We know, in fact, that compounds formed directly, by the combination of a solid with a gaseous body, are partially decomposed by heat, the decomposition being measured by a tension invariable for each temperature and independent of the quantity of undecomposed product; it is the tension of dissociation of the chemical combination. On the contrary, substances

* M. Favre has not indicated the volumes of gas corresponding to his different determinations.

which have dissolved gases (such as water charged with carbonic acid), and those which have condensed it (such as platinum-black charged with hydrogen), emit gases which, for one and the same temperature, have tensions variable with the state of saturation of the material. It is by the study of the tensions that we have arrived at the recognition of the simultaneous production of a definite compound and of a solution of hydrogen gas.

The palladium, saturated with hydrogen at the negative pole of a voltameter, was introduced into a glass tube communicating at one extremity with a manometer, and at the other with a Sprengel pump, which permitted a vacuum to be produced at the commencement, and afterwards, in the course of the experiment, determined volumes of gas to be expelled*.

If the operation take place at about 100°, successively increasing quantities of gas being taken away, we obtain, with either cast or spongy palladium, the results inscribed in the following Table, which contains the observed pressures for different degrees of saturation.

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The Table shows us :-(1) that as long as the volume of hydrogen fixed is more than 600 times the volume of the palladium, the pressure decreases very rapidly at each subtraction of hydrogen, which is the character of a solution; (2) that the pressure becomes con

As palladium saturated with gas disengages hydrogen at the ordinary temperature, it is necessary, if we wish to know exactly the total volume of gas absorbed, to put the metal, on taking it out of the voltameter, into a small balloon full of boiling water and furnished with a discharge-tube, The temperature of the balloon is raised to the boiling-point of water, and the gas discharged is collected. After cooling, the metal can be handled in order to introduce it into the manometric apparatus without fear of losing any gas. This preliminary operation is indispensable when the forged metal is operated on, which, when withdrawn from the voltameter, grows rapidly hot in contact with the air, in consequence of the combustion of the hydrogen.

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