solution of iodine, incandescence was obtained at the invisible focus of the lens on the roof of the Royal Institution. Knowing the permeability of good glass to the solar rays, I requested Mr. Mayall to permit me to make a few experiments with his fine photographic lens at Brighton. Though exceedingly busy at the time, he in the kindest manner abandoned to my assistant, Mr. Barrett, the use of his apparatus for the three best hours of a bright summer's day. A red heat was obtained at the focus of the lens after the complete withdrawal of the luminous portion of the radiation. § 9. Black paper has been very frequently employed in the foregoing experiments, the action of the invisible rays upon it being most energetic. This suggests that the absorption of those rays is not independent of colour. A red powder is red because of the entrance and absorption of the luminous rays of higher refrangibility than the red, and the ejection of the unabsorbed red light by reflexion at the limiting surfaces of the particles of the red body. This feebleness of absorption of the red rays extends to the rays of greater length beyond the red; and the consequence is that red paper when exposed at the focus of invisible rays is scarcely charred, while black paper bursts in a moment into flame. The following Table exhibits the condition of paper of various kinds when exposed at the dark focus of an electric light of moderate intensity. We have here an almost total absence of absorption on the part of the red paper. Even white absorbs more, and is consequently more easily charred. Rubbing the red iodide of mercury over paper, and exposing the reddened surface at the focus, a thermograph of the coal-points is obtained, which shows itself by the discharge of the colour at the place on which the invisible image falls. Expecting that this change of colour would be immediate, I was at first surprised at the time necessary to produce it. We are here reminded of Franklin's experiments on cloths of different colours, and his conclusion that dark colours are the best absorbers. This conclusion, however, might readily be pushed too far. Franklin's colours were of a special kind, and their deportment by no means warrants a general conclusion. The invisible rays of the sun possess, according to Müller, twice the energy of the visible ones. A white substance may absorb the former, while a dark substance-dark because of its absorption of the feeblest portion of the radiation-may not do so. The white powder of alum and the dark powder of iodine, exposed to the action of a source in which the invisible rays greatly surpass the visible in calorific power, exhibit a deportment at direct variance with the popular notion that dark colours are the best absorbers. § 10. In conclusion, I would briefly refer to a few experiments made to determine the calorescence obtainable through glasses of various colours. In the first column of the subjoined Table the colour of the glass is given; in the second column the effect observed when a brilliant spectrum was regarded through the glass is stated; and in the third column the appearance of a leaf of platinized platinum when placed at the focus, after the converged beam had passed through the glass, is mentioned. Another blue glass. JYellow intercepted with greatest power. All the blue end absorbed Besides the green, a dull red Extreme blue and red trans- Central portion of spectrum cut out. Dims the whole spectrum, but chiefly absorbs the green. Transmits the blue,a green band, and a band in the extreme red. Blue; ayellowish-green band and the extreme red transmitted. Dims all the spectrum: white Calorescence. Bright white. Vivid red with bright yellow in centre. No incandescence. Vivid orange. Vivid orange. Vivid orange. Red heat. Reddish-pink heat. Pink heat, passing into red. Pink heat. Barely visible red. Dull red. red, orange The extremely remarkable fact here reveals itself, that when the beam of the electric lamp is sifted by certain blue glasses, the platinum at the focus glows with a distinct pink colour. Every care was taken to avoid subjective illusion here. The pink colour was also obtained at the focus of invisible rays. Withdrawing all the glasses, and filtering the beam by a solution of iodine alone, platinum was raised nearly to whiteness at the focus. On introducing the pale-blue glass between the iodinecell and the focus, the calorescence of the platinum was greatly enfeebled-so much so, that a darkened room was necessary to bring it out in full distinctness; when seen, however, the thermograph was pink. A disk of carbonized paper being exposed at the obscure focus, rose at once to vivid whiteness when the blue glass was absent; but when present, the colour of the light emitted by the carbon was first a distinct pink; the attack of the atmospheric oxygen soon changes this colour, the combustion of the carbon extending on all sides as a white-hot circle. If subsequent experiments should confirm this result, it would follow that there is a gap in the calorescence, the atoms of the platinum vibrating in red and blue periods, and not in intermediate ones. But I wish here to say that further experiments, which I hope shortly to make, are necessary to satisfy my own mind as to the cause of this phenomenon. The incandescent thermograph of the coal-points being obtained, a very light-red glass introduced between the opake solution and the platinum reduced the thermograph both in size and brilliancy. A second red glass, of deeper colour, rendered the thermograph still smaller and feebler. A dark-red glass reduced it still more-the visible surface being in this case extremely minute, and the heat a dull red merely. When, instead of the coloured glass, a sheet of pure-white glass was introduced, the image of the coal-points stamped upon the platinum-foil was scarcely diminished in brilliancy. A thick piece of glass of deep ruby-red proved equally transparent; its introduction scarcely changed the vividness of the thermograph. The colouringmatter in this instance was the element gold, not the compound suboxide of copper employed in the other red glasses. Many specimens of gold-jelly, prepared by Mr. Faraday for his investigation of the colours of gold, though of a depth approaching to absolute blackness, showed themselves eminently transparent to the obscure heat-rays; their introduction scarcely dimmed the brilliancy of the thermograph. Hence it would appear that even the metals themselves, in certain states of aggregation, share that high diathermic power which the elementary metalloids have been found to display. I have just said that a sheet of pure-white glass, when inter posed in the path of the condensed invisible beam, scarcely dimmed the brilliancy of the thermograph. The intense calorific rays of the electric light pass through such glass with freedom. We here come to a point of considerable practical importance to meteorologists. When such pure-white glass has carbon mixed with it when in a molten condition, the resulting black glass is still eminently transparent to those invisible heat-rays which constitute the greater part of the sun's radiation. I have pieces of glass, to all appearance black, which transmit 63 per cent. of the total heat of the electric light; and there is not the slightest doubt that, in thicknesses sufficient to quench entirely the light of the sun, such glass would transmit a large portion of his invisible heat-rays. This is the glass often, if not uniformly, employed in the construction of our black-bulb thermometers, under the impression that the blackening secures the entire absorption of the solar rays. This conclusion is fallacious, and the instruments are correspondingly defective. A large portion of the sun's rays pass through such black glass, impinge upon the mercury within the bulb, and are ejected by reflexion. Such rays contribute nothing to the heating of the thermo meter. When a sheet of common window-glass, apparently transparent, was placed between the iodine solution and the platinumleaf at the focus, the thermograph was more dimmed than by the black glass last referred to. The window-glass here employed, when looked at edgeways, was green; and this experi ment proves how powerfully this green colouring-matter, even in infinitesimal quantity, absorbs the invisible heat-rays. Perfect imperviousness might doubtless be secured by augmenting the quantity of green colouring-matter. It is with glass of this description that the carbon should be mixed in the construction of black-bulb thermometers; on entering such glass the solar rays would be entirely absorbed, and greater differences than those now observed would probably be found to exist between the black-bulb and the ordinary thermometer. In conclusion, it gives me pleasure to mention the intelligence and skill displayed by my assistant, Mr. Barrett, in executing the numerous experiments committed to his care during the progress of this investigation. FELD LXVI. Chemical Notices from Foreign Journals. [Continued from p. 315.] ELDMANN* has investigated the roots of the Laserpitium latifolium, L., and has found that this plant, like many others which are botanically allied, contains a peculiar new crystallized substance to which he has assigned the name Laserpitine. The finely-cut roots are exhausted by alcohol of 80 per cent. at a temperature of 60°, and of the filtered extracts as much alcohol as possible distilled off in the water-bath. On cooling, the liquid separates into two layers—a lower aqueous layer, and an upper brownish-coloured resinous one. This latter contains, along with some resin, nearly all the laserpitine; on separating it from the other and exposing it in shallow vessels, it forms a crumbly mass of crystals. By appropriate purification these are obtained quite pure, in colourless rhombic prisms without smell or taste. When they contain some brown resin they taste bitter. They are insoluble in water, but easily soluble in alcohol, ether, benzole, &c. Chloroform dissolves more than its own weight of laserpitine. It melts at 114°; and the melted mass cools to an amorphous mass, which sooner or later passes into the crystallized condition. Amorphous laserpitine has a far lower fusing-point than crystallized, dissolves more easily in solvents, and separates again in the amorphous form. If laserpitine is heated beyond its melting-point, it volatilizes and sublimes, without decomposition, in oily drops. Laserpitine is not altered by treatment with dilute hydrochloric acid, or by being heated in a current of hydrochloric acid gas. Heated in a closed tube with concentrated aqueous hydrochloric acid, a decomposition takes place, but without any definite results. This applies also to treatment with dilute sulphuric acid. Laserpitine is decomposed by fusion with caustic potash, and also by treatment with strong alcoholic potash. To a saturated alcoholic solution of laserpitine, concentrated caustic potash was added until the precipitate at first formed was redissolved. This solution was then heated on the water-bath until all alcohol was removed. The solution was then neutralized with sulphuric acid, by which some resinous matter was separated, and the filtered solution supersaturated with dilute hydrochloric acid and distilled in a condenser as long as any acid passed over; the acid distillate was shaken with ether until the acidity was removed, and on leaving the ether to spontaneous evaporation, fine long needles Liebig's Annalen, August 1865. |