oxalate, forming water; the carbon is saturated with oxygen, forming carbonic acid gas; and a part, if not the whole of the nitrogen of the ætherized gas is separated in the state of nitrogen gas; both which gases are evidently produced after the decomposition of the powder. The mercury is now revived, and converted into vapour, as may be gathered from the immense quantity of caloric extricated, by adding concentrate sulphuric acid to the mercurial powder. On a more minute analysis he finds the proportion of these ingredients in 100 grains of the mercurial powder to be as follows: Of pure oxalic acid .... Of mercury formerly united to the oxalic acid....60·72 Total of mercury. Of nitrous ætherized gas and excess of oxygen Grains. 21.28 2.00 2:00 64.72 14:00 100.00 The following are the principal properties of this singular powder: -it takes fire at the temperature of 368° of Fahrenheit; it explodes by friction, by flint and steel, and by being thrown into concentrate sulphuric acid. It is equally inflammable under the exhausted receiver of an air-pump, as when surrounded by atmospheric air; and it detonates loudly, both by the blow of a hammer and a strong electrical shock Its action, though extremely powerful, is however confined within a very limited sphere. It will burst a gun-barrel, though it will not carry a ball to any considerable distance. Mr. Howard does not fail to caution future operators concerning the experiments they may be tempted to make on this powerful agent, having himself suffered considerably from an instantaneous explosion, produced by pouring six drams of concentrated sulphuric acid upon fifty grains of the powder, which wounded him severely, and destroyed the best part of his apparatus. This uncommon elastic power is ultimately ascribed to the gas and caloric suddenly set at liberty, and to the mercury and some portion of water being converted into vapour. The paper concludes with some observations on other fulminating powders, where the author acknowledges that he has in vain attempted to communicate fulminating properties, by the mercurial process, to gold, platina, silver, antimony, tin, copper, iron, lead, zinc, nickel, bismuth, cobalt, arsenic, and manganese; mercury being as yet the only metal which he has found to have a joint affinity with nitrous ætherized gas and oxalic acid, or to be capable of combining with nitrous ætherized gas. Investigation of the Powers of the prismatic Colours to heat and illuminate Objects; with Remarks, that prove the different Refrangibility of radiant Heat. To which is added, an Inquiry into the Method of viewing the Sun advantageously, with Telescopes of large Apertures and high magnifying Powers. By William Herschel, LL.D. F.R.S. Part I. Read March 27, 1800. [Phil. Trans. 1800, p. 255.] After recommending a cautious circumspection in admitting specious appearances and plausible inferences in our researches both after physical and moral truth, the Doctor acknowledges that a general diffidence of this nature had raised a doubt in his mind, that the power of heating and illuminating objects is not equally distributed among the various coloured rays. This surmise received some confirmation from the different sensations he experienced on viewing the sun with his large telescopes, and through various combinations of differently coloured glasses. With some of these combinations he felt a sensation of heat, though he had but little light; while others gave much light, with scarce any degree of heat. Suspecting hence that perhaps certain coloured rays may be more apt to occasion heat, while others, on the contrary, may be more fit for vision, he resolved to put this conjecture to the test of experiments. The first set of these experiments relate to the heating power of coloured rays. They were made by admitting successively each differently coloured ray of a prismatic spectrum, through a proper aperture in a pasteboard, on a thermometer whose bulb was blackened, while another similar thermometer, at a certain distance, showed the temperature of the ambient air. The general results here were, that the temperature, or rather the power of heating of the red ray, is greater than any other, bearing a proportion to that of the green ray as 9 to 4, and to the violet, the least calorific, as 13 to 4. The next series of experiments was on the illuminating power of coloured rays. These were simply made by viewing through a microscope certain opaque bodies, consisting of minute particles, and illuminated successively by different coloured rays. These substances were red, green, and black paper, a piece of brass, a nail, and a guinea. The uncommon variegated appearances of the metals, and especially of the iron nails, occasioned by their very minute and differently arranged particles, is here mentioned both as an object of admiration, and as singularly conducive to the purposes of the present inquiry; the greater or smaller number of these particles that became discernible by the different coloured rays affording a kind of scale of comparison which led to the inferences here laid down. These general inferences are, that the red-making rays are very far from having the illuminating power in any eminent degree; that the orange possesses more of it than the red, and the yellow still more; that the maximum of illumination lies in the brightest yellow or palest green; that the green itself is nearly equally bright with the yellow; but that, from the full deep green, the illuminating power decreases very sensibly, the blue being nearly upon a par with the red, while the violet is still more, and the purple the most deficient of any. From a collective view and a due contemplation of these well established facts, it is inferred, that perhaps they may lead to the discovery of different chemical properties in the different coloured rays; and that the various degrees of heat in different coloured flames will probably be fully explained on the principles that may be deduced from them. Here follows a section in which the author advances a position, that radiant heat is of different refrangibility, and that it is subject to the laws of the dispersion arising from this different refrangibility. We perceive from the drift of his arguments here advanced, that he inclines in favour of a distinction between calorific rays and colorific light, both being liable to refraction, but under different angles, and hence probably possessed of different momenta. This, if admitted, will apply to the invisible heat of red-hot iron gradually cooled till it ceases to shine, and likewise affords a solution of the reflection of invisible heat by concave mirrors. We come next to an application of the result of the foregoing observations to the method of viewing the sun advantageously, with telescopes of large apertures and high magnifying powers. It is well known, that even with the naked eye, and much less with telescopes of large apertures, the sun cannot be viewed without a darkening apparatus. Dr. Herschel had repeatedly used red glasses for this purpose; but always found, that though the lustre was thereby sufficiently abated, yet they did not prevent an irritation on the eye, which was manifestly the effect of heat. This induced him to make experiments with glasses of various other colours, the result of which was, that, as might have been inferred from the above investigation, dark-green glasses are the most efficacious for intercepting the red or more calorific rays, and will therefore answer one of the purposes of the darkening apparatus. But as in viewing the sun we have, besides the heat, also its splendour to contend with, further trials were made to obviate this inconvenience: and for this purpose, common smoked glasses were found the most efficacious. Some directions are here given for smoking glasses uniformly, and in the most convenient manner. From a series of teiescopic experiments respecting this darkening apparatus, which concludes the paper, we learn, that as the heat will often crack the glasses when placed at or near the focus of the pencils of rays, a safer and more advantageous way of applying them is to place them before and near the small speculum, or immediately behind the second eye-glass. In this last position a single dark-green glass was found of great utility; but a deep blue glass, with a blueishgreen smoked one upon it, was still preferable: the sun appeared of a whiter colour than with any other composition, and the sensation of heat was by no means inconvenient. Experiments on the Refrangibility of the invisible Rays of the Sun. By William Herschel, LL.D. F.R.S. Read April 24, 1800. [Phil. Trans. 1800, p. 284.] In a paper read to the Society at a former meeting, the Doctor announced some observations which seemed to indicate that there are two sorts of rays proceeding from the sun; the one the calorific rays, which are luminous and refrangible into a variegated spectrum; and the other the invisible rays, which produce no illumination, but create a sensible degree of heat, and appear to have a greater range of refrangibility than the colorific rays. To the latter he assigns the name of radiant heat. Having lately had some favourable opportunities to prosecute this investigation, he here delivers an account of the series of experiments he made on the subject, which seem to him to confirm the above conjecture. The mode of conducting these experiments was simply this: On a horizontal tablet covered with white paper, and divided into squares, for the conveniency of measurement, a part of the extreme colour of a prismatic spectrum was suffered to fall, the remainder of the coloured rays passing by the edge of the tablet, so as not to interfere with the experiment. Three thermometers were placed on the tablet, at different distances from the termination of coloured rays. The general results of the ten experiments here described were as follows: From the four first it appears, that there actually are rays coming from the sun which are less refrangible than any of those which affect the sight; that they are invested with a high power of heating bodies, but with none of illuminating objects, which probably is the reason why they have hitherto escaped unnoticed. The fifth and sixth experiments showed that the power of heating is extended, though in a feeble degree, to the utmost limits of the most refrangible or visible purple rays, but not beyond them; and that it is gradually increased as the coloured rays grow less refrangible. And from the four last experiments we gather that the maximum of the heating power resides among the invisible rays without the prismatic spectrum, and is probably about half an inch beyond the last visible one, or from the confines of the red ray. These likewise show that the sun's invisible rays, in their less refrangible state, still exert a heating power, considerably beyond this maximum, fully equal to that of the red-coloured light; and that consequently, if we may infer the quantity of the efficient from the effect produced, these invisible rays of the sun probably far exceed the visible ones in number. The inferences deduced from these results are, that the range of refrangibility of radiant heat, or colorific rays, when dispersed by a prism, begins at the purple-coloured light, where they are most refracted, and have the least efficacy; and that their refrangibility lessens and their power increases as they approach the confines of the red-coloured light, but that these confines are not the limits of their decreasing refrangibility and increasing power, these having been traced far beyond the prismatic spectrum in an invisible state; that as their density gradually decreases, their energy at last vanishes, till at length the thermometrical spectrum, as the Doctor is willing to call it, becomes wholly imperceptible. Hitherto the effects of these heating rays have been observed as far as one inch and a half from the confines of the red ray. If this be a true account of solar heat, (says our author at the close of his paper,) it remains only for us to admit, that such of the rays of the sun as have the refrangibility of those which are contained in the prismatic spectrum, by the construction of the organs of sight, are admitted under the appearance of light and colours; and that the rest, being stopped in the coats and humours of the eye, act upon them, as they are known to do upon all the parts of our body, by occasioning a sensation of heat. Experiments on the solar, and on the terrestrial Rays that occasion Heat; with a comparative View of the Laws to which Light and Heat, or rather the Rays which occasion them, are subject, in order to determine whether they are the same, or different. By William Herschel, LL.D. F.R.S. Read May 15, 1800. [Phil. Trans. 1800, p. 293.] In the prefatory part of this paper, the author found it necessary to limit the sense he affixes to the word heat; and after excluding the late terminology of latent, absolute, specific, sensible heat, the matter of heat, caloric, and even radiant heat, which last, however, comes nearest to the expression he has adopted, he desires to be understood, that, in speaking of rays which occasion heat, he does not mean that those rays themselves are heat, but that he here considers heat merely as the effect of a cause, the nature of which is no part of his present inquiry. Having thus determined the subject of his investigation, the Doctor distinguishes heat into six different kinds; whereof three are solar, and three terrestrial. These, however, are reducible into three general divisions, each of the solar and terrestrial kinds resembling each other respectively. The first is the heat produced by luminous bodies, whether by the sun or by terrestrial flames. The second comprehends the heat of coloured radiants, such as that of the sun separated by a prism, and that of culinary fires openly exposed. And the third relates to heat from radiants, where neither light nor colour can be perceived; such as the heat of invisible solar rays, refracted by a prism, which have been the subject of a former paper; and the terrestrial heat from fires inclosed in stoves, and from metals heated short of the lowest degree of incandescence. The chief object of the present inquiry being to give a comparative view of the operations that may be performed on the rays that occasion heat, and of those which we know to have been effected on the rays that occasion light, a short detail is given of the principal facts |