The rings may be formed without thick or thin plates of glass or air the secondary set of rings in view, I found their shape and colour always completely well formed. This experiment was also repeated with a small plain glass instead of the metalline mirror put under the large plate. In this manner it still gave the same result, with no other difference but that only six rings could be distinctly seen in the secondary set, on account of the inferior reflection of the subjacent glass, XXXIII. Coloured Rings may be completely formed without the Assistance of any thin or thick Plates, either of Glass or of Air. The experiment I am now to relate was at first intended to be reserved for the second part of this paper, because it properly belongs to the subject of the flection of the rays of light, which is not at present under consideration; but as it particularly opposes the admission of alternate fits of easy reflection and easy transmission of these rays in their passage through plates of air or glass, by proving, that their assistance in the formation of rings is not required, and also throws light upon a subject, that has at different times been considered by some of our most acute experimentalists, Į have used it at present, though only in one of the various arrangements, in which I shall have occasion to recur to it hereafter. Experiment of Sir I. Newton placed a concave glass mirror at double its Sir I. Newton, focal length from a chart, and observed, that the reflection of a beam of light admitted into a dark room, when thrown upon this mirror, gave four or five concentric irises or વ rings of colours like rainbows." He accounts for them by alternate fits of easy reflection and easy transmission exerted in their passage through the glass plate of the concave mirror f. of the duke of Chaulnes, The Duke de Chaulnes concluded from his own experiments of the same phenomena, "that these coloured rings depended upon the first surface of the mirror, and that the second surface, or that which reflects them after they had passed the first, only served to collect them and throw them "upon the pasteboard, in a quantity sufficient to make them 66 visible *." Mr. Brougham, after having considered what the two au- of Brougham, thors I have mentioned had done, says, "that upon the whole "there appears every reason to believe, that the rings are "formed by the first surface out of the light, which, after "reflection from the second surface, is scattered, and passes "on to the chart †." My own experiment is as follows. I placed a highly po- of the author. lished 7 feet mirror, but of metal instead of glass, that I might not have two surfaces, at the distance of 14 feet from a white screen, and through a hole in the middle of it one tenth of an inch in diameter I admitted a beam of the sun into my dark room, directed so as to fall perpendicularly on the mirror. In this arrangement the whole screen remained perfectly free from light, because the focus of all the rays, which came to the mirror, was by reflection thrown back into the hole, through which they entered. When all was duly prepared, I made an assistant strew some hair-powder with a puff into the beam of light, while I kept my attention fixed upon the screen. As soon as the hair-powder reached. the beam of light, the screen was suddenly covered with the most beautiful arrangement of concentric circles, dişplaying all the brilliant colours of the rainbow. A great variety in the size of the rings was obtained by making the assistant strew the powder into the beam at a greater distance from the mirror; for the rings contract by an increase of the distance, and dilate on a nearer approach of the powder.' This experiment is so simple, and points out the general causes of the rings, which are here produced,, in so plain a manner, that we may confidently say they arise from the flection of the rays of light on the particles of the floating powder, modified by the curvature of the reflecting surface of the mirror. Here we have no interposed plate of glass of a given thickness between one surface and another, that might pro Priestley's History, &c. on the Colours of thin Plates, p. 515. duce and interstices duce the colours by reflecti me rays of light and transmitting others; and if we were inclined to look upon the distance of the particles of the floating powder from the mirror as plates of air, it would not be possible to assign auy certain thickness to them, since these particles may be spread in the beam of light over a considerable space, and perhaps none of them will be exactly at the same distance from the mirror. I shall not enter into a further analysis of this experiment, as the only purpose for which it is given in this place is to show, that the principle of thin or thick plates, either of air or glass, on which the rays might alternately exert their fits of easy reflection and easy transmission, must be given up, and that the fits themselves of course cannot be shown to have any existence. Newton's the- " It will hardly be necessary to say, that all the theory reory of the size. lating to the size of the parts of natural bodies and their inof bodies, terstices, which Sir I. Newton has founded upon the existfounded on fits ence of fits of easy reflection and easy trasmission, exerted of easy reflection and trans-differently, according to the different thickness of the thin mission of -plates of which he supposes the parts of natural bodies to light, unsupported by fact consist, will remain unsupported; for if the above mentioned fits have no existence, the whole foundation, on which the theory of the size of such parts is placed, will be taken away; and we shall consequently have to look out for a more firm basis, on which a similar edifice may be placed. That there is such a one we cannot doubt; and what I have already said will lead us to look for it in the modifying power, which the two surfaces, that have been proved to be essential to the formation of rings, exert upon the rays of light. The Second - Part of this Paper, therefore, will enter into an examination of the various modifications, that light receives in its approach to, entrance into, or passage by, differently disposed surfaces or bodies; in order to discover, if possible, which of them may be the immediate cause of the coloured rings that are formed between glasses. VI. Description of a newly invented Calorimeter; with Experiments to prove, that an increased Capacity for Caloric accompanies an Increase of Temperature. By JOSEPH READE, M. D. I SIR, Edinburgh, Jan. 22, 1808. Lavoisier and Beg leave to communicate, through the medium of your very interesting and scientific Journal, the invention of a calorimeter, free from those inaccuracies incident to the ap- Defects of the paratus of Messrs. Lavoisier and Laplace, in which it was apparatus of impossible to guard against errours arising from capillary Laplace. attraction, from the process of freezing and thawing proceeding at the same period, and likewise from the influence of a current of atmospheric air. In this communication I will confine myself to a summary description of the apparatus, and of a discovery deduced from it, which must influence in a most important manner, if proved, the investigations of caloric; that, contrary to received opinion, water Capacity of increases in capacity from the thermometric range of 32 to 212, in a just rate for every degree of temperature commu- uniformly with nicated. water for caloric increases it; temperature. Description of the Calorimeter, which is to be formed of thin sheets of Brass or Tin. The innermost compartment No. 1, Pl. V, fig. 1, designed The calorimefor the fluid to be subjected to experiment, is to be stopped ter describd. with a thermometric cork, a, or, what is better, a thermometer surrounded with chamois leather, and made to fit accurately the aperture. The second compartment, No. 2, holds a quantity of water, and is likewise to be stopped by a thermometric cork, b, made air-tight by sealing wax, as this water is not to be removed from the compartment during the course of the experiments. The external compartment, 3, is designed to act as an imperfect conductor of caloric, and is to have a coating of Method of finding the comparative list or flannel between the sheets of brass, which, combined with the confined air, renders the instrument extremely accurate, a minute elapsing before the thermometer fell 1 degree at 150°. Therefore in experiments scarcely requiring that time, there can be no abstraction of any consequence by the atmosphere. When we wish to estimate the specific caloric of two fluids, suppose oil and water, we bring the calorimeter to specific heat of the precise temperature of 32°, 40°, 50°, or any other we two fluids. desire, indicated by the two thermometers. We then fill the interior compartment, No. 1, with water at 212o, and immediately stop it with the thermometric cork, a. After agitating the apparatus for about the space of 1 minute in a horizontal position, the thermometers indicate the rise experienced by the water at 50° in the second compartment, and the number of degrees lost by the water at 212° in the interior. Suppose the calorimeter be raised from 50° to 80°, we take that number as the specific caloric of water. We then pour the water from the interior compartiment, and again reduce the temperature of the apparatus to 50%, which is speedily accomplished, by pouring cold water into the innermost compartment, until the thermometers are reduced to the desired point. We are next to fill the interior compartment with oil at 212°; and if, after agitation, on examining the two thermometers, we find the temperature raised, suppose to 60°, we easily find the specific caloric of oil compared with water. Thus taking water as the standard, in a short time all fluids may be examined. By substituting an iron cage, solids may be subjected to experiment; so likewise may fluids, which act chemically on metals, be examined. by enclosing them in a glass vessel. Solids and cor rosive fluids may likewise The author en I am at present engaged in a series of experiments, which gaged in a se- I hope soon to be enabled to lay before the public. Here nes of experi- the reader is to take notice, that I have only used ideal ments. numbers, more clearly to illustrate the mode of operating with the apparatus, and by no means indicative of the real specific caloric of oil and water. I will end this part of my communication by remarking, that in this instrument the inaccuracies arising from abstraction of caloric by the atmosphere and vessel are obviated, which was impossible by means |