in opposition to the explanation which Berthollet has attempted to give of chemical affinity. But if Berthollet's opinions with regard to the effect of quantity on what he understands to be the relative forces of chemical affinity were established, not only Mr. Dalton's observations, but almost all the conclusions of chemists, respecting composition, would be overturned. The errors of Berthollet, however, have been distinctly pointed out by Proust (Jour. de Phys. lix. 1804); and it is surprising that so good a chemist, and so accurate a reasoner, as Berthollet, should have mistaken so many mechanical mixtures for chemical compounds. The agreements observed by Gay-Lussac in the relative bulks of gases which enter into combination with each other are in some instances in conformity with Mr. Dalton's observations respecting their relative weights. In other instances, when he states certain relations to exist between the bulk of the compound and its elements, he is at variance with Mr. Dalton; and the question between them can be determined only by repeated experiments. It is obvious, however, that whatever agreements may be traced in the relative bulks of the elementary gases, all such proportions must be lost when the gases are changed into fluids or solids. Mr. Dalton's explanation has the advantage of not being affected by the specific gravities of the elements or of the compound; and it applies equally to gases, fluids, and solids. ARTICLE VII. Magnetical Observations at Hackney Wick. By Col. Beaufoy. 15 48 Ditto 20 8 25 24 14 43 Sept. 18 8h 20′ 24° 15′ 41′′ 1h 55' 24° 18′ 51′′ 6 15′ 24° 17′ 17′′ Ditte 198 15 06 1 25 24 26 In deducing the mean, the observation on the noon of the 30th and 26th, and the observations on the evening of the 27th, 28th, 29th, and 30th, are rejected. On Sept. 26, rain fell. On the 27th, hard rain, with thunder and strong wind. Sept. 29, the sky was very black in the west, and hard rain fell afterwards. Sept. 30, rain. Rain fallen { Between noon of the 1st Sept. 1.100 inch. .2.600 1 ARTICLE VIII. Some Remarks on the Theory of the Equilibrium of Radiant Heat, and on some Difficulties started against that Theory. By M. P. Prevost, Professor of Philosophy at Geneva. * I. In the Annals of Philosophy for May, 1815, vol. v. p. 338, there is a very good refutation of some objections to the theory of the equilibrium of radiant heat. The objections mentioned by the author of that memoir (Mr. Richard Davenport) are three in number. The first, extracted from the new Edinburgh Encyclopædia, is announced in these terms: "On this hypothesis a hot body ought to cool more slowly when it is placed near a large body of inferior It may be proper to state, that this paper was drawn up by the author in the French language, and that I have translated it into English.-T. temperature than when near a small one; because in the former case it must receive more calorific emanations than in the latter." The second is repeated in the same work from Mr. Murray. It is drawn from the difference in the radiation of two bodies, whose surfaces are different; such, for example, as a metallic surface and a blackened surface. "Of different surfaces which at a given temperature radiate different quantities of caloric, that which radiates least must be least powerful in returning caloric to the thermometer, and must therefore have least effect in counteracting the reduction of temperature." And in applying this general remark, the author of the objection concludes from it, that if the theory of the equilibrium were true, it would follow that the blackened surface (which radiates most) ought to produce a less degree of cold than the metallic surface (which radiates least). I may attempt shortly to explain what is merely hinted at in the objection such as I have transcribed it. Two bodies colder than the room are supposed, I conceive, to be presented to a thermometer, one of them terminated by a metallic surface, the other by a blackened surface. It is known that the blackened body will soonest acquire the temperature of the place, and therefore will sink the thermometer most powerfully during the time of its heating. The author of the objection seems to think that, according to the theory of equilibrium, the contrary ought to happen, because the black body radiates more powerfully than the metallic surface; and because this radiation, in part compensating the loss which the thermometer experiences from its own radiation, ought to be most efficacious in that of the two bodies, which radiates most abundantly. The third objection is likewise by Mr. Murray. It is drawn from the following experiment. A conical metallic tube, about 18 inches long, one inch in diameter at its narrowest extremity, and five inches at its widest, polished internally, so as to make a good reflector, is placed in a horizontal situation. A very sensible thermometer is placed at the widest end, and a matrass full of ice at the other. The thermometer sinks a very little. The experiment is now reversed; so that the thermometer occupies the narrow end, while the matrass is placed at the widest extremity. In this case the thermometer sinks much more rapidly than in the preceding. This appears to the author of the objection incompatible with the theory of the equilibrium; doubtless because he conceives that the calorific rays ought to be condensed in the second situation of the tube, and thereby render the cooling of the thermometer less sensible. This experiment originated with Count Rumford (Memoir on Heat, 1804, p. 146); and he proposed it as a proof of the frigorific undulations, which he admitted, and which he compared to the sonorous waves. This objection may be proposed in a much more simple form. In a place where the temperature is uniform, let a thermometer be presented to the narrow end of the tube: no rise whatever will ensue. Those who make the preceding objection ought to be astonished at this result, and to blame the theory for not explaining it. Many other objections may be started, and have indeed been raised, in consequence of the same imperfect and erroneous conceptions. I shall only mention one, which, like the preceding, has only become known to me by means of a good refutation. : The refutation is by M. Tremery, who has inserted it in the Nouveau Bulletin des Sciences (Août, 1813, No. 71, p. 323) and the objection, the author of which he does not name, is relative to the reflection of cold by means of two concave mirrors. It is known that the theory of the equilibrium explains the result of this experiment respecting the reflection of cold with the same facility as it does the reflection of heat. It is needless to state this part of the theory, which is, I believe, pretty generally known. To this explanation it is objected, that the matrass of ice or snow placed at one of the foci, being supposed to radiate, ought to send by double reflection some rays to the thermometer placed in the other focus. If the mirrors were withdrawn, these rays would be dissipated, and would not come to the thermometer. Therefore when the mirrors are removed, the thermometer ought to sink, and it ought to rise again when the mirrors are replaced, which is contrary to the matter of fact. II. These objections have been foreseen and refuted long ago, in a work entitled Du Calorique Rayonnant, which I published in 1809 (at Geneva and in Paris; Paschoud). Some of them are even peculiarly answered, particularly, that one drawn from the experiment with the conical tube (Du Calorique Rayonnant, § 113). I have therefore only to refer to that work. But as philosophers occupied with this subject have been obliged to enter into considerable details in order to get rid of these difficulties, started frequently without any regard to the previous solutions of them, it will not be without utility to state here as simply as possible the principles on which the theory depends, and on which the answers to these objections depend. These principles are at bottom the same as those explained by Messrs. Tremery and Davenport; and I shall state them more shortly, and perhaps more generally. 1. I suppose that constitution of caloric which agrees best with the phenomena of radiation to be known and admitted. It is a discrete fluid, every particle of which moves rapidly in a straight line. These particles go, one in one direction, and another in another; so that every sensible point of the hot space is a centre, from which depart, and to which arrive, rows of particles or calorific rays. 2. A reflector in a place of uniform temperature sends neither more nor fewer calorific rays than another body.-In fact, the reflector will not be called of the temperature of the place till the assertion which I have just made be verified; and in a short time this cannot fail to happen from the laws of the equilibrium of heat. As to the thermometrical effect, it is of no consequence whether the rays passing from the body be transmitted (that is to say, emanated from the interior of the body,) or reflected. If the reflector is perfect, the whole current is composed of reflected rays; if it is imperfect, it is composed of reflected and transmitted rays. The most convenient way of representing to oneself an imperfect reflector is to conceive its surface decomposed into two parts, one of which is a perfect reflector, while the other does not reflect at all. We must here apply the laws of the reflection of light. In particular we must observe that the surface reflects inwards as well as outwards. 3. Every calorific ray which a body sends by emission or by reflection, only replaces another ray, which would take the same direction if the body were withdrawn.* This is a necessary result of the constitution of caloric; for whatever be the direction of the rays emitted or reflected, there is one which follows the same route, and which the body intercepts. 4. It follows from this, 1. That in a place of uniform temperature, a reflector of whatever form does not affect a thermometer subjected to its influence. 2. That if it reflect rays emanated from a body more or less hot than the place, it will raise or depress repectively the thermometer subjected to its influence. III. The application of these principles to the objections detailed offers no difficulty. Let us take for an example the first two objections stated in the New Edinburgh Encyclopædia. 1. A hot body, it is said, ought to cool slower before a large cold body than before a small. The objector forgets that each of the rays which the cold body sends merely replaces the ray which the cold body intercepts. The intercepted ray being hotter than that which comes in its place, it is easy to see that the more of these substitutions take place (or, in other words, the larger the cold body is) the greater will the cooling effect be. 2. Two bodies, the one with a metallic, the other with a blackened, surface, are presented to a thermometer. It is alleged that the blackened body ought to cool the thermometer least, because it radiates most. Here the objector has not thought of the portion of radiant heat which these bodies give out by reflection. This portion is not changed by the change of temperature of the body. It subsists quite entire. The portion emitted only is diminished. Therefore by the same diminution of temperature, that one of the bodies which emits the most (the blackened surface) ought to radiate least; that one, on the contrary, which is the best reflector (the metallic surface) ought to radiate most, which is conformable to experience. *It is to be understood that we speak of a hot place, that is to say, where caloric radiates. If the intercepting body is of the same temperature with the place, the ray which it replaces is equal to itself. If not, this ray or row of particles, is more or less abundant in caloric. |