this effect, if it were to take place, would increase almost as the loss of heat of the body, so that the error produced would affect equally all the results. It was easy to raise the temperature of the surrounding medium, by passing vapour into the water through the tube SUV, plunging to the bottom of the liquid. The orifice of the balloon is shut by a thick plate of glass, A B, ground with the greatest care upon the edge of the balloon itself. The surfaces in contact have besides, in consequence of the thickness of the neck, a sufficient extent, so that the interposition of a small quantity of hog's lard renders the contact close, and prevents all communication from without. This plate is perforated at its centre by a circular opening, into which a cork is firmly put, which contains the tube of the thermometer; and the intermediate tube, CO, is of such a length that the bulb is precisely in the centre of the balloon. By on By giving this intermediate tube a very small diameter, the quantity of mercury without the bulb is diminished, and the swelling which takes place at the commencement of the scale enables us to fix the tube more firmly in the cork. Thus the thermometer is fixed in the plate, and this disposition is shown particularly in fig. 6, where the bulb of the instrument is placed above the furnace, which serves to heat it. The screens, A A', are lea leaves of tin plate, separated from each other, which serve to screen the plate, A B, from the action of the heat. Let us now return to fig. 5. The stem of the thermometer, which is without the balloon, as is evident from the figure, is covered by a hollow tube, ST, the ground bottom of which is applied to the upper surface of the glass plate. This kind of vessel is terminated above by a stop-cock, to which is cemented the end of the very flexible leaden tube, DE F. The other extremity of this tube is firmly fixed to the plate of an air-pump, HK. The canal, which in this machine makes the communication between the centre of the plate and the barometer, is connected with another tube with a stop-cock, to which is cemented a tube filled with muriate of lime. It is through this tube that the gas passes by way of the bent tube, mp The glass air holder, being moveable up and down, enables us to make the elasticity of the gas introduced the same as that of the atmosphere. We shall now describe our mode of proceeding in each experiment. The water in the trough being brought to the requisite temperature, and the thermometer fixed in the glass plate being heated to nearly the boiling point of mercury, it was transported rapidly into the balloon. The glass, S T, already cemented to the leaden tube, was then drawn down over the stem. While the surfaces in contact were carefully luted, d. an assistant ra rapidly exhausted the balloon, by means of the air-pump. The commaad yo Podlud edy quifas nication between the balloon and the glass tube was rendered very free by the central opening Pemings, a and b, made in the plate near the If the cooling was to be observed, in vacuo, the process was stopped when the machine ceased to dilate the air, and we measured immediately the tension of what remained in the balloon. The stop-cock was then shut, and the observations commenced. When the experiment was to be conducted in air, that of the balloon was at first dilated, in order to facilitate the contact of surfaces, and then the proper quantity was allowed to enter. When the cooling was to be observed in a gas, the balloon was first emptied of air, gas was then allowed to enter, and a vacuum was again made, after which the requisite quantity of gas was introduced. By this contrivance, it was mixed with only an inappreciable quantity of air. We shall terminate this description by saying, that the dimensions of the thermometer had been calculated, so that the observation of the cooling could begin at about 300°. The experiments in air and in the gases require rather a longer preparation, and cannot be commenced with safety till the equilibrium is restored through the whole extent of fluid. The series of observations belonging to them commence at about 250°. The experiment for cooling in vacuo, or in gases, being thus prepared, it remained merely to observe the rate of cooling by means of a watch with a second's hand at equal intervals of time. But these temperatures require two corrections, which we shall point out. In the first place, it is obvious, from the nature of our apparatus, that after a short time the stem of the thermometer was cooled down to the temperature of the surrounding air. Every temperature observed, therefore, was too low, by a number of degrees equal to that to which the mercury in the stem would dilate, when heated from the temperature of the surrounding atmosphere to that of the bulb. This correction was easily calculated, and was applied to all the temperatures observed. The object of the second correction was to reduce the indications of the mercurial thermometer to that of the air thermometer. For this we employed the table given in the first part of this memoir. حمد Having thus obtained a series of consecutive temperatures of the thermometer, it only remained to apply to that series the mode of calculation which we have explained above. We divided it then into two parts, which were represented each by expressions of the form m, a2+ in which t denotes the time; and these formulas served to calculate the velocity of cooling for the but velocities different required a diminution easily determined in each case. That it may be conceived in what this consists, we must remark, that the cooling of the bulb of the thermometer, arising from the loss of heat which takes place at the surface, is always a little excesses of temperature? each cas 7 augmented by the entrance of cold mercury from the stem of the thermometer. But the volume of mercury being known, and likewise its temperature, it was easy to estimate exactly the amount of this correction, which, though very small, ought not to have been neglected. Such is the mode which we always followed, in conducting and calculating all our experiments. We satisfied ourselves with determining the velocity of cooling for excesses of temperature, differing from each other by 20 degrees. And that we might not make this memoir too tedious, we have withheld all the intermediate calculations which led to our determinations. We shall now enter upon a detail of our experiments, stating them in the order in which they were made. Our preliminary researches having made us acquainted with the influence of the nature of the surface upon the law of cooling, it was necessary to study that law under different states of the surface of our thermometers. But it was necessary likewise that these surfaces should not experience any alteration from the highest temperatures to which they should be exposed. The only two which appeared to us to answer this condition are surfaces of glass and of silver. Accordingly most of our experiments were made, first preserving to the thermometer its natural surface, and then covering it with a very thin leaf of silver. These two surfaces possess, as is known, very different radiating powers; glass being one of the bodies which radiate most, and silver of those which radiate least. The laws to which we have arrived, by comparing the cooling of these two surfaces, are of such simplicity that there can be no doubt of their being applicable to all other bodies. (To be continued.) ARTICLE II, On Mr. Tennent's Bleaching Salt; known by the Name of Oxymuriate of Lime. By Thomas Thomson, M.D. F.R.S. WHEN I was drawing up the fifth edition of my System of Chemistry, one of the substances, respecting which I found myself unable to form a definite opinion, was the bleaching salt, originally invented by Charles Tennent, Esq. and well known in commerce under the name of oxymuriate of lime. I found myself, therefore, obliged to omit the substance altogether, resolving, however, to ascertain its nature as soon as I should have sufficient leisure for that purpose. I got a quantity of it accordingly from Mr. Tennent, last autumn, quite fresh, and subjected it to the requisite experiments. I shall in this paper give a short sketch of the principal facts which I observed, reserving a more detailed account of the experiments till some future opportunity; for more time than I can at present command would be requisite to disentangle the useful from the immaterial or indecisive experiments, with which they are mixed in the journal which was written down at the time. 1. Oxymuriate of lime, when recently prepared, is quite dry to the feel. It has a peculiar smell, bearing some relation to that of chlorine, but not so offensive. Its taste is hot and astringent. The hot taste is probably owing to the uncombined quick-lime contained in the powder; for when the oxymuriate of lime is dissolved in water, the taste of the solution is merely astringent. 2. Fifty grains of the powder being digested in a sufficient quantity of water to dissolve the soluble part of the salt, and poured upon a filter, left a quantity of lime, partly in the state of quick-lime, and partly in the state of carbonate. Concluding that the carbonic acid had been absorbed during the drying of the lime, I digested the insoluble residue from other 50 gr. of the salt in diluted sulphuric acid, evaporated the liquid to dryness, and exposed the sulphate of lime, thus obtained, to a red heat. It weighed 27:8 gr. indicating 11.68 gr. of lime. 3. The portion of the oxymuriate of lime dissolved in water reddened turmeric paper, and when left exposed to the air, a crust of carbonate of lime was formed in the surface of the liquid. Hence it was obvious, that besides the bleaching salt, or oxymuriate of lime, the water had dissolved likewise a portion of lime, and was, therefore, in the state of lime water. 4. The solution obtained by digesting 50 gr. of the bleaching powder in water and filtering, was mixed with an excess of sulphuric acid, evaporated to dryness, and exposed to a red heat. The sulphate of lime formed weighed 31-5 gr. indicating 13-23 gr. of lime. 5. Another similar solution, obtained from 50 gr. of the bleaching powder, was precipitated by nitrate of silver. The chloride of silver obtained weighed 55 gr. indicating 13.56 gr. of chlorine.. 6. Now if we consider the bleaching salt to be a combination of chlorine and lime, or a chloride of lime, as I shall afterwards show it to be, we shall find the quantity of uncombined lime in the solution by subtracting from the total quantity of dissolved lime that portion of it which is in combination with the chlorine. This quantity may be found as follows: The weight, of an atom of chlorine is 4.5, and of an atom of lime 3.625; the quantity of chlorine in 50 gr. of the bleaching powder is 13.56 gr.; therefore, 4.5: 3.625 :: 13.56: 10.92 lime united to the chlorine. 7. From the preceding experiments, it follows (supposing what is called oxymuriate of lime to be a chloride of lime), that the composition of the bleaching powder which I examined was as follows: ・・ 1. Undissolved portion; viz. lime.. વે 13.56 32ยา 2. Combined lime' .... 10.92 2.31H 5038-47 Hence the water and the impurities in 50 gr. of the bleaching powder, which I examined, amounted to 11-53 gr. or somewhat more than 4th of the total weight. } 8. The uncombined lime in the specimen which I examined, which was newly prepared, amounted to 15.87 gr., while the portion united to the chlorine was 10.92 gr., so that the uncombined lime was to the combined nearly in the proportion of three to two. In the experiments which Mr. Dalton published on this salt, in the first number of the Annals of Philosophy, he found one half the lime united to the chlorine. I made some attempts to repeat his mode, of analysis, but did not succeed. When protosulphate of iron was added to the solution of the bleaching salt, it acquired a deep red colour from a few drops, and it speedily became impossible for me to determine whether I had added a sufficient quantity of protosulphate or not. Mr. Dalton has not been explicit enough in his description of his mode of analysis, to enable other chemists to repeat his process. Of course I am unable to state, whether the difference between Mr. Dalton's results and mine be owing to the different modes of analysis which we followed, or to a difference in the composition of the bleaching powders which we examined. I think it very likely that both causes contributed to produce the difference which exists between us. II. Nature of Oxymuriate of Lime. But the principal object of my experiments on the bleaching powder was to ascertain the nature of the compound formed when chlorine is made to pass through hydrate of lime. اور 1. 1160 gr. of the dry powder were put into a glass retort, the beak of which was luted to a bent tube, the end of which was plunged into a water trough, and a glass jar full of water was inverted above it. The retort being heated on a sand bath, gas came over, and continued to come over for some hours. The whole quantity thus extricated measured 164 cubic inches, and was pure oxygen gas. The dry salt in the retort had lost its smell and its action on vegetable colours; and when digested in water, a solution of common muriate of lime was obtained. I conclude from this experiment that, in the bleaching salt, the |