have merely to square the respective distances of these luminiferous foci from the screen, to obtain an exact measure of their relative intensities. For example, when a wax candle of three to the pound, and a good mechanical or Carcel lamp, are placed respectively at distances of three and of ten feet from the screen, their flames will be found to project shadows from a slender cylinder of equal depth. Now, the squares of these numbers being 9 and 100, will denote the relative illuminating powers of the candle and the lamp. When the tint of the light, however, is very different, as with the blueish flame of gas, the vivid white of an argand lamp fed with oxygen (called the Bude light), or the gray light of the sky, it becomes very difficult to measure the respective intensities of such lights, by comparing the shadows which they project, with the shadows projected from the flame of a standard or wax candle, or a mechanical lamp. I experienced this difficulty, (Dr. Ure observed,) of late upon two interesting occasions. The first was in trying to measure the relative illu, minating powers of the Bude light, in subserviency to my examination before the Select Committee of the House of Commons, on lighting the House. The second was in estimating the degree of obscuration of daylight, produced by a high wooden wall or hoard, recently erected in a garden behind two valuable houses in George-street, Hanover-square, London. The landlords of these houses, justly conceiving that such a wall, if permanently built, (as was intended,) would greatly deteriorate the value of their property, were advised to sue for an injunction against it. As certain surveyors and architects had taken upon them to declare in their affidavits, that the obscuration produced by the said hoard was inconsiderable in itself, and did not materially impair the comfort of the tenants of the houses, I was called upon, by the solicitors for the plaintiffs, to investigate scientifically the darkening effects upon the windows and principal apartments of the said houses produced by the hoard, and to show that they were not matters of vague opinion, but were points susceptible of physical demonstration and accurate measurement. After several trials, I satisfied myself that the method of comparing the shadows projected by the windows opposite the hoard, and those from the windows of an adjoining house, not opposite to it, with the shadow of a standard wax candle, was not capable of affording uniform results, even when the yellow light of the candle was modified by a plate of pale blue glass. I had recourse to another photometric plan, which appears to be free from all ambiguity or source of error. The chloride

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and several other salts of silver are so very sensitive to light, as to take a dark grey tint from exposure to diffuse daylight in a very short time. When the aspect of the sky continues uniform for two or three hours, paper imbued with the nitrate, carbonate, chloride, or phosphate of silver, will assume a depth of grey or brown tint proportional to the time of its free exposure to the day. Availing myself of this principle, I simultaneously placed pieces of paper so prepared, in the apartments subject to the darkening influence of the wall, and in apartments of the adjoining bouse, not under that influence. The papers which enjoyed the free aspect of the sky, having assumed a decided depth of hue in half an hour, I folded them up from the light, and proceeded to watch the papers placed opposite to the windows less or more obscured, till I observed them to take the same depth of tint. I found that in the dining-room of the house No. 16, in Georgestreet, the paper exposed at the determinate distance from the windows, required three hours to acquire the same dark colour, and in the dining room of the house No. 17, it took an hour and a half. Hence it may be certainly concluded, that the daylight in the said apartment of No. 16 was diminished six-fold, and in that of No. 17, three-fold, by the interposition of the hoard. A piece of the paper laid upon the sideboard of the dining-room in No. 16, continued during the whole of a bright summer's day, without being perceptibly darkened in its hue; proving very manifestly that hardly any sky light at all was allowed to reach the bottom of this fine room, which is thirty-two feet long. The relative degrees of diurnal illumination in different apartments of any house-in different countries-or on different days in the same place or country, may thus be accurately measured and permanently registered by a series of photogenic impressions of any form, which will exhibit the progressive depths of tint, after an exposure during a certain number of minutes to diffuse daylight; care being always had to prevent the direct or reflected impulsion of the sunbeams. The tints thereby produced being fixed by water of ammonia, hyposulphite of soda, or any of the well-known photogenic expedients, will serve as standards of comparison to enable us to estimate the vivacity of daylight in any region of the globe, from the time in which paper similarly prepared with a standard salt of silver, acquires from exposure to daylight the same hue. since the comparison of tints may be made with considerable precision by an experienced eye, this photometric method may prove a valuable addition to the scientific resources of the meteorologist.

And

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Measuring Fluidity or Viscidity of Oils, &c.-Dr. Ure also detailed a number of "experiments to determine the fluency or viscidity of different liquids at the same temperature, and of the same liquids at different temperatures. Having been employed professionally, (Dr. Ure observed,) to investigate the operation and merits of a new lamp, recently patented by Mr. Parker, in which the oil is heated by the flame of the lamp to the temperature of from 200° to 250°, before arriving at the wick, it was desirable to determine the degree of fluency imparted to oils by a certain elevation of temperature. The light emitted by this lamp, when supplied with the viscid, and very cheap, but nearly scentless, southern whale oil, surpasses, in purity and whiteness, the light of the best mechanical lamp, though it be fed with the best vegetable or even sperm oil. This superiority is in part due to the form of the chimney, and to the oil being maintained uniformly at the level of the bottom of the flame; but it must also be ascribed, in a certain measure, to the high temperature and fluency of the oil, by which it enters more readily into complete combustion, than cold and viscid oil could possibly do. The preparatory heating seems to act on the same principle here, as it does in the smelting of iron by the hot blast. Having presented a memoir upon the subject of the lamp to the Society of Civil Engineers, which will appear in their Transactions, I shall decline entering at present further into its merits. In that memoir, I

2,000 grain-measures of water, at

When the funnel and glass tube were faintly smeared with oil, though perfectly 2,000 grain-measures of water, at

stated the results of some experiments which I had then made upon the fluency of liquids, by means of an apparatus, consisting of a small glass syphon, and a platina capsule, containing a measured quantity of the liquid to be run off through the syphon. Having since had reason to imagine that certain numerical errors had been occasioned by variations in the position of the syphon, though the general results are true, I have recently repeated the experiments with another form of apparatus free from that fallacy, and submit the following brief account of them. Upon this occasion I put the liquid, either cold or heated to a certain temperature, into a glass funnel, terminated at its beak with a glass tube of uniform bore, about one-eighth of an inch in diameter, and three inches long. The funnel was supported in a chemical stand, and discharged its contents, on withdrawing a wooden pin from the beak, into a glass goblet placed beneath, alongside of which a chronometer was placed to indicate, in seconds, the time of efflux. The volume of liquid used in each case was the same,-viz., 2,000 grain measures, at 65° Fahr. The times of efflux with liquids of the same specific gravity and bulk, in the same vessel, vary with the viscidity of the liquids, and serve to measure it. A correction ought to be introduced in estimating the times of efflux of hot liquids, on account of the enlargement, by expansion, of the bore of the glass tube; but this, being a point of little consequence in the practical application of this inquiry, has been neglected. 60° Fahr., ran off in 14 seconds.

Pyroxylic spirit

The rape-seed oil is so viscid, as to burn with difficulty in lamps of the ordinary construction, but in the hot oil lamp of Parker it affords a very vivid light. In my former apparatus, the difference of level between the two legs of the syphon, which constituted the effective pressure of efflux, was only half an inch, whereby 2,000 grain-measures of sperm oil, at 64°, took no less than 2,700 seconds to run off, while that volume of oil of turpentine ran off in 95 seconds. It would, therefore, appear that the fluency of a viscid oil diminishes in a very rapid ratio with the diminution of pressure. Hence, an oil will burn well in a mechanical lamp, where it is raised to the level of the bottom of the flame by pump work, which will afford a very indifferent light in one of the French Annular, or Sinumbral lamps, where the supply is given by a very slight pressure.

Prof. Forbes asked what was the diameter of the orifice of the funnel through which the fluids ran, and what was the quantity of each fluid experimented upon.-Dr. Ure said, that the orifice had a diameter of oneeighth of an inch, and the quantity of each fluid used was 2,000 grains.-Prof. Forbes said, that the interest he felt in these experiments arose from the great importance, in some of the researches he was engaged in, of sustaining the action of the lamp in an unchanged state during the progress of an experiment, and sometimes during a series of experiments; but the changes to which even the best ordinary lamp was subject were so anomalous, and the causes so beyond the reach of discovery, as to be quite disheartening. The instrument that he used for measuring minute changes of temperature was so delicate, that the heat given out by even an apparently well-burning lamp, often appeared to be in a constant state of change, the needle indicating it by an almost constant oscillating motion of even several degrees; even lamps, whose wicks were made with the greatest care, were subject to these changes; and the only wicks which he had found to furnish a sufficiently steady temperature for his purposes, were those made by Locatelli,

of Paris.

Manufacture of White Lead.-Mr. Benson read a paper on the theory of the formation of white lead. The carbonate of lead formed by the common process is anhydrous, amorphous, and contains one proportional each of carbonic acid, oxygen, and lead. In order to explain my views (said the writer), I shall have to advert, for a moment, to the

Dr.

oxide of lead, termed litharge. Litharge is seldom manufactured with a view to its ulterior application, as it is produced as a secondary product in the separation of silver from lead, even under Mr. Pattison's arrangements, in quantities too large for the demands of commerce. A considerable proportion of the litharge made must, therefore, necessarily be reduced again into the metallic state, and, during this reduction, 7 per cent. of the weight of the metallic lead is lost by sublimation, and by combining with the earthy matter of the coal. Now, as litharge is a protoxide of lead, it has been thought that in order to effect its conversion into white lead, nothing more was requisite than to combine it with a due proportion of carbonic acid; and from this mistake, a variety of fallacious processes have been projected. The processes to which I allude, are founded upon bringing the litharge into solution as a basic salt, and then precipitating it as a carbonate by the injection of carbonic acid. Painters maintained that this precipitate was not white lead. Chemists, finding, by analysis, the correct proportions of protoxide of lead and carbonic acid, attributed the opinion of painters to prejudice. Ure was, I believe, the first to discover in what the difference between the precipitated carbonate and white lead consisted. I have already stated, that white lead produced by the usual mode of manufacture is amorphous, and, in oil, opaque; whilst, by microscopic observations, Dr. Ure has ascertained, that the former is semi-crystalline, and, to a certain degree, transparent. The difference between white lead and precipitated carbonate may be illustrated by comparing them to pulverized chalk and powdered marble; both are carbonates, but the one is crystalline, the other is not, and one is, consequently, less opaque than the other; and this difference is, of course, more appreciable, when the powders are diffused through highlyrefracting media, such as oil. There is one mode by which this difficulty may be avoided, and this was arrived at previously to the discovery of Dr. Ure, as a deduction from a different train of reasoning, The rationale of both the processes, of that which produces the crystalline, and of that which produces the amorphous carbonate, is the same. In both, the lead is converted into basic acetate-in both, the salt is decomposed by carbonic acid, but in the former the process is modified by the pressure of water. In the one the carbonate has been

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deposited from a solution-in the other, the particles, never having departed from the solid state, have not been at liberty to arlarge themselves symmetrically. In order, therefore, to produce amorphous carbonate, or white lead, from litharge, it became necessary to present the oxide of lead with a quantity of acetic acid so minute, that an insoluble basic salt should be formed, with a quantity of moisture merely sufficient to determine the action of the carbonic acid. The process would then resemble, in all respects, the ordinary one, except that in the one the lead has been previously converted into oxide, in the other, the formation of the oxide goes on simultaneously with that of the carbonate. The process has been carried out on a scale of considerable magnitude at Birmingham Heath. The quantity of acetic acid used is less than 1.300th of the weight of the litharge, and the quantity of moisture found to be most advantageous is such as will just render the litharge sensibly damp to the touch. A purer and more economical source of carbonic acid than bark has been found in the combustion of coke, and powerful machinery has been applied to facilitating the process, by exposing new surfaces to the action of the gas. The result has been, that the process is completed in as many days as the ordinary one requires months, and the product is of a purer white, and in opacity or body, and all other respects, at least equal to the usual white lead of commerce. One or two other facts deserve mention, which are not generally known. It is singular, that if the protoxide of lead known as massicot, and the protoxide known as litharge, be exposed to a high temperature, approaching to a red heat, the massicot will rapidly absorb oxygen, and become the ordinary red lead of commerce; while the same process goes on exceedingly slowly with the litharge, if at all; but, on the other hand, if massicot and litharge be moistened with dilute acetic acid, and exposed to carbonic acid, the litharge will be converted into carbonate before the massicot is much affected. Another fact is, that white lead and oil combine with so much energy, that if linseed oil be poured upon a large quantity of white lead, and the mass be left undisturbed for a few hours, the temperature will become so elevated, as to carbonize the oil, and render the whole perfectly black. It seems also not generally known, that white lead possesses the power of destroying the colouring matter of linseed oil. If sulphate of barytes be mixed with one portion of oil, and white lead with another portion, the latter will appear comparatively white. If the two mixtures be allowed to remain for some days undisturbed, a quantity of oil will gradually

rise to the surface of both. In the former, the supernatant oil will have undergone no change in the latter, the oil will be nearly deprived of colour, and will have acquired the degree of rancidity, termed by painters fat. The colouring matter has not combined, as might have been expected, with the white lead, for if this be dissolved by the agency of a weak acid, the disengaged oil will also be found to have been bleached. A large quantity of white lead is required to produce this effect, and the precipitated carbonate is less efficient than the white lead of com

merce.

Resistance of Atmosphere to Railway Trains. Dr. Lardner communicated at great length, the experiments he performed with Mr. Wood on the Great Western and other railways, on atmospheric resistance, and the particulars of which we published (vol. xxx, pp. 266, 297, 370) at the time of their being made; further details the limits of our publication will not allow, but we shall give a re-statement of his conclusions. He said that he reserved to himself the power, when the experiments should be all reduced, of modifying these conclusions, if it should appear necessary to do so. He stated, that many of the experiments had been only recently made, and had, consequently, not been submitted to mathematical analysis. Meanwhile he had taken care to lay nothing before the section, except what had been fully borne out by the experiments themselves. He regarded the following conclusions as established by his experiments :

1. That the resistance to a railway train, other things being the same, depends on the speed.

2. That at the same speed the resistance will be in the ratio of the load, if the carriages remain unaltered.

3. That if the number of carriages be increased the resistance is increased, but not in so great a ratio as the load.

4. That, therefore, the resistance does not, as has been hitherto supposed, bear an invariable ratio to the load, and ought not to be expressed at so much per ton.

5. That the amount of the resistance of ordinary loads carried on railways at the ordinary speeds, more especially of passenger trains, is very much greater than engineers have hitherto supposed.

6. That a considerable, but not exactly ascertained, proportion of this resistance is due to the air.

7. That the shape of the front or hind part of the train has no observable effect on the resistance.

8. That the spaces between the carriages of the train have no observable effect on the resistance,