Letter from Edward Turner, M. D. Lecturer on Chemistry, Edinburgh, and Robert Christison, M. D. Professor of Medical Jurisprudence in the Univer sity of Edinburgh. Edinburgh, 9th Sept. 1824. GENTLEMEN, We have considered, at your request, Professor Leslie's Letter of the 19th July to the Coal Gas Proprietors, relative to the illuminating power of Oil and Coal Gases, and have made a variety of experiments, with the view of ascertaining what weight may be attached to the statement there given. Before the appearance of his letter, the public were in possession of results differing materially from those obtained by Professor Leslie, but agreeing very closely with each other, although procured by various unconnected experiments. Mr Brande found the relation between the light of Oil Gas and Coal Gas to be as 2 to 1; Messrs Philips and Farady as 3 to 1; and Mr Dewey, who was sent from the United States to examine the chief Gas Works in Britain, confirmed the results of the two last Gentlemen. Other persons of less note have likewise published on the subject, some of whom assign to the Oil Gas so high a comparative power as 4, and others one so low as 9-5ths. But, for several reasons, the opinions of Messrs Brande, Phillips, Farady, and Dewey, appear to us most worthy of confidence. The singular and unexpected conclusions, at which Professor Leslie has arrived, could not fail, therefore, to excite some distrust of their legitimacy; and accordingly, we apprehend, he has allowed himself to commit an important oversight, in relying on the indications of his Photometer. Although many objections exist to the employment of this instrument as a measurer of light, (so that, after being known upwards of 20 years, it has not hitherto been recognized by the scientific world,) yet these objections have nowhere been very forcibly stated. This it is high time to do, now that an attempt is made to apply it to a practical purpose of such vast consequence. We have examined, with some care, the fallacies to which it is liable; and have thus been led to the following conclusions, which you will at once perceive render the Photometer totally incapable of determining the point in question. I. It is not a sufficiently delicate instrument. It indicates total darkness in the brightest moonshine, and at the distance of a few feet from four wax candles burning with their fullest intensity. II. When placed at different distances from a light, it does not indicate the cor responding intensities. Thus, at the half of any given distance, instead of indica ting four times the light, as it ought to do, it indicates only a little more than twice the light. III. It is powerfully affected by heat, as well as by light; a fact which is easily proved, by placing it before a ball of iron heated, but not so as to be luminous, or even before a vessel of boiling water. This objection has great force, because it is well known that the relation of the light to the heat, emitted by burning bo dies, is very far from being the same; and, consequently, a body burning with less light and more heat may influence the Photometer as much as, or even more, than another which burns with greater light, and less heat. Professor Leslie, obviously aware of this difficulty, says, he has arranged his apparatus so as to "exclude the irregu lar influence of heat;" and although no mention is made of the manner in which this was effected, it is understood to have been done by interposing skreens of some transparent material between the lights and the instrument, under the belief that the heat (for a certain period at least) is absorbed by the skreens, while the light alone passes through to the Photometer. It has been shown, however, by a late celebrated chemist, (De Laroche,) that a large proportion of the heat is transmitted, or radiated, at once through such a skreen, just as through the air itself. As we are not aware that the experiments now referred to have ever been publicly confirmed, we have investigated the subject anew, and in a different way from that resorted to by De Laroche; and the results obtained agree with his, both in substance, and even also, in some instances, numerically. Hence it is impossible to exclude the irregular influence of heat in the way attempted. The Committee will readily perceive how much this fact takes away from the value of Professor Leslie's results, since the Coal Gas, which gives, according to his Photometer, so brilliant a light, greatly exceeds the Oil Gas in its heating power. We apprehend, therefore, that the Company must recur, for the present, to the statements given by Messrs Brande, Phillips, Farady, and Dewey. We have made preparations for repeating and varying the researches of these gentlemen, and will acquaint you with the results as soon as possible. It is our purpose to support the conclusions stated above, by publishing, at an early opportunity, the experiments on which they are founded. may mention, for your satisfaction, that the essential part of them have been exhibited before Dr Brewster, and Dr Duncan, jun. who expressed themselves satisfied of their accuracy. Meanwhile, we 5. When Mr Leslie's instrument was first made for sale, the illustrious philosopher, Sir William Herschel, examined it with great care; but he found it to give such inconsistent and absurd results, that he rejected it as unfit for any useful purpose. This fact Sir William communicated to me in a letter written soon after he made the experiments. We have the honour to be, GENTLEMEN, Your most obedient Servants, (Signed) EDWARD TURNER. R. CHRISTISON, To the Directors of the Edinburgh Oil Gas Light Company. Letter from Dr D. Brewster, F. R. S. &c. &c. Allerly, September 14, 1924. GENTLEMEN, IN compliance with your request, I have examined the results obtained by Professor Leslie, respecting the illuminating powers of Oil and Coal Gas, and I now beg leave to state to you my opinion of them, and the grounds upon which it has been formed. 1. The instrument, called a Photometer, by which these results were obtained, though bearing the name of Professor Leslie, was proposed in 1760 by the celebrated Prussian philosopher, M. Lambert; but he proposed it only to point out its inaccuracy, and he never used it in his numerous experiments on ths Mensuration of Light, which he has recorded in his admirable work on Photometry. 2. This instrument, even if it were correct in theory, is entirely useless in practice, as it indicates total darkness when exposed to the light of the brightest moon, though condensed many hundred times by the most powerful burning lenses or mirrors. 3. The Photometer under consideraation is founded on the assumption that the quantity of heat in any beam of light increases and diminishes in the same proportion as the intensity of the light; whereas it is well known, that between a mass of hot iron, where there is plenty of heat and no light, and the brightest moonbeam, where there is plenty of light and no heat, there is an infinite number of cases of combustion where the light and heat are combined in various proportions. 4. Although the Thermometrical Photometer has been known to the public for sixty-four years, and Mr Leslie's revived instrument for more than twenty, yet it has never been regarded by men of science as of any utility; and, so far as I know, has never been used by any philosopher whatever. 6. As the Coal Gas is known to give out, during its combustion, a much greater quantity of heat than the Oil Gas, any person acquainted with the Thermo. metrical Photometer could have foretold that such an instrument would ascribe to the Coal Gas a much greater, and to the Oil Gas a much less, illuminating power than actually belongs to them. 7. For the reasons above assigned, I am decidedly of opinion that there is no method which can be relied upon for determining the relative degrees of illumination of Oil and Coal Gas, but the method of shadows, devised by Count Rumford, which measures the actual light emitted by each flame, independent of all theories and suppositions; and as I believe Mr Brande and other chemists used this method in their comparative trials, I am of opinion that the ratio of the illuminating powers of Oil and Coal Gas should be considered as between 24 to 1, and 3 to 1, till other experiments, made by the method of shadows, shall establish a different proportion. In concluding this letter, I beg leave to state, that I was present at the repetition of several well-devised and accuratelyconducted experiments with Mr Leslie's Photometer, made by Drs Turner and Christison, and that I entirely concur in the conclusions which these Gentlemen have drawn from them, respecting the incorrectness of the instrument, and the fallacy of its results. I have the honour to be, Your most obedient and humble servant, To the Directors of the Letter from John Pollock, Esq. Secretary The Dublin Coal Gas Company was formed in 1821. They have lately, however, sold their works and whole concern to the General Gas Company of London. The Hibernian Coal Gas Com. pany was instituted in 1823. The Dublin Oil Gas Light Company commenced their regular operations in the beginning of last winter; and I am happy to say, that, notwithstanding every effort by the two opposing Coal Gas Companies, we have succeeded beyond our expectations, -although the Coal Gas has been improved as far as it is believed possible to improve it. We have already actually gained upwards of 700 lights from Coal Gas, in addition to our own regular consumers; and at the very moment I am writing, the requisite measures are proceeding, to make a change from Coal Gas to Oil Gas, on the 29th instant, (quarterday,) in a very large establishment, where there are upwards of 100 lights. This is in the Royal Arcade, the property of a very intelligent citizen, Mr Home. There are in this concern a number of fancy and other shops, the occupiers of which have found so much inconvenience from the use of Coal Gas, that they have addressed a letter to Mr Home, (a copy of which is annexed,) requesting him to change to the Oil Gas, with which request he has complied. Every quarter-day we obtain from ten to twenty shops and houses from the Coal Gas. The churches of St. John's, St. Bride's, St. Thomas's, and St. Catherine's, with several dissenting places of worship, are lighted with Oil Gas. The two first-mentioned churches were lighted with Coal Gas, but the ma nagers were compelled to turn it out. Only one church, St. Andrew's, is now lighted with Coal Gas. Within the last week we have received orders from the governors of several hospitals to light them with Oil Gas. The private houses of many gentlemen of rank in this city have been lighted by us for some time, all of whom express the greatest satisfaction in the use of Oil Gas, on account of its freedom from all injurious qualities. Among others, are lighted the houses of Sir J. K. James, Bart.; Hon. and Rev. J. Pomeroy; Sir William Betham; S. Kildalk, Esq.; J. Beaty, Esq. M. D.; &c. &c. &c. Nearly all the public and most fashionable hotels and public buildings are lighted by Oil Gas; namely, Gresham's, O'Dienne's, Bittow's, Ryeland's, Dwyer's, and Hope's hotels; the Mansion-house, the interior of the PostOffice, the Rotunda Rooms and Gardens, the Club House Sackville Street, and the Dublin Institution. The latter establishment is scientific; the managers paid at first a fixed rate for the Oil Gas, but were so pleased with it, that they agreed to pay by measure, on their being informed that we could not continue to supply them at a fixed rate. The interior of the Post Office was for. merly lighted with Coal Gas, but on representation of the clerks, who stated that they could not continue in the Office if Coal Gas was used, the Post-Master-Ge. neral ordered it to be lighted with Oil Gas,-you will find subjoined a certificate by Sir E. S. Lees, the Secretary, relative thereto, which he wrote after Oil Gas had been used a considerable time. It may be well to mention, that last Session of Parliament the sameGentleman was called on to furnish Parliament with a certificate to the same effect, which he did in a much stronger manner in favour of Oil Gas than the one now subjoined. I should suppose that the foregoing detail, as to the use of Oil Gas in this city, will be ample; but if you wish for more evidence, I shall feel no difficulty in supply. ing you. I shall only add, that the only part of the city lighted by Gas at present is lighted by Oil Gas, that is Rutland Square. I was very much surprised by the statements made by Professor Leslie, in his letter to the Edinburgh Coal Gas Company, and cannot conceive how he found the results he has published, they differ so materially from all who have gone before him, and can be so easily controverted. The authority of Professor Brande, of Dr Henry, of M. Ricardo, Esq., Michael Farady, Esq. F.R.S., Richard Phillips, Esq. F. R. S., Benjamin Hawes, Esq., and Dr Barker, Professor of Chemistry, Trinity College, Dublin, not one of whom make the relative illuminating powers of Oil and Coal Gases less than 34 to 1, are not so easily overturned as to be esteemed as nothing, when compared with the isolated statement of Professor Leslie. It has never been disputed in Dublin, notwithstanding the conflicting interests, that Oil Gas, in illuminating power, is to Coal Gas as 3 or 34 to 1. If Professor Leslie would condescend to resume his experiments on Oil Gas, manufactured on a large scale, I am quite confident that he would find the results far different from those he has published. He is evidently not aware that Oil Gas, manufactured on a small scale, is so deficient in specific gravity, that it consumes in much larger quantity, in proportion to the light given, than when it is manufac tured on a large scale. The condensed Oil, which goes to waste, is also much greater, in proportion, in a small, than in a large quantity of Gas. As to the produce of Gas from a given quantity of Oil, I can only state from actual experience, that from a gallon, Irish gauge, of good Whale Oil, I have got even 110 cubic feet. The Irish gauge is one-sixteenth less than the English. Oil Gas advocates in England found their calculations, on 100 cubic feet only being produced from one gallon of Oil, English gauge. The point I have stated I am willing to verify. The charge for Coal Gas in Dublin is 15s. per 1000 cubic feet, and for Oil Gas, 54s. 2d. We do not supply the Oil Gas in any case except by the meter; and yet we are gaining ground in the manner stated in the beginning of this letter. The Coal Gas Companies charge by a fixed rate for each burner; our charge of 54s. 2d. per 1000 feet is regulated by the relative proportion of the Gases generally acknowledged ;-54s. 2d. being to 15s. about as 3 is to 1. And it is my decided opinion, that in all cases where Oil Gas is burned fairly, and not allowed to run to waste, it will be found as cheap, if not cheaper, than Coal Gas. But the strongest fact, as to this point, will be found in the Parliamentary documents, which appear in the Report relative to the Bristol Oil Gas Bill, when applied for two Sessions ago. The Collectors of some of the Oil, and also of the Coal Gas Works, exhibited their different rates before a Committee of the House, when it appeared, that, on an average, the Oil Gas was quite as low as Coal Gas. I am sorry my time is so limited that I cannot go farther into detail at present, and I remain. GENTLEMEN, Your very obedient Servant, (Signed) JOHN POLLOCK, Sec. To the Directors of the Edinburgh Gas-Light Company. Extract of a Letter from Dr Barker, Professor of Chemistry, Trinity College, Dublin, to the Directors of the Dublin Oil Gas Company. "I FEEL myself bound to declare, that I believe this to be the best, the most economical, and the safest mode of obtaining light; and I am also confident that a Lecturer on Chemistry, and his pupils, would derive many advantages from a supply of Oil Gas to be used at Lecture." Extract of a Letter from Sir E. S. Lees, Secretary General Post-Office, Dublin, to the Directors of the Dublin Oil Gas Company. "I CAN have no hesitation in stating, that, so far as the experiment of produ Report of the Bishop of Kildare, and Drs Brooke and Pentland, a Committee of the Managers of the Dublin Lying-in-Hospital, appointed to examine the Dublin Gas Lights, and to report. Lying-in-Hospital, 9th Aug. 1822. YOUR Committee, &c. have carefully examined the Gas Lights produced by the Oil and Coal Companies, respectively. There can be no doubt, that both kinds of light are applicable to the public streets; but we find that in the intensity of light, the Oil Gas has the superiority at least of three to one. The escape of Coal Gas not ignited is attended with a very disagreeable smell; but that of the Oil is little or nothing unpleasant. We are of opinion that the Oil Gas may be safely and beneficially introduced into any rooms or passages whatsoever; but we are aware that the light of the Coal Gas is not so free from noxious effluvia; and, upon the whole, your Committee are of opinion, that if any light by Gas shall be used in, or near this Hospital, the Oil Gas ought to have the preference. CHARLES KILDARE. Vinder's Letter to the Editor of the Ca ledonian Mercury. " SIR, "The Oil Gas Company of Edinburgh, in their eagerness to occupy the market, having circulated industriously a paper injurious to the interests of the Coal Gas Company, the Directors of this establishment found themselves reluctantly compelled to publish an unvarnished statement of the facts in their possession. After the lapse of two months, the rival company has returned to the charge, and brought forth a very bulky and patched report, which, except an attempt to deny the originality and accuracy of the Photometer invented by Professor Leslie, and employed by him in comparing the illuminating power of the two sorts of Gas, contains nothing new, and leaves the main question exactly where it stood. Instead of plunging into a philosophical dispute, one should have expected that they would have instituted accurate trials of the illumination of the Gas manufactured at Edinburgh, by the favourite method of shadows; but if any such experiments have been yet made, we must conclude that these turned out unfavourably, else the Company would surely not have failed to communicate the results. In the mean time, they would persuade the public, that though the superior illuminating power of Oil Gas above Coal Gas has been reduced, by successive observers, from 4 to 3, and even to 2, these various statements are not discrepant, "but agree closely ;" and they modestly take three to be the just proportion. They very prudently pass over in silence the recent and careful experiments of Dr Fyfe, who has obtained, both by a comparison of shadows, and by the relative measure of Olifiant Gas, almost the same results as Professor Leslie derived from a very different principle. That those experiments should give 14 instead of 2 or 24, assigned as the comparative illumination of the two kinds of Gas by the trials at Bristol-the most correct of all those previously made-may be fairly attributed to the superior quality of the Edinburgh Coal Gas, which is generally allowed to be better than any other, except, perhaps, that of Glasgow. "Not content with depreciating the Photometer of Mr Leslie, the report labours, with singular consistency, to convey a pretty intelligible hint, that the Professor's judgment was biassed by the circumstance of his holding some shares in the stock of the Coal Gas Company. Such insinuations savour more of the spirit of petty traders, than becomes the high cha racter maintained by several of the rival Directors. "To set the public right, therefore, they have employed two young chemists, not proprietors of the Coal Gas Company, and have joined with them, as a sort of champion, the "celebrated" Dr Brewster, who, though a proprietor of that Company, comes forward to bear his testimony, it would seem, out of pure zeal for the dissemination of truth. Though he brings up the rear, I shall honour his communication with the first notice, and examine in their order the series of aphorisms in which it is contained. "1st, Dr Brewster displays the extent of his research, by acquainting the public with a fact which none of the learned of the present age had ever surmised, that the Photometer invented by Professor Leslie was really discovered before the year 1760, by Lambert, (a French, and not a Prussian philosopher, as Dr B., with his usual accuracy, asserts,) and described by him in his Photometria. Dr B. may deem himself singularly fortunate, if ever he met with that ingenious work, which is so extremely rare, that Dr Priestly, at the end of his History of Light and Colours, printed in 1772, declares that he could not, by his active correspondence, procure it, and of which no copy is known to exist in Edinburgh or London, except in the library of the Royal Institution. So concise and acute a writer as Lambert must have taken very needless trouble, if, as here alleged, after proposing the Thermometric Photometer, he immediately rejected it as useless. But how could that philosopher describe an instrument which depends essentially on a fine appli cation of the Differential Thermometer, with which he is acquainted? I will leave it to the penetration of Dr B. to explain this small difficulty. "2d, Dr B. says that the Photometer is "entirely useless in practice," because it does not indicate the light of the moon, "though condensed many hundred times" by powerful lenses or mirrors. Dr B. can possibly tell the name of the fortu. nate man who possessed those extraordinary glasses, and applied them to such a delicate experiment. The Photometer was designed to measure the illumination of the sun, of the sky, or that of artificial lights which bear a sensible proportion to those standards; it is not fitted to indicate, immediately, the faint glimmer of the moon, for this obvious reason, that its scale could not be divided into three hundred thousand visible parts. No method, indeed, is likely to be ever devised for the direct comparison of lights so extremely different in their intensities. |