up, with their own absurd inventions, some portion of the divine doctrine, imposed, with little difficulty, the incongruous mass upon the credulity of their ignorant followers. It would be difficult to decide whether this superstition has been, in proportion to its extent, more destructive to the morals of society than the false system which had previously established itself in the hearts of men. It is most likely, that, upon the whole, some benefit accrued from the purer parts of its composition; but this advantage was probably greatly overbalanced by its more durable hold upon the least tractable of the human passions, and the strong barrier which it was consequently calculated for a considerable time to interpose against the universal reception of Christianity. Independently of their intrinsic merits, which it is not our business to discuss, the respective claims of the Christian and Mahometan religions may be fairly estimated by their separate effects upon the course of social improveinent. In adopting this criterion, we discover at once the wide and marked distinction between the Divine and the human institution, the benevolent work of God and the interested fabrication of the most abandoned of his creatures. But Christianity and Mahometanism have been the subjects of attempted revision, in times subsequent to those of their founders: tradition has in both instances been prodigal of its aid; in the one case to vilify and corrupt what was for all its legitimate objects infinitely perfect; in the other to soften and explain what could not but appear to the awakened apprehension of fanaticism itself impious and absurd. But how dissimilar the results of this identical process! Christianity, secure in the plenitude of innate worth, has survived unhurt the mighty efforts of its numerous assailants : abuses, corruptions of all kinds, open and disguised, have repeatedly beset it, and threatened to hurry back the societies committed in its fate to the semi-barbarism from which it had relieved them. In vain! It has triumphed over difficulties surpassing the support of a corruptible power. Nations, in proportion as they become enlightened by knowledge, learn, one after the other, to shake off the errors which had fastened upon its substance: they retain with affectionate ardour the purer elements, and under their fostering influence advance with an assured step to the higher stages of civilization. What, on the other hand, has Islamism to offer to its enslaved votaries ? It has long since seen its zenith: it began near its highest point of attainable prosperity,-shone for a few centuries in the vicinity of that narrow circle, and has since gradually declined into a mingled state of infamy and contempt. It is totally divested of all power of self-elevation. Ignorance and political thraldom are the marks which it fixes upon the people benight ed under its impenetrable shade. To them the only avenue of hope is through the prospect of all possible degradation; when reverting, from the insane exaltation of fanaticism, to the stupid indifference of savages, and totally divested of all secular power, they may at length find a cure for all their ills under the salutary wings of that religion which was before the ob ject of their hatred and scorn. Mahometanism, the patron of indolence, sensuality, ignorance, and despotism, is in modern times the presiding genius of the Third Era. Even the obstinacy of Oriental polytheism, interwoven as it unfortunately is with all' the civil offices of life, may sooner yield to the kind and unremitted persua sion of truth, seconded by the practical lessons of a highly-civilized popu lation, providentially united to them by social ties. Shamanism, a peculiar superstition, possessing very extensive influence among the eastern nations, all the intricate and various machinery of Hindoo mythology,-will proba bly have ceased to influence the conduct of men some time before the final extinction of the Arabian superstition. It is therefore peculiarly necessary® to advert to the operation of the latter, upon the course of civilization. A principal feature of the Third Stage is the abject submission of the society to one or more tyrants, who rule without laws, or whose conduct such laws as chance may have established arc insufficient to controul. However a power so constituted may be divided, the effect is the same. Whether, as in the Ottoman empire, it is alternately in the hands of the Sultan and of the ferocious soldiery; or as in Morocco, in the sole possession of the despot, the people are equally deprived of all appeal from oppression. A dominion of this description was familiar to the ancients; but their religion presented no decided barrier against reform, and their consequent passage into that ameliorated state which has been alluded to under the head of the Fourth Stage, when the legislative enactments are binding in a sensible and important degree upon the conduct of the sovereign. In this stage, accordingly, Christianity found many of the nations at the time of its public introduction into the Roman states, about the middle of the fourth century. Not so the Mahometans. In the infancy of their religion, the societies which adopted it soon assumed their appropriate station in the Third Stage; and they have continued there ever since, without any prospect during their political existence of advancing beyond it. Unrestrained tyranny, and the profound ignorance under which alone that tyranny can be tolerated, are indispensable constituents of this deplorable superstition: a free admission of light, and the consequent imposition of impartial laws, extending alike to the humble cabin and the mysterious recesses of the seraglio, would at once dissipate the spell, and soon remove every vestige of its dominion over the human intellect. Among existing Mahometan states, unquestionably, an able observer might discover many points of difference. They have for the most part been better than they now are. They have fluctuated between the opposite confines of the Third Stage; but their motion has been for a considerable time past uniformly retrograde. The differences between them are accordingly to be estimated by the comparative extent of their losses, rather than the amount of their respective acquirements. From these considerations, we deduce that the wars incident to the Third Stage will linger in their last resort among the Mahometan nations. But can they be eternal? or rather, will they endure to the end of that term which a reasonable probability allows to the existence of the world? May we not, with greater reason, hope that the uncontrollable force of knowledge will sooner than is generally supposed reach these unhallowed retreats, and annihilate at once the empire of fanaticism, and the cause of those sanguinary contests which are the inevitable results of institutions so peculiarly unfavourable to innocence and peace? The Christian nations have almost all arrived at that point of social ele vation in which the government is compelled to pay some deference to public opinion. Abyssinia, the chief striking exception to this rule, was probably at no time enlightened by the pure spirit of Christianity; and the vast Russian empire can yet be considered only as a great motley association of sectarian Christians, Mahometans, and Idolators. Yet even in the latter case, it may well be questioned whether the Autocrat would, if he were inclined to hazard the experiment, be long suffered to lavish with impunity the blood of his subjects, for the avowed object of gratifying either his caprice or his ambition. What may be briefly termed tyrant-wars-the contests excited by the mere will and pleasure of the sovereign-must, it is evident, be expected in no very distant day altogether to cease. They are totally inconsistent with the genius of Christianity, and that spirit of true liberty of which Christianity is the parent and support: we have therefore only to point to the wellknown fact, that Christianity is actually on all sides gaining ground, as a full warrant for the foregoing inference. Wherever those wars are still enabled to preserve an equivocal existence, the scene of their enterprise becomes yearly more and more narrowed in the natural course of transactions connected with a superior stage of civilization. It is thus that their power over the great question of the ultimate disuse of all wars diminishes in weight, so as to afford the highest assurance of their complete extinction long before the more intricate, although less degrading, causes of contention incident to a better era have been finally suppressed. (To be continued.) DOCUMENTS IN THE COAL AND OIL GAS CONTROVERSY. (Continued from Vol. XV. page 744.) [The following Letter, which appeared in the Caledonian Mercury, and purports to be a further defence of Mr Leslie's deductions, we print here, as a sequel to the Letter of "Vindex."] OIL AND COAL GAS.-In our paper of This result is certainly unex- VOL. XVI. more than double the price of the Coal. On the subject of Coal and Oil Gas, a While Mr Milne's contains 32 parts of Olifiant Gas, the Leith Oil Gas only contains 16, which is just one part more than the Coal Gas itself. The next paper that we shall notice is that which was lately published by the Directors of the Edinburgh Oil Gas Com pany. It consists of a report by the Directors, and of the documents on which that report proceeds, viz. a letter from Dr Turner and Professor Christison, chemists, another from Dr Brewster, and a third from the Secretary of the Dublin Oil Gas Company; together with others of less importance. The third is the only one of these letters which contains any thing new on the subject of Gas; and this is, that in Dublin, Coal Gas sells at 15s. per 1000 feet, and Oil at 54s. 2d.; and that the Oil Gas Company is, notwithstanding, in a prosperous condition, and daily gaining ground, as they state, upon their rivals. In Dublin, however, coal is dear, and of a quality inferior to the Edinburgh for Gas making. The object of the other letters appears to be, not so much to throw any new light on this important subject, as to question the accuracy of Professor Leslie's results, by shewing that the instrument which he used for measuring the light of the Gases was not adapted for the purpose. It wants sensibility, it seems, and, according to Dr Turner and Professor Christison, it is "powerfully affected by heat, as well as by light," and therefore not to be trusted, where heat and light are both thrown off together from the Gas burners. Dr Brewster's opinion is to the same effect; and the Directors, improving the hint, state, that the photometer is affected" in a much greater degree by heat than by light;" and as it is known, they continue, that Coal Gas gives out much more heat than Oil, it is no way surprising that Professor Leslie, mistaking heat for light, should ascribe so much,illumination to the one Gas, and so little to the other. His results, therefore, are rejected as of no authority, and here we leave the parties at issue, referring to a letter on the subject, which will be found in another part of our paper. In justice to ourselves, however, we must remark, that, in recommending the photometer to the attention of experi menters, we did so from our own experience of its utility: and we have never found in it any of these defects with which it is now charged. Instead of being any way affected by heat, one of these instruments, which we have used for years, provided light be excluded from it, remains constantly at the same degree of heat, winter and summer ; and whether it be standing in a warm apartment, or exposed to the coldest atmosphere; but whenever light is admitted, if we only remove it, for example, from the interior of the house to the window, it rises sensibly, in consequence of the increase of light which it is now receiving. If we take it to the open sir, it rises still higher, in consequence of the light which is reflected on it from the sky and the clouds; and if we place it upon the snow, instead of sinking from the cold to which it is thus exposed, it actually continues to rise from the effect of the light which is so copiously reflected from the snowy surface around it. Again, if we set it before a common fire, it rises in consequence of the light which issues from the flame, and from the red hot coals but whenever we interpose a thin opaque skreen to shut out the light, and placing it so near the fire as still to throw out much heat, the instrument sinks to nothing—it stands at the same degree of heat as if it were plunged into a freezing mixture of salt and snow. This instrument, therefore, is very unlike a thermometer, which is so alive to all the vi cissitudes of heat and cold. It acts upon quite a different principle-it measures heat, no doubt, but it is only that heat which is derived from light, while the heat which flows from any other source has, under proper management, no effect on it whatever. On these grounds, we still think that this instrument is an accurate measurer of light, and that there must surely have been some fallacy in the alleged experiments to prove the con. trary. We would again, therefore, recommend the photometer to the attention of every experimenter on the illuminating powers of the Gases. Dr Brewster's Reply to "Vindex's" Letter, and Challenge to Professor Leslie. N. B. Were it not for the circumstance of its bearing the author's signature, the following angry epistle, written in a spirit of jealous resentment, incompatible (we had hoped) with the peaceful pursuits of science, and disfigured by paltry and abortive attempts at wit, should have been excluded from our record of the memorials of this controversy. To the Editor of the Courant. SIR,-Having learned, since my arrival in town, that there appeared in the Edinburgh papers an anonymous and abusive article on the subject of my letter to the Oil Gas Company, I trust you will give a place to the following observations: That this anonymous letter was written by Professor Leslie, is, I understand, well known, but as it does not bear the sanction of his name, I cannot permit myself either to read it or to answer its scurrilities. Having, long ago, resolved never to peruse any such anonymous productions, I have not even seen the newspaper lucubrations of the learned Professor; and the sole object of the present communication is, to confirm the opinion which I was called upon to give, in the discharge of a duty to a respectable public body. As I have for many years had no communication with Professor Leslie, I was aware that any opinion of mine would be misconstrued, that was unfavourable to the photometer, to which he has ventured to affix his name; and hence it became necessary to corroborate it by the high authority of M. Lambert and Sir W. Herschel, both of whom had been particularly occupied in the comparison and mensuration of lights. Such a line of argument was perhaps unnecessary, after Dr Turner and Dr Christison had demonstrated by actual experiment the errors of the thermometrical photometer; but as my opinion was intended for the information of the purchasers of Oil and Coal Gas, the authority of two celebrated names could not fail to have its influence with those who were incapable of appreciating the details of experimental science. The authority of Lambert against the thermometrical photometer is the most unequivocal of any that could have been adduced. Mr Leslie himself has read the photometria of that author, and characterises it as a work "displaying the various resources of his ardent and fertile genius ;" and as Lambert undoubtedly invented the thermometrical photometer, which now bears Mr Leslie's name, his unfavourable opinion of the very instrument which he himself proposed must be held decisive against its accuracy. I have been informed that Mr Leslie has rashly asserted, both in public and in private, that I never read the photometria of Lambert; that I could not possibly have seen a copy of it, as there is only one in the kingdom, viz. in the British Museum, and that this work contains no allusion whatever to the thermometrical photometer. As Mr Leslie does not ap pear to have read the photometria for 20 years, and as neither he nor any of his friends have seen the copy in the British Museum since this controversy began, it would be interesting to know how they have become acquainted with the fact that the thermometrical photometer is not mentioned in that work! Into this mystery, however, I shall not inquire. It is necessary for me only to state, that I studied the photometria of Lambert for some months, from a copy belonging to the reverend Dr Macknight, and that I could at any time have commanded the use of another copy in the possession of Mr Sivright of Meggatland. I have not only read this work with care, but I have given an analysis of the most valuable portion of it in the article OPTICS in the EDINBURGH ENCYCLOPÆDIA, which is the only account of Lambert's best experiments that has appeared in our language, or indeed in any foreign work with which I am acquainted. But, what is of more importance for Mr Leslie to know, I have in the same article quoted, in Lambert's own words, his proposal and his rejection of the thermometrical photometer, and I have mentioned its priority and similarity to his own instrument. Lambert does not say whether the thermometer that he mentions was the mercurial thermometer, or the differential thermometer, which was long ago invented by Van Helmont, as Sir Humphry Davy first shewed, though it also bears Professor Leslie's name; but whether it was the common thermometer, or any other, is of no importance, as he distinctly states, "that if we assume that the heat and light of a solar beam increase and diminish in the same proportion, then the thermometer will perform all the functions of a photometer." Now, whether Mr Leslie invented the differential ther |