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On the important subject of coal-gas two treatises have just appeared, which contain much valuable information. One of these is by Mr. Accum, whose former publication is probably well known to such of our readers as may be concerned in this branch of inquiry. The present may be considered as a new work, superseding the former, and abounding in practical details which the author's extended experience has put him in possession of. It is illustrated with seven useful plates.

After pointing out the nature and advantages of the art of procuring light from coal-gas, Mr. Accum gives an account of the different kinds of coal employed in Great Britain, and of their relative value for producing gas; he then treats of the form and dimensions of the retorts, and of the management of the distillatory process. We were here surprised to find so favourable an account of Mr. Clegg's horizontal rotary retorts, a description of which, furnished by that gentleman, will be found in the 2d volume of this Journal. We say surprised, from having been informed of the total failure of the plan at the Westminster gas-works, although erected under the inventor's superintendence; whereas, it now appears to have been carried into execution at Chester, Birmingham, and Bristol, and to be employed at the Royal Mint under the direction of Mr. Accum.

The 8th part of the author's treatise relates to the apparatus, and means of purifying gas; and in the 9th chapter he treats at length of gas-holders, or, as we have generally been in the habit of calling them, gasometers. A curious collapsing gas-holder, contrived by Mr. Clegg, is here described, especially applicable in those cases where difficulty attends the construction of a well, or tank, for the usual gasometer to work in. It requires a sheet of water no more than 18 inches deep.

Mr. Accum then proceeds to describe the gas-metre, a very curious and useful invention, derived also from the ingenuity of Mr. Clegg. It is a cylinder so constructed as to be kept revolving upon its axis, by the passage of the gas through it, the number of its revolutions depending upon the quantity of gas which goes through its various cavities in a given time; so that if

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is a self-acting register, which, if placed between the purifying vessels and the gasometer, shews the quantity of gas generated daily; and if between the gasometer and the mains, it tells the quantity of gas which is consumed by the burners, or otherwise disposed of.

The remaining sections treat of the pipes, burners, and a variety of other minor, but important subjects.

Mr. Peckston's work on gas illumination also abounds in useful details. It treats of the theory of the production of artificial light, the cost of various modes of obtaining it, the natural history of pit-coal, its various combinations and uses, especially in the production of gas. In this volume there are also fourteen useful plates, and the author seems to have attained the object he had in view, that of describing plainly and methodically every part of the gas-light apparatus, so as to enable any one acquainted with machinery to erect such, either for supplying his own premises with gas, or for lighting up manufactories, streets, or towns.

In both these treatises we are glad to find concise, but explicit directions to workmen, for the erection and construction of different parts of the apparatus; with much theoretical knowledge there are abundant practical details; and though Mr. Accum and Mr. Peckston widely differ upon certain minor points, they agree in their principal conclusions. If we understand rightly, Mr. Accum prefers cylindrical, and Mr. Peckston elliptical retorts. The latter, in his chapter on the gas-meter, has discussed, at some length, the title of Mr. Clegg to its invention, and considers the gas-meter contrived by Mr. Malam as not only an original invention, but as infinitely preferable, which it really appears to be, to the prior contrivance of Mr. Clegg.

These works on the manufacture of coal-gas will not only be found useful to those who wish to avail themselves practically of their contents, but they abound in statements and information which will probably interest a much more numerous class of general readers.

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ART. XIII. Outline of a Theory of Meteors. By Wм. С. REYNOLDS, M.D. Middletown Point, New Jersey.

(The following paper is from the Quarterly Journal of Science, edited by Dr. Silliman, at New York. We have republished it in the hope that it may call the attention of our readers to the very interesting and curious subject of which it treats, and in the expectation of its inducing some of our Correspondents to resume the discussion. We do not consider the hypothesis of Dr. Reynolds as the most happy that has been invented to account for the production of meteoric bodies, though it appears to us quite as plausible as the idea that they are of lunar origin, a notion sanctioned by some eminent mathematicians and meteorologists. ED.]

SHOULD the progress of science, for a century to come, keep pace with its rapid advancement for the last fifty years, many appearances in the physical world, now enveloped in obscurity, will then admit of as easy solution as the combustion of inflammable substances, or any familiar process in chemistry, does at this day. Among the many subjects from which the veil of mystery would thus be raised, we may include those luminous appearances in the aërial regions, called meteors, which I am about to consider in the following essay; and which seems to constitute a distinct class of bodies of considerable variety.

Meteors were regarded by the ancients as the sure prognostics of great and awful events in the moral and physical world; and were divided by them into several species, receiving names characteristic of the various forms and appearances they assumed; but of their opinions, as to the physical cause of these phenomena, the ancients have left us nothing solid or instructive. The moderns, more enlightened, have ceased to regard these bodies with the superstitious awe of former ages; but in respect to the cause thereof, are perhaps but little in advance of their predecessors, having, I believe, produced nothing yet that will bear the test of philosophical investigation.

Doctor Blagden (Philosophical Transactions, 1784,) considers electricity as the general cause of these phenomena; Doctor Gregory, and others, think they depend upon collections of highly inflammable matter, as phosphorus, phosphorated hydrogen, &c.

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being volatilized and congregated in the upper regions of the air. Doctor Halley ascribes them to a fortuitous concourse of atoms, which the earth meets in her annual track through the ecliptic; and Sir John Pringle seems to regard them as bodies of a celestial character, revolving round centres, and intended by the Creator for wise and beneficent purposes, perhaps to our atmosphere, to free it of noxious qualities, or supply such as are salutary. Many other theories, as ingenious as fanciful, might be enumerated: but without commenting on their comparative merit, I must acknowledge that none of them have yet impressed my mind with a conviction of their truth. A series of observations, however, have enabled the moderns to ascertain, with apparent accuracy, several particulars relative to these stupendous bodies, which add much to our knowledge of their general character: - their velocity, equal to 30, and even 40 miles in a second of time; their altitude, from 20 to 100 miles; and their diameter, in some instances, more than a mile, are facts we derive from respectable authority, and may aid us, essentially, in forming just conceptions of their nature and properties.

I believe meteoric stones to result from all meteoric explosions; limiting, however, the term meteor, to those phenomena, in the higher regions of the air, denominated fire-balls, shooting-stars, &c. That these bodies move in a resisting medium, must be evident to every attentive observer; and that this medium is our atmosphere, is pretty certain, 1st. Because we know of no other resisting medium round the earth; 2dly. Because the same kind of resistance is apparent at every intermediate altitude, from their greatest to their least, which last we know to be far within our atmospheric bounds; and, 3dly. Calculation has, in no instance, assigned them an elevation beyond the probable height of the atmosphere.

That meteors proceed from the earth, that they arise from certain combinations of its elements with heat, and that meteoric stones are the necessary result of the decompositions of these combinations, are opinions I will endeavour to support, by the following considerations.

1st. The properties and habitudes of matter, under certain conditions and combinations.

2dly. The situation of the earth's surface in respect to the sun, the influence of his rays thereon, and the nature of the elements of compounds on which these rays act:

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And, 3dly. The identity that exists between the component parts of meteoric stones, and the elements that enter abundantly into the composition of our globe; and, by several other facts and arguments.

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Under my first general specification, I will select such principles from the established doctrines of philosophy, as have an immediate bearing on the subject; without engaging in any of those subtle speculations in which certain recondite properties of matter, or the identities of quality and body are affirmed or denied.

Thus, 1st. Heat is the universal cause of fluidity and volatility in bodies; hence no solid can assume the state of gas, until it absorbs, or unites with, a certain portion of caloric; and the subtilty and volatility of compounds thus formed, will be in a due ratio to the quantity of caloric they employ.

2dly. The heat employed to maintain a body in the gaseous state, is said to be latent or fixed, and may be regarded as an ocean or atmosphere of fire, holding the ultimate particles of the body in a state of extreme division, and wide separation, from which they can be driven only by some change in the affinities or condition of the compound.

3dly. If the latent heat in a gaseous compound be suddenly abstracted, as in explosion, its escape is attended with the emission of light and sensible heat, when the volatilized particles held in solution being no longer able to maintain the state of gas, suffer approximation in a due proportion to the quantity of caloric they have lost.

4thly. Caloric, in reducing solids to the state of gas, lessens, but cannot in any case, as far as we know, totally destroy their gravitating force; the diminution of this force, however, being in a direct proportion to the quantity of heat employed.--Hence the following inferences may be fairly drawn, as they seem to be in unison with the relative dependence and harmony existing between the material elements of this globe, and, I believe, are contradicted by no direct experiments; viz., that the expansion of volume, specific

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