The heating of the gas furnace, the charging of the retorts with coal, the closing them up air-tight, and keeping them red-hot, are the only operations required in this art; and these demand no more skill than a few practical lessons can teach to the meanest capacity.

The diversified experiments which have been made by different individuals unconnected with each other have now sufficiently established the perfect safety of the new lights, and numerous manufactories might be named in which the gas lights have been in use for upwards of seven years, where nothing like an accident has occurred, though the apparatus in all of them is entrusted to the most ignorant man.

That coal gas, when mixed with a certain portion of common air in close vessels, may be inflamed by the contact of a lighted body, is sufficiently known. But the means of preventing such an occurrence in the common application of this species of light are so simple, easy, and effectual, that it would be ridiculous to dread dangers where there is nothing to be apprehended.

In speaking thus of the safety of this new art of illumination, it would nevertheless be easy to name instances where explosions have been occasioned, but solely through egregious mistakes having been committed in the erection of the gas light machinery, were this a subject on which I meant to speak; but as I do not, I shall merely mention, on the present occasion, that an explosion very lately took place in a manufactory lighted with coal gas, in consequence of a large quantity of gas escaping (from the gazometer being overcharged with gas) into the gazometer house, where it mingled with common air, and was set on fire by the approach of a lighted candle. That such an accident could happen, is an evident proof that the apparatus for preparing the gas was a bad one, because such an accident might have been prevented effectually by adapting a waste pipe to the gazometer, as well as to the gazometer house. By this means, if more gas had been prepared by a careless operator than the gazometer could contain, the superfluous quantity could never have accumulated, but must have been transported out of the building into the open air, in as effectual a manner as the wastepipe of a water cistern conveys away the superfluous quantity of water when the cistern is full.

In answer to the second question made by your Correspondent, namely, what sort of coal is to be prepared for producing the gas, it remains to be observed, that Cannel coal produces the very best gas; or at least the gas which it affords requires the least trouble of being purified and rendered fit for illumination; though Newcastle coal is employed for illumination in this metropolis. But the

The public buildings already illuminated in this town with coal gas are the following: the church of St. John the Evangelist, the avenues to the House of Lords and House of Commons, Wesminster Hall, the Admiralty, the house and offices of the Speaker of the House of Commons, the Mansion House, the whole liberty of Norton Falgate, &c.; and the total length of pipe laid down as mains in the streets of London amounts already to 15 miles,

nature of the gas obtained from the same coal varies considerably, according to the conditions under which it is obtainable. 112 lb. of common Cannel coal produce at the minimum from 350 to 360 cubic feet of carbureted hydrogen; but the same quantity of the best Newcastle coal, that is to say, such as coke readily, and send out brilliant streams of flame, which undergo a kind of semifusion when laid on the fire, produce upon an average 300 cubic feet of this gaseous fluid, besides a large portion of sulphureted hydrogen, carbonic acid, and carbonic oxide.

Half a cubic foot of this gas, when fresh prepared, that is to say, holding in solution or suspension a portion of the essential oil which is generated during the production of the gas, is equal in illuminating power to from 170 to 180 grs. of tallow, which is the quantity of this material consumed in one hour by a well snuffed tallow candle six to the pound. Now 1 lb. avoirdupois is equal to 7000 grs., and consequently 1 lb. of candles of six to the pound, burning one at a time in succession, would last 700 = 40 hours. To produce the same light, we must burn one half of a cubic foot of coal gas per hour; therefore one half multiplied by 40 hours is equal to 20 cubic feet of gas in 40 hours, and consequently equal to 1 lb. of candles, six to the pound, provided they were burnt one after another.

Further, 112 lb. of Cannel coal produce at the minimum 350 cubic feet of gas, and are equal to 350 divided by 20, which last is equivalent to 1 lb. of tallow, making therefore 112 lb. of coal equal to 250 174 lb. of tallow; and 112 lb. of coal divided by 17 of tallow gives six and four-tenths of coal equal to 1 lb. of tallow.

With regard to Newcastle coals, it may be stated that one chaldron of Wall's End coal produces in this large way upwards of 11,000 cubic feet of crude gas, which when purified diminish to nearly 10,000 cubie feet. But the quantity and quality of the gas, as stated already, is much influenced by circumstances attending the formation of it. If the tar and oil produced during the evolution of the gas in its nascent state be made to come in contact with the sides of the red-hot iron retorts; or, better, if it be made to pass through an iron cylinder or other vessel heated red-hot, a large portion of it becomes decomposed into carbureted hydrogen and olefiant gas; and thus a much greater quantity of gas is produced than would be obtained without such precautions. If the coal be distilled with a very low red heat, scarcely observable by day-light, the gas produced gives but a feeble light if this distillatory vessel be of a dull redness, the light produced by the burning gas is more brilliant if a bright, or cherry-red, heat be employed, the gas produced burns with a brilliant white flame: and if the heat be increased so far that the retort is almost white hot, and consequently in danger of melting, the gas given out has little illuminating power, and burns with a clear bluish flame: and if this coal abounds in pyrites, a large portion of sulphureted hydrogen gas is

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then produced, which has the capital disadvantage of affording a suffocating odour when the gas is burnt.

I need scarcely mention that it makes no difference in what form the coal is used, and that the very refuse or small coal, which passes through the screen at the pit's mouth, and which finds no market, nay, even the sweepings of the pit, which are thrown away, may be employed for the production of the gas.

With regard to the pressure of the gazometer, your Correspondent is informed that experience has shown that a pressure of a column of water from an half to one inch is sufficient for regulating the proper supply of the gas to the lamps and burners; but this pressure must be constant and uniform. It is obvious that the weight of the gazometer or vessel which contains the gas is constantly increasing in proportion as it fills with gas and rises out of the water or cistern in which it is immersed; and consequently, if a constant or uniform balance weight equal only to that of the gazometer in the first moment of its immersion be employed, the gas becomes gradually more and more compressed by that part of the weight of the gazometer which is not counterpoised; therefore insurmountable difficulties would follow, because it would be impossible to regulate the size of the flames, &c. To compensate for this increasing weight of the gazometer, the chain by which this vessel is suspended, or at least such a part of it as is equal in length to the height of the gazometer (measured at right angles to the axis of the wheel over which it passes downwards) must be loaded with a weight equal to the quantity of water which the gazometer displaces; and thus the density of the gas will be uniform, or at all times the same.

The diameter of the pipes which convey the gas is not taken at random, as your Correspondent imagines. Their diameters is a simple matter of calculation, depending upon the quantity of gas which they have to deliver in a given time, and the diameters of the branch pipes proceeding from them,

Further information concerning the general nature of the gas light illumination, together with a description of the best machineries employed in this new branch of civil economy, your Correspondent will find in a Treatise on Gas Light, illustrated with copper plates, which will be published on the 10th of next month, by, Sir, Your most obedient humble servant,

Compton-street, Soho,
April 22, 1815,

FREDERICK ACCUM.

* For this elegant contrivance we are indebted to Mr. Clegg, the engineer of the Gas Light Company.

ARTICLE V.

Remarks on the Older Floetz Strata of England.
By J. C. Prichard, M. D. F.L.S. F.W.S. &c.

SIR,

(To Dr. Thomson.)

I HAVE long entertained a suspicion that it may be possible by comparing the organic remains found in the lime-stones, which are connected with coal-fields, with those which characterize some other rocks, to elucidate the series of secondary strata, which our island presents, and especially to determine the era of the independent coal formation. On reading Dr. Fleming's late communication on the fossils found by him in Linlithgowshire, I was so strongly confirmed in this persuasion that I have ventured to submit the following remarks on the subject to your inspection, and to that of the public if you think them worth inserting in your Journal.

It seems improbable that a single species of organized beings should appear in one stratum, and then vanish entirely during an interval, and afterwards show itself again. It is contrary to what we find in nature. A fossil which abounds in one formation is often seen more scantily dispersed through a second, in a third it is scarcely found, and at length withdraws itself altogether from our view. A continual progress seems to have been made from the more simple to the more complex forms. We observe no retrograde changes. But if the extinction and revival of a single animal be thus improbable, how much more difficult is it to suppose that an entire assemblage of co-existent beings should disappear altogether, that their place should be filled during an interval by creatures of a totally different character, and that these should become extinct to make way for a reproduction of the former class? The supposition is so contrary to the usual course of our observations, that I think we may conclude, when we discover two formations to abound with similar fossils, and a third to be characterized by remains of a different description, that the two former belong to one era, and that the latter is either more ancient or more recent than both of them. If this conclusion be allowed, it will enable us to ascertain the relative age of the independent coal formation, or at least of the coal-fields in Britain.

I shall first enumerate the extraneous fossils found in the oldest class of rocks which contains any, viz. those of the transition formation, and chiefly the transition lime-stone.

Mr. Jameson mentions among the fossils of this rock encrinites, madreporites, tubiporites, corallites, and trochites.

Von Buch found in the transition lime-stone of Norway, Sweden, and Finland, which lies under granite, a great abundance and

variety of orthoceratites, some of which were many feet in length. He observes that they distinguish this formation throughout Europe. He notices also pectinites, the oniscus, trilobites, a number of large madreporites, a great many trochites, entrochites, patella, a few ammonites, and a great number of other univalves.

Saussure found in the lower chains of the Alps, between Mont Blanc and Geneva, pectinites, terebratulites, gryphites, entrochites, a great many corallites and madreporites, turbinites, and ammonites.

I shall now mention some of the fossils found in the lime-stone rocks which accompany the coal formation in Britain, and which generally shut in or inclose the coal-fields.

Orthoceratites, as observed by Dr. Fleming. Their existence in the coal-field of Linlithgowshire is not a solitary fact. I have seen one which was found in St. Vincent's Rock, in the boundary of the Somersetshire coal basin. It was in the possession of Mr. Cumberland.

Encrinites and trochites occur in astonishing abundance in all the rocks of this class in South Britain. Dr. Fleming has mentioned them in Linlithgowshire.

A great variety of madreporites is commonly seen.
Tubiporites are mentioned by Mr. Townsend.

Pectinites are often found in the rocks near Bristol.

The trilobite is well known in the lime-stone rocks at Dudley, in Staffordshire.

Ammonites occur, though more rarely, in the lime-stone of the coal formation. They are mentioned by Mr. Aikin in the coal-field of Shropshire.

Terebratulites are found very commonly in all the lime-stones of the coal formation.

I might enlarge this catalogue to a much greater extent; but what I have said will suffice to show that there is a general conformity between the animal remains found in the transition limestone and the lime-stones of the coal-fields. Hence it appears that at the periods when these two formations were deposited, the ocean was filled with organized beings of the same description. The astonishing abundance of these relics in the rocks of both orders testifies the vast profusion of animal life which the sea contained at each of the periods in question.

That the whole of this assemblage of animals became extinct, and were afterwards produced anew, and that the ocean in the interval was filled with a different set of creatures, which suddenly vanished when their predecessors appeared for the second time, can scarcely be imagined. It follows, therefore, that the first floetz lime-stone of the Wernerian series, to which fossils of a different character are assigned, is more recent than the rocks of the independent coal formation.

This conclusion is confirmed by considering the situation in which