vacuum, and, at others, four inch pressure. pressure is in the retort that the gas leaks out.

It is when the

MR. GRAY-What about the comparative time for cleaning the retorts under pressure and without pressure, and also the comparative time of cleaning the stand-pipes with and without pressure?

MR. FORSTALL-The first portion of your question has been answered-three months with dips and six months without; the second part of the question I can answer at the next meeting of this Association; but, at present, I am not prepared to to do it. I am keeping a daily record of everything connected with this subject, the results of which I will endeavor to present at the next meeting.

MR. BATTIN—Was there an analysis of the gas made at different times?

MR. FORSTALL-No; not an exhaustive analysis; but the specific gravity was taken and found to be very heavy-more than our average.

MR. BATTIN-I suppose, then, you were perfectly satisfied that the gas was heavier?

MR. FORESTALL-Yes, sir; I had no doubt about it.

THE PRESIDENT here stated that about as much time had been consumed by the discussion of Mr. Forstall's paper as it was possible for the Convention to bestow upon it; and he, therefore, announced the next paper, which was read by Mr. Rankin, on "Carbonizing Coal."

Before commencing to read, Mr. Rankin said:

I want to request the Association to make as little disturbance as possible, because my voice is not very strong, and the less confusion there is the more apt you are to hear. [A voice —put on more pressure.] I will state that we always work under a low pressure.

MR. RANKIN's paper on

CARBONIZING COAL.

It has been said, that "Science and Art differ in this, that Art says-give us Rules; while Science demands reasons." If

so, ours then may be deemed a profession, as we require both rules and reasons. But are we not too often content to work by rule, rather than seek for and demand substantial reasons.

The subject that I have chosen for this paper, viz., carbonizing coal, lies at the very beginning, in fact, out of it spring all others connected with our calling. Much profound thought, and patient investigation, has been bestowed on this subject by those who were far more competent than the writer, therefore, I could not expect to say much that has not already been said; but feeling that there is room for improvement both in the apparatus and process, as usually employed, and with a desire and hope that the discussion of the subject may suggest something practical, and a decided step in advance. I would, with becoming modesty ask your attention for a short time.

In the manufacture of illuminating gas the proper means of carbonizing coal is of the first importance, ignorance on this point has often entailed very serious loss, when by a little intelligence, satisfactory profits would have resulted.

Heat being the important agent in this operation, we can but regret that science has not furnished us with some convenient and ready method of estimating high temperatures; and, until something capable of being reduced to daily practice is discovered, we must be guided by the colors presented by the retorts. This, however, is uncertain, from the fact that when heated retorts are exposed to daylight, the color or temperature appears lower than it really is; or, when they are observed in the dark; the temperature seems higher than it actually is. But constant observation improves the faculty of estimating high temperatures by the shades of color, and continued practice enables the intelligent observer to regulate the working of his furnaces.

The apparatus now used for carbonizing are too familiar to require any description at this time, so we will at once proceed to consider the silent, but powerful, forces that are at work within. There are three of them-heat, cohesion and chemical affinity, especially the first two, as it is by the mutual action of these two antagonistic forces, which have been appropriately termed the attractive and repulsive forces, that the state or condition of all matter is determined; a preponderance of the co

hesive force producing the solid condition of matter; the equilibrium of the two forces the liquid state; and an excess of the repulsive force the gaseous or vaporous condition.

The proper degree of heat to be employed in carbonizing coal in gas making we will not consider in this connection, but will first notice its effect upon the charge of coal which, we assume, is just being introduced into the retort. "The constituents of the coal are, by the first application of the heat, torn asunder, and converted from the solid into the liquid state, by a further increase of the heat these liquids are converted into vapors and gases, which are, in turn, further torn asunder, by the action of higher temperatures, into simpler compounds; at each step, carbon, in the solid form, being more or less eliminated, and the liquids and gases become simpler in their constitution."

Now, the uniformity of these results depend upon certain unvarying conditions. When coal is distilled at a very low temperature the result is the production of oil vapors; if these vapors be passed through an empty retort, or pipe, of considerable diameter, heated to bright redness, only a very small amount of the vapors are decomposed, and the oil vapors pass almost unchanged. If, on the other hand, the retort or pipe be filled with other pipes, each one of less diameter than the last, thereby dividing the large space into a number of narrow annular passages presenting a large surface, and these oil vapors be passed through this when heated to bright redness, the decomposition is very much increased; in fact, only a very small quantity of these vapors escape decomposition, you will perceive it is not owing to any change in the temperature that produces such widely differing results but simply a change of conditions. "This effect of surface in bringing about decomposition, results from the fact that gases and vapors are comparatively feeble conductors of heat, they allow the heat rays, or pulsations sent into them from the heated walls of the retort, to pass freely through them, without being themselves heated to the extent necessary to bring about decomposition, and, therefore, it is only by direct contact that the necessary amount of heat can be communicated rapidly, in order to bring about the decomposi

tion of organic substance when in the vapor state." But, while surface is the medium through which the heat is transmitted to the elements of coal to bring about decomposition, due regard must be had to the temperature of that surface. If the heat be too low, decomposition is not effected; if too high, decomposition is carried too far, and the gases become too simple in their constitution. Seeing, then, that the heat limits are comparatively narrow, within which carbonization must be conducted, it is evident we must have some certain means, or arrangements connected with our retorts by which we may secure both the requisite amount of properly heated surface, and insure the contact of the vapors with that surface, if we ever attain the results that experiment seems to indicate as being within easy accomplishment.

This brings us to the consideration of the apparatus now employed, and more especially the mode of using them in this most important department of gas making. The clay retorts, now so universally adopted, I think, are altogether satisfactory, so far as durability is concerned, any lack of efficiency is, in my opinion, almost, if not entirely, owing to a want of subdivision of the processes which actually take place in every coal retort. As we have already seen, coal is always distilled at a low heat, and the condensable vapors are subsequently converted into permanent gases.

We may now consider, whether it is possible with the means employed to maintain nearly a uniform temperature, which is indispensable to uniform results. In practice, we take a retort, say 12"x20"x8'6" long, heated to bright redness, into which we shovel, as rapidly as possible, from 150 to 200 pounds of coal; the consequence is, that, within from twenty to thirty minutes, the temperature of the inner surface of the retort is so much reduced that it is incapable of doing little more than change the solid elements of the coal into the liquid or vaporous state; this process, however, goes on very rapidly, even at the reduced temperature, which fact may be easily determined. With a suitable apparatus, I found by a number of experiments recently made on good Pittsburgh coal, that with the retort heated to the lowest visible red heat, that the volatile matter of the coal'

was expelled at the following average rate. At the end of the first hour after introducing the charge that fifty-eight per cent. of the whole gas producing matter had passed over; at the end of the second hour eighty-five per cent.; and at the end of the third hour, ninety-six per cent., or nearly the whole of the volatile matters, and with the cannel coals the rate was even more rapid. This, then, being the fact, we can very readily forsee what must of necessity be the result. While the heat is so low, the great bulk of the volatile matter is driven over, and, failing to meet with heated surface to decompose them, these vapors are carried forward and gradually condensed into liquids, whose composition is the very elements that constitute good illuminating gas; with different heat, at the proper time, these same vapors could be measurably converted into permanent gases. This low temperature continues till the charge of coal, by contact with the bottom and sides, and by radiation from the top of the retort, has become heated to redness, which usually requires from one to one and a half hours, at which time things are in good condition for efficient work. With a large per cent. of the gas producing material wasted, so far as lighting is concerned, we have now attained the most favorable conditions possible with the retorts as now used for carbonizing coal; and could we maintain just this condition of things, to the end of the charge, it would be well. But, no; as the outer surface of the coal in the retort has become sufficiently heated to melt and form an envelope, or crust, of coke, and this crust having parted with its vapors, is soon raised to a high temperature ; through this countless jets of vapor are constantly forced from the enclosed portion of the charge. The temperature of this crust is not only increasing, but the heat is penetrating to a greater depth; the vapors from the interior having to pass through this excess of heat, decomposition is carried to far, carbon is deposited in the form of exceedingly fine particles, imparting to the coke that peculiar white, metallic lustre always observed in coke from hot retorts; and the gas produced from this time to the drawing of the charge becomes gradually poorer in quality. This, however, I think, should not be so. As the coal put into the retort was all alike, it seems