stimulates me to this action is, that so far as opinions upon it are antagonistic to the one I hold, my researches upon the subjects connected with it are either not taken into proper account, or else are completely ignored.

Thus the Rev. Mr. Highton, in communicating the fact instanced to the British Association, sitting at Brighton in 1872, referred the origin of the gas evolved to a decomposition of a portion of the sulphuric acid used, the constituents of this acid being, as he considered, hydrized to water and sulphuretted hydrogen by the nascent hydrogen liberated in the carbon pores during the process.

This statement, though eliciting much discussion at the meeting instanced, did not induce any one there to attempt the application of these published researches of mine thus indicated to the explanation of this phenomenon, but the majority of those who took part in this discussion sought rather to attribute this evolution to the decomposition of sulphide of iron in the graphite used, this substance being in fact a general constituent of graphite, and easily decomposed by sulphuric acid with liberation of the gas mentioned.

Both these assumptions appearing to me not only radically erroneous, but altogether superfluous, in face of the facts described in the published paper of mine instanced, I ventured in the “Chemical News” ^* to refer the Rev. Mr. Highton to these facts, and to suggest that the phenomena in question could be easily explained by taking these into proper consideration, and I received an answer ^† from that gentleman, which, except for certain suggestions, does little more than inform me that his opinions on this subject are unchanged.

This is the precise position of the question in dispute between us, and as it appears of some importance, from a chemical point of view, and may be also in connection with the very desirable object of freeing coke from sulphur, I now beg leave to treat the whole of it in a paper to you, such a course allowing me to discuss it at greater length and with more justice to myself than can well be attained within the limits which a correspondence column would necessarily impose upon me.

It will be seen, then, that there are three theories propounded for explaining the evolution of sulphuretted hydrogen from graphite or carbon under the circumstances named:—

In relation to the first, I have ascertained that the gas in question is not evolved from carbon when paired with zinc in pure sulphuric acid, if only the

carbon is free from sulphur; but if we allow the carbon a few seconds contact with sulphuretted hydrogen, wash it, connect as before, and even use pure hydrochloric acid as the exciting fluid, a great quantity of this gas is given off at the surface of such carbon.

These experiments are very simple ones, and appear to be decisive—at least they clearly show, when taken in connection with the prevalence of sulphuretted hydrogen in the air and the absorptive power of carbon for it, that the evolution of this gas described by Mr. Highton does not necessitate the decomposition of sulphuric acid. But, in answer to this, Mr. Highton asserts that “if any one will take a porous jar with a plate of carbon in the middle packed round with broken pieces of carbon, form it into a battery with amalgamated zinc, dilute sulphuric acid for an electrolyte, and set it to work through say twenty Ohms for twelve hours, I think he will find the evolution of sulphuretted hydrogen too great to be thus accounted for.”

Now, I contend that if the suggestions conveyed in this paragraph were carried out, their only effect would be to encumber the case, and further, that the opinion Mr. Highton arrives at, being based upon hypothetical results merely, has not that force or certainty proper to any opinion the object of which is to controvert one affirmed to rest upon experimental evidence, as this does. If Mr. Highton is so sure of his ground that he thinks he can afford to exercise his courtesy by toning down in this manner what he conceives to be truth, I thank him for this manifestation towards me, but, while appreciating the kindly motive itself, I must express a wish that he had been more specific; and I may state here that my only reasons for delaying so long as I have in answering this letter have been the lack of this, quality in it, and the hopes which I entertained that the question in dispute between us might ere this have been taken up by some one who would be unbiassed to either side by previous utterance or thought of his with regard to it.

However, to go back to the allegations I then made in reference to the paragraph itself, I would ask Mr. Highton what necessity, if any, is there to pile the carbon pole around with pieces of carbon? If juxtaposition of pieces of carbon with the carbon pole fundamentally affects the character of the results in regard to the question at issue, then I submit that Mr. Highton is leading me off from this question to the consideration of a phenomenon I have neither contemplated or disputed, which is, besides, new to chemistry, and so one that Mr. Highton, being the first to indicate, has the honourable right to be the first to investigate.

This modification by Mr. Highton would certainly have the tendency to largely increase the evolution of the gas in question and so to increase its duration, but still I contend this would not be to a greater extent than can be accounted for upon other grounds than those Mr. Highton has taken up. It

would, I think, be proper for Mr. Highton to determine the quantity of this gas which can be liberated from a certain weight of carbon by the means I have indicated before assigning a limit to it in the manner he has. In regard now to the “experimentum crucis” which Mr. Highton suggested to me for settling the question, viz., “to keep renewing the sulphuric acid” (of the battery of his just described) “till all sulphides accidentally present must be fully decomposed, and seeing whether evolution of sulphuretted hydrogen still went on.” Upon the theory (not yet exploded) as to the infinite divisibility of matter, and owing to the length of time which would be necessary to detach by this means the bulk of the gas in question from the centre of the carbons used, I might with propriety decline to allow my theory upon this subject to stand or fall by the results of such an experiment. I have, however, tried the experiments more for relative than absolute results, and, as I expected, found a well-marked diminution in the yield of sulphuretted hydrogen as time proceeded.

This diminution is manifested very conspicuously to prepared lead paper when held over the battery. At first this is rapidly blackened, but after twenty-four hours have elapsed a light brown tint only is communicated after it has been in contact one hour with the gas evolved, and when the acid was renewed, the zinc cleaned, and the battery allowed to work another six hours, lead paper suspended in the same position as before did not manifest a change in colour for twelve hours; and so, making renewals of acid, but still retaining the same carbon pole, I at last got the evolution of sulphuretted hydrogen so far reduced that even when the battery was in good action it required two days' contact of the test paper with the gas issuing from the battery in order to get an indication of the presence of sulphuretted hydrogen. As above inferred, I do not expect that this gas could ever be wholly removed from a plate of carbon by this process, but I think my results show that it is given off in constantly diminishing quantity, no matter how often we renew the sulphuric acid, and this is all I contend for, as it shows clearly that this acid cannot be the source of the sulphur of the H²S given off, as Mr. Highton asserts, since it is obvious that were it so the gas should be given off in quantities nearly uniform for equal times.

Regarding the supposition that nascent hydrogen in carbon pores should, according to the analogy of other chemical facts, form, with sulphuric acid, water and sulphuretted hydrogen, I am at a loss to know what facts are referred to by Mr. Highton, and therefore cannot debate this point. I admit, however, that porous carbon or platinum is capable of exercising decomposing and recomposing effects upon certain substances by what is termed provisionally catalytic action; but there is a line above which this action is not exercised, and it, seems to me that the decomposition of sulphuric

acid in the way indicated is above it. Analogy cannot be always trusted as a guide, though as a useful suggestor of probabilities it is unrivalled.

As to the theory that the sulphide of iron in the carbon or graphite used have is concerned in the production of this gas, Mr. Highton himself appears to have thoroughly settled this by proving that sulphuretted hydrogen, is not generated when the poles of his battery are unconnected, sulphide of iron being, as is well known, decomposed with evolution of this gas on the mere contact of sulphuric acid with it, and this would of course go on with still more energy and rapidity when the iron compound is in juxtaposition with a negative substance, as carbon.

Having thus endeavoured to show the inability of either of these two theories to explain the phenomena described, I will now proceed to the next one, which is that I have always held and now hold upon this subject, viz., that the gas in question is derived, as to its sulphur at least, from the carbon itself.

I base this conclusion upon the following data:—

Of these data the only ones requiring consideration here, as not being already treated or detailed, is the metallic nature of carbon, and (in relation to the absorptive power of this substance for sulphuretted hydrogen) the form in which this gas or its sulphur is absorbed.

That carbon in the graphitic state is a metal has long since been promulgated, and without, as I believe, eliciting any controversy of a nature at all subversive to such an idea, indeed its relations when in this state, both to light and electricity, are so precisely those of metals generally to these forces, that, according to the manner by which we as yet distinguish metals from metalloids, we cannot avoid accepting this idea. Graphite, the substance here in view, is not, as we know, pure carbon, but it certainly bears the same relation to light and electricity as pure graphitic carbon does; therefore we may, I think, correctly class it as a metal too (though an impure one) and possibly sustaining the same relations to pure graphitic carbon as hydrogenized palladium bears to pure palladium, both metals being impure although truly metallic.

Now, as to the form in which H²S is absorbed by charcoal—that is, as we have seen, by impure metallic carbon—it is well known that those metals which

are somewhat difficult to oxidize can absorb H²S, for instance, silver, mercury, gold, and platinum. It is further known that the removal is, in these cases, chemically brought about, the sulphur of the gas combining with the metal forming a true sulphide; why not, then, continue the analogy of graphitic carbon with these metals, by supposing that the absorption of this gas by such carbon is also a chemical action resulting in the formation of a sulphide of carbon analogous in some measure with, the sulphides of the metals instanced? We can suppose that, according to the proclivities of these sulphides, they may be hydrous or anhydrous, but any way we must account for the hydrogen dissociated from the sulphur in this absorption, and this we cannot do better, I think, than by supposing it combined with oxygen to form water, free or combined, according as the sulphides are anhydrous or not.

I cannot see but that this is a legitimate conclusion; and though it would be very difficult to prove by analytical results that such is the case, it would, I believe, be quite as difficult to prove the reverse.

It may be averred, in response to this, that there is no physical change manifested in the carbon, and that the absorption is slight as computed upon the weight of solid material used; but we must consider that physical change, of a magnitude so great as to manifest itself to us, is not necessarily the consequence of even a profound chemical change in structure or composition. The sulphurizing of gold or platina for instance at common temperatures by the gaseous method does not exhibit itself optically, nor even gravimetrically to any ordinary weighing apparatus, yet the surface of these metals can, as we know, be covered with the compound thus formed, and so completely that they will not analgamate while in this condition.

Again we must consider that the comparatively small extent to which this absorption can be carried on may be due to carbon, being less quantivalent when in the graphitic or metallic state than it is now assumed to have, this being certainly computed for it as existing in the metalloidal state. Further, we know that hydrogen and other elements form a notable part of most graphitic carbons, especially of charcoal, and as these are, without doubt, in greater part combined with the carbon, we may have a compound to deal with in place of an element, and so a substance having yet a higher atomic equivalent. Besides, the greater part of this compound may, like that of gold and platinum with sulphur, be so situated as to be out of contact with the gas; also, the sulphuretted compound thus formed is possibly so firmly adherent to the carbon that it cannot scale off and so expose fresh surfaces (as rust of iron does), and it may be so insoluble in the menstruum in which the gas is administered that the thinnest films of this compound upon the carbon act as an effectual bar to further action of this kind, graphitic carbon in these particular relations of it to sulphur occupying in