warmed the paper and left the wire less to do, but also enabled me to bring the wire somewhat over the flame, and thus it was more thoroughly heated, and a double advantage gained. The copper, however, soon became oxidized, and another improvement was made, by replacing the copper wire with one of platina much thinner. It was sooner heated, gave a better writing, and kept quite clean. No doubt the pen remains open to many improvements. Having thus given the experiments made to eliminate the carbon from the paper by heat alone, we have now to give an account of the other method proposed, namely, by means of chemical agents. Among the substances known to chemists two offered themselves to notice as worthy of a trial; both of them most energetic in overturning almost all organic compounds, and setting their carbon free. Destructives they were held to be, yet it might be that for our purpose that was their good property. A solution of phosphorus in bisulphuret of carbon was one, and strong sulphuric acid was the other. Any one who has had the pleasure of hearing our worthy Vice-President, Dr Stevenson Macadam, lecture on carbon, and of seeing his beautiful experiments, will have no doubt of the capability of either to eliminate black carbon. The solution of phosphorus, however, must be dismissed for being too energetic. A stream of this solution, by no means saturated, flowing from the orifice of a tube, estimated about the th of an inch in diameter, gave broad black lines more than a quarter of an inch across. It permeates on every side, and is so violent in its action that the thickest paper and strong thick card is carbonized through to the back. Besides, it is a most dangerous substance to work with. Means may perhaps be found to control it and render it useful, but for the reasons stated it was set aside. We now come to sulphuric acid. The acid used was the ordinary acid of the shops, of specific gravity 1825-1830, or thereby, and free from all solid particles, so that the fine bore of the tube employed to write with might not get choked. The bore of the tube used as the pen may be as fine almost as it can be made, but with strong sides, so as to resist even considerable force. The acid goes a long way in filling up the paper; a very few minims will fill a large page. Precautions in dealing with such a corrosive fluid are necessary, and will suggest themselves to everybody who knows what oil of vitriol is. When first written with the sulphuric acid does not bring out the dark carbon; and if, after being written with, it is left in the paper, it spreads, and the whole writing becomes one blurred, watery-looking blot. But if written with on good thick brown or white paper, and then, some time within half an hour after, a moderate heat be given to the sheet by holding it near a decent fire, a deep black writing is in a second or two produced or developed. The same result follows, though perhaps not so perfectly, if the paper be slightly warm when written upon. The written sheet, when thus produced, may be washed in water, in aqua potassæ, nitric acid, and in chlorine water, without effacing the writing; but if put into a milk of bleaching-powder the black writing disappears; yet it leaves its marks behind it, in deeply cut lines in the paper wherever it has been. Looked at through a glass of no great power, these lines are seen in all their integrity, and they may be filled up again with any coloured powder, by sprinkling it over the paper, and brushing off what remains on the surface; in fact, the acid has engraven the paper, giving deep, well-defined canals, out of which the carbon and all else have been removed. So sharp are these lines, that it seems possible the use of sulphuric acid on a properly adapted paper may serve some of the purposes of the engraver. They can be drawn with all the ease and speed employed in using a pen or pencil, and they look as if it would not be difficult to get a cast out of them. It may very reasonably be asked, why bleaching-powder should thus act, while free chlorine has almost no effect? The answer seems to be, that the excess of acid left in the paper reacts upon the proximate constituents of the bleaching-powder-namely, hypochlorite of lime and chloride of calcium-setting free both hypochlorous and hydrochloric acids as gases, which, as they escape, lift up the carbonized paper and shoulder it out of its place; the formation of sulphate of lime may also have something to do with it. The action seems to be mechanical; for if we first wash the paper well before we put it into bleaching-powder, the effect produced is very slight; we might from that fact be led to believe, that by simply washing the paper we should escape the only chance of obliteration. But no; for if the explanation given be correct, it would be easy to soak the paper either with an alkali or an acid, and to evolve carbonic acid in it, and then the mechanical effort of the escaping gas, would lift up the carbonised lines out from the substance of the paper. It is right, however, to say, that the writing is not easily deprived of its carbonaceous markings; it took rubbing and beating under a stream of warm water to bring them out. Various suggestions might be made as to the possibility of preventing this evil, but none of them offer anything like certainty. Objections may be urged against the use of sulphuric acid : it is corrosive and destructive to many things very likely to come in contact with it; it is colourless, and the progress of the writing is not seen. The reply to the first is, that we must take things as we find them and make the most of them; it is its destructive power, as we call it, which makes it useful. So far as the free use of it is concerned, no other precautions were taken than to wipe well the outside of the tube, and when not in use, to lay it down with the capillary end lowest; so dealt with, no annoyance was occasioned by it. Less than ten minims will fill a large page; it goes far, so that a comparatively small tube will hold enough for almost a day's work; with ordinary care, therefore, the risk is small. The second objection, that it is colourless, is met by the fact that it can be easily coloured by many substances. But it is well to notice here, that there must be nothing added to it which may choke the tube; solid particles in the tube or in the acid will make their use very troublesome, and may lead to accidents; a clean tube and clean acid are both indispensable. The most serious objection against the use of sulphuric acid, as a writing material, lies in the destruction it would afterwards occasion if left in the paper, and in the difficulty of getting it out. Certainly a considerable portion of the acid is so much destroyed, as to be effete for any purpose save to attract moisture; but that is so serious an evil that a remedy must be found; fortunately it is not far to seek. VOL. VI. P Washing with water removes nearly the whole acid, and a steep in some weak alkaline solution would neutralize it in toto. All this is of course troublesome; and it remains to be seen if the object sought be worth that trouble or no. One is apt to say, that if a thing be worth keeping for a thousand years, or thereby, some pains in preparing it ought to be taken. In the first part of this communication it was mentioned that cards had been embued with a variety of carbon compounds. That was done in the hope of finding one so easily decomposed by heat, that it might be used, either as an ink, and then to be developed by heat, or, being first in the paper, decomposed by the hot wire used as a pen. Some experiments tend to show, that it is not impossible some such substances may exist; at present, however, no more can be said about them. And now to sum up. So far as experiment has yet gone, heat appears to give the best result. Sulphuric acid possesses the good property, that it can be used very much in the same way, and with the same freedom, as ordinary ink; it also gives a much better defined and blacker writing than the hot wire. But the hot wire gives a writing which, when the heat has been full enough, refuses to be altogether effaced by any means yet tried; so much cannot be said of the sulphuric acid manuscripts. It would be wrong, however, at present to pronounce the wire writing quite indelible. By the heat employed, the carbon of the paper is not wholly evolved in its black amorphous form; it partially assumes that intermediate condition, in which it has lost the white colour it has, or gives, or helps to give, for we know nothing about it-when combined, and takes on the deep-brown hue, which is so common an accompaniment of the oxidation of carbon compounds. When in that condition, it is sometimes soluble in water-as for instance in molasses, and sometimes not, as in tar; but in either state it can very easily be destroyed by chemical means. some such condition it partially exists in the paper, after the passage over it of the hot wire, for experiment shows that chemical substances do carry some of it away. It is of importance, therefore, to use as full a heat as we safely can. In Improvements on the paper and pen may, in time, give us all the advantages of the sulphuric acid. It may be proper here to mention, that the time taken to fill a page with the hot-wire pen is not quite double that taken to write the same number of words with ordinary pen and ink. Before concluding, it may be stated that a portion of this communication has been written with the wire heated by galvanism; another with the wire heated by the jet of gas; a third, with sulphuric acid, then heated and left as written; a fourth, with the same acid, then heated and washed in water; a fifth, similarly treated as the fourth, but in addition, soaked in, and well washed with weak ammonia, and then in water; and yet a sixth, written with Dr Stark's ink. I am quite sure you will bear with me a little longer, while I bear witness to the useful labours of Dr Stark in his endeavours to give us the best ordinary writing ink. An impossible problem has been put before him; for we have sought hitherto that the chemist shall give us a substance for ink which is soluble in some simple fluid, such as water, but which, when once dry, shall become insoluble, and unalterable in all menstrua and under all circumstances. These demands are self-destructive. No one has wrought more successfully than Dr Stark to accomplish the best results, from what he wisely believes the best materials, namely, iron, and tannin or its products. No one has made a better use of these materials than himself. In conclusion, this communication has been thus crudely brought before you that others, possessed of more leisure, and better means, may enter further into the subject. Nothing is ever brought to perfection by one man's labour; in perfecting small matters as well as great, man needs his brother. On Chromo-Lithography. By Messrs MACLURE and GENTLEMEN, In responding to the general invitation contained in the prospectus of the Royal Scottish Society of Arts * Read before the Society, and specimens exhibited, 8th April 1861, |