JAN THE CHEMICAL NEWS. VOLUMEFRXXIII. EDITED BY WILLIAM CROOKES, F.R.S., &c. No. 841.-JANUARY 7, 1876. IN a paper read before the Society of Telegraph Engineerst on February 12th, 1873. Mr. Willoughby Smith announced the remarkable fact, that a bar of crystalline selenium, through which a current of electricity passes, has its conductivity increased 15 to 100 per cent when the bar is exposed to light. The light from an ordinary gas-burner placed at a distance of several feet increased the conductivity 15 to 20 per cent. Mr. Smith satisfied himself that alterations in temperature in no way affected this result, by placing the selenium in water, in such a manner that the light from burning magnesium ribbon held some inches above the bar passed through about an inch of water before falling upon the selenium. Under these circumstances, the conductivity of the bar was found to increase more than two-thirds, returning to the normal conductivity when the light was withdrawn. Lieutenant Sale, in a communication made to the Royal Society, describes a series of experiments undertaken with the object of ascertaining the relative effect upon the electric resistance of selenium of the light in different parts of the spectrum. He found that in the solar spectrum the conductivity is least in the violet, and increases as the red is approached, attaining its maximum in a position just on the outside edge of the red rays at the red side. The conductivity in this position is greater than in diffuse daylight, but very considerably less than when the selenium is exposed to full sunlight. Mr. Sale observed that the effect of light is apparently instantaneous, but that the return in darkness to the normal resistance is not so rapid. He corroborates the statement of Mr. Willoughby Smith, already cited, that the varying resistance is in no way due to alteration of temperature of the selenium.§ Soon after the publication of Mr. Smith's observations, we undertook a series of experiments with the object of, if possible, determining the precise molecular state of selenium, which exhibited this phenomenon of diminished electrical resistance under the action of light, and the conditions necessary for its production. It would here appear necessary to give a brief resumé of the state of our knowledge of the physical properties and relations of selenium. This is of the more importance From the Proceedings of the Royal Irish Academy, vol. i., Ser. II. (Sci.). + Nature, vol. vii., p. 203. * Proceedings of the Royal Society of London, vol. xxi., p. 283. § No experiments are adduced in support of this statement. because little, if anything, has been added to that knowledge for nearly twenty years, and because the statements sufficient, but often discordant with the results obtained in some of the acknowledged text-books are not only inby the savants to whom we owe all that up to this time has been done in relation to the subject. As we would desire to avoid matter which is supplied by studied by that chemist, and it is through his researches, Selenium may exist in several different forms: 1. As a vitreous mass, with conchoidal fracture. enious acid or selenites by the action of reducing 2. As a red amorphous powder precipitated from se agents. solution in bisulphide of carbon. selenides exposed to the air. metallic cobalt or cast-iron, and obtained by the heating The first three modifications, it should be mentioned, resemble one another in their physical and chemical rela tions, and must be regarded as different conditions of the same allotropic form of selenium. It is here only necessary to speak of vitreous selenium and of its heat-produced allotropic modification, the granular variety, or as it has been felicitously called by Regnault, metallic selenium. Vitreous selenium has no definite point of fusion. At temperatures exceeding 60° C. it softens, becoming gradually softer with increased heat, and being perfectly fluid at 250°. When rapidly cooled from this temperature, it returns to its original condition. At normal temperatures it may be kept without change of state for many years, and is probably under these conditions perfectly stable. It is, though very sparingly, soluble in bisulphide of carbon. In thin films, it appears by transmitted light of a ing to Schaffgottsch, 4'276. beautiful ruby-red colour. Its specific gravity is, accord When this vitreous selenium is maintained for some time at any temperature between 94° and 200° C., and is then slowly cooled, it is found to have assumed a metallic ap 2 Pyrology, or Analysis and Synthesis by the Blowpipe. pearance, and to have a grey granular fracture. It is now, we ourselves find, perfectly opaque to light, even in the thinnest films. Its specific gravity has increased to 4'796. When heated it does not soften, but at 217° fuses without taking any intermediate pasty condition. At 250° it is perfectly fluid, even when the mass is considerable; and when rapidly cooled returns, without any tendency to crystallise, to the vitreous, non-metallic modification. All that has up to the present been made known as to the electrical relations of selenium may be very shortly told. Solid vitreous selenium cannot, according to Berzelius,* be rendered electrical by friction, but, on the contrary, Bondsdorff states that when rubbed in very dry air it has this property. Knox found that fused selenium conducted the current of a battery of sixty pairs. Hittorf found that granular selenium at normal temperatures con. ducted sufficient of the current of one Grove's element to deflect the astatic needle of a galvanometer having 200 convolutions 17°, and that when the selenium was heated to 210° in a small crucible the needle marked 80°. But when the temperature reached 217° (the point of fusion of granular selenium), the needle went back suddenly to 20°. The action of light as probably effecting some change in the allotropy of selenium was not wholly unsuspected prior to Mr. Smith's observations. Gmelin mentions exposure to sunlight as a favourable condition for the precipitation of selenium from dilute solutions of selenious acid by sulphurous acid; and Hittorf, while noting the likelihood of such an influence, was unable to detect it, and was obliged to attribute the observed change of amorphous into crystalline selenium, while drying in sunlight, entirely to the effect of heat. In pursuing the line of research we have marked out for ourselves, we have been obliged to repeat much already published work, which with improved means of experiment has lost somewhat of its significance. We have thus encountered several apparently contradictory statements, some of which our experience has either failed to verify, or has placed in a new light; and we have been convinced that the properties of this remarkable substance are but imperfectly understood, and still present a wide field for investigation. Vitreous selenium is, we should say at the outset, apparently an absolute non-conductor of electricity. We have been unable to obtain any deflection of the very sensitive astatic needle of a high resistance galvanometer, when the thinnest films of selenium, of the continuity of which we could assure ourselves, are interposed in the circuit of ten Leclanché elements. The difficulty of producing very thin films of absolute continuity disposes us nevertheless to state our belief as to the complete non-conductivity of vitreous selenium with some reservation. CHEMICAL NEWS, the extreme range of the mercurial thermometer, but so much beyond that it is still diminishing when, owing to the rapid volatilisation of the selenium, it has been found necessary to terminate the experiment. It has hitherto been assumed that there is but one granular form of selenium, and that this is a conductor of electricity. We have, however, obtained a granular form in no way optically differing from granular selenium of comparatively low resistance, through a rod of which, 16 m.m. long and 3 m.m. diameter, the current of ten Leclanché cells does not in the least deflect the needle of our highly sensitive galvanometer. Nor does light, so far as our experiments have yet gone, diminish the resistance of this modification. We have, on the other hand, succeeded in obtaining bars of granular selenium through which the current of one Leclanché cell causes a very considerable deflection of the needle. This form is, we find, in its electric resistance almost unaffected by light. Between these two forms of granular selenium-the apparently non-conducting and the comparatively highly conducting-there is another of intermediate resistance. This modification is highly sensitive to light; its conductivity when in the form of flat bars increasing in direct sunlight 75 to 100 per cent, and in artificial light in ratios ranging from 10 to 50 per cent. One form of granular selenium, as Hittorf first showed, has its resistance considerably diminished by heat: indeed, he says that could it be heated red hot its conductivity would not be inferior to that of the metals. Our own experiments confirm the diminution of conductivity by heat, but we have found, in at least one modification which we have produced, a body which so far conforms to the metallic type as to have its resistance strikingly increased by heat. We have made bars of selenium which when placed in the circuit of a battery and galvanometer, have shown a deflection of 48°, while upon completing the circuit of a nitric acid battery, the current of which heated a spiral platinum wire surrounding the bar of selenium, the needle gradually fell to 15°, as the temperature of the bar became greater. For the present we refrain from comment upon these results; and although we have been engaged during many months in this investigation, we defer details of our experiments, and especially of the conditions under which the different electrically resisting and light-sensitive forms of selenium are obtained, until with larger experience we may hope to bring before the Academy results tending more closely to the solution of the questions we have proposed to ourselves. BY MEANS OF THE BLOWPIPE.* As might be expected from this character, selenium in ON PYROLOGY, OR ANALYSIS AND SYNTHESIS the vitreous form becomes electric by friction. So easily indeed have we invariably obtained this result that we find it difficult to understand the contrary conclusion of Berzelius. Hittorf, as has been stated, found that when vitreous selenium is converted into the granular form its electrical resistance diminishes directly with its temperature, but that when 217° is attained the resistance is suddenly and largely augmented. In repeating this experiment we have obtained results concordant with those of Hittorf, who appears, however, not to have exceeded the temperature of 217. Going beyond this point, we find that the resist ance diminishes up to the point of complete fusion of the selenium, being at its maximum at 250°. We have obtained also the remarkable result that when the vitreous selenium resulting from the rapid cooling of the completely fused granular form is quickly heated, it begins to conduct the current at a temperature between 165° and 175°, and that its resistance diminishes, not only up to By W. A. ROSS. (Continued from vol. xxxii., p. 252.) there are English chemists-accomplished ones too-and *The term "pyrological" (" pyrologischen Versuche ") has been adopted by Professor Richter, of Freiberg, in a letter to the writer. is not a sufficient reply to say that "there is a course of | be correct, to what may be termed the metaphysical blowpipe instruction in almost every English chemical class or college." Every chemist who has been to Freiberg, or even to the American colleges, knows how inefficiently the blowpipe is taught, or rather, how effectually it is neglected in our otherwise excellent schools of chemistry. If this point be controverted, it will not be difficult to take one of our most generally received analytical textbooks, and show what extraordinary and incorrect statements are there made with regard to blowpipe analysis, nor does an inspection of some of the English pyrological instruments and apparatus at all reassure us. I have lately, incredible as it will seem, been shown an instrument as the only kind of blowpipe in use in a public laboratory in London, having an aperture at the jet, at least a quarter of an inch in diameter; and, to make matters worse, filled both with the blast and with aërated coal gas! I saw a youth blowing through such a machine as this by means of a mouthpiece cleverly extemporised with a glass funnel, and warned him that he would thus probably injure his lungs, a misfortune which would of course be attributed to "the blowpipe," instead of to the personal use of such a tuyere as this. (8). It may be as well to mention here that blowpipes like that devised by Von Frick, having the operator's breath, or blast from any other source, and the gas for ignition conveyed by the same tube, are utterly useless for these purposes; that Bunsen burners are useless for these purposes; that Fletcher's most ingenious, and no doubt otherwise invaluable "hot blast blowpipe is almost useless for these purposes, which are most erroneously supposed to be always best effected by the greatest heat. Let any student who really wishes to learn this science produce a blue pyrocone from a candle by a hand blower, and then with a mouth blowpipe held in the other hand, direct a blast across this blue pyrocone, so as to dissect it; he finds it to be a solid mass of blue flame. If he now perform the same operation on the pyrocone from a gas jet, the pyrocone is found to be slightly hollow, while the pyrocone from a Bunsen burner has only the merest thin shell of circumscribing ignited gaseous matter. It is obvious, therefore, that the effect of holding the fragment or paste of a mineral or other essay, in the middle of these three pyrocones, must be different in each case; yet how seldom in England do we see a candle or oil lamp used for important results which can be obtained solely by its use? (9). It is obvious that in order to properly systematise the science of blowpipe analysis, we ought to reduce it to as close analogy as may be with the procedure in ordinary chemical analysis, which has been found to answer so well. I have, therefore, in my method discarded the preliminary use of the salts borax and microcosmic sal', and commenced attacking substances with the pyracids "Boric and phosphoric anhydride." Boric acid, indeed, contrary to the account found in most of our chemical works, dissolves before the blowpipe no oxide whatever completely, except those of the alkaline metals, and that of silver a little, but its reactions thus are a thousand times more valuable than if it really (as we are told) dissolved all those, while, by adding pyrologically a very small proportion of alkali (not quite 5 per cent) we obtain a still acid menstruum which is itself extremely soluble in water. Phosphoric acid, on the other hand, is, before the blowpipe, the most powerful unmixed solvent in the world, dissolving gold leaf to a bluish violet glass quite rapidly. In either case we can further, if we like, obtain an acidulated water solution, in which precipitates can be produced by alkalies, &c., just as in "the wet way," but this process requires to be worked out, and shall not therefore be further alluded to here. (10). It is necessary, however, to lightly t uch upon one part of the subject, not invidiously but conscientiously, before proceeding to details. However invaluable the application of the symbolical and algebraical process o ratiocination may be, assuming its groundwork of facts to chemistry of modern times, it seems insufficient when absolutely and completely substituted for practical knowledge, and reasoning in mere English, and, indeed, it is obvious that, in England at least, there is a tendency among our best writers, even on chemical metaphysics, to express their thoughts, not only in English, but in the very plainest English which has ever been in the possession of Her Majesty or her predecessors. There is no art so difficult of attainment as simplicity, and we may safely regard the value of an invention (or the description of one) in the inverse ratio to its complication. Whoever takes the trouble to compare the amazing hieroglyphical work of the communistical chemist Nacquet, translated into English the other day, with the luminous "essays" of the marvellous apothecary Scheele ;† or Axiom V., Book I. of Newton's Optics, on the ratio between the sines of incidence and refraction in a ray of light, with the ordinary scientific account of the same simple law given in our modern mathematical or physical works, will soon see that the advantage in every essential particular remains with the adopters of expression by means of language. Besides, no one, not even the most eminent philosopher, can afford to discard the very great advantage of address. ing himself to thousands-perhaps millions-of his fellow creatures, instead of to a few transcendentalists who cannot spread his opinions beyond their own clique. REPORT ON THE DEVELOPMENT OF THE CHEMICAL ARTS By Dr. A. W. HOFMANN. THENARD, on bringing peroxide of hydrogen into contact with his tongue, in order to ascertain its taste, found that it was whitened. The cuticle was also blackened, and at the same time a violent itching was excited. Litmus paper without any previous reddening was at once decolourised, as was also turmeric paper. In 1863 Chevreul undertook comparative experiments on the bleaching power of hydrogen peroxide. Its concentrated solution speedily turned syrup of violets green, oxygen being set at liberty. For the following experiments dilute colour-solutions were used-namely, syrup of violets, tincture of litmus, extract of peach-wood, and extract of logwood. The results were as follows:Litmus. Peach-wood. Logwood Slight Change to bleaching. rose. Time. 10 mins. 24 hrs... 80 hrs... Violets. Imperceptible. Complete bleaching. Almost complete Turns yellow Complete bleaching of all the solutions. Decolorisation is therefore effected less rapidly by peroxide of hydrogen than by chlorine. Tessié du Motay and Maréchal§ mention it as one of the agents which they propose for bleaching tissues, which, after treatment with permanganate of potash, they recommend to be steeped in a solution of peroxide of hydrogen. But it had been much earlier applied as a bleaching-agent by Thénard himself for a particular purpose-namely, for restoring old The controversy at present proceeding between Dr. Frankland and Mr. Wanklyn in the CHEMICAL NEWS, with reference to water analysis, may be cited as an illustration of this point. † Schedle seems to have been a German, not a Swede, settled as a common apothecary, at Köping, in Sweden. "Berichte über die Entwickelung der Chemischen Industr Während des Letzten Jahrzehends." Chevreul, Comptes Rendus, lv., 735. Bull. Soc. d'Encouragement, 1867, 472. cxxc(?)iv., 526. Dingler, Polyt. Journal Pélouze and Frémy, Traité de Chimie, 1861. NEWS "Jan. 7,1876 pearance, and to have a grey granular fracture. It is now, we ourselves find, perfectly opaque to light, even in the thinnest films. Its specific gravity has increased to 4'796. When heated it does not soften, but at 217 fuses without taking any intermediate pasty condition. At 250° it is perfectly fluid, even when the mass is considerable; and when rapidly cooled returns, without any tendency to crystallise, to the vitreous, non-metallic modification. All that has up to the present been made known as to the electrical relations of selenium may be very shortly told. the extreme range of the mercurial thermometer, but so much beyond that it is still diminishing when, owing to the rapid volatilisation of the selenium, it has been found necessary to terminate the experiment. It has hitherto been assumed that there is but one granular form of selenium, and that this is a conductor of electricity. We have, however, obtained a granular form in no way optically differing from granular selenium of comparatively low resistance, through a rod of which, 16 m.m. long and 3 m.m. diameter, the current of ten Leclanché cells does not in the least deflect the needle of our highly sensitive galvanometer. Nor does light, so far as our experiments have yet gone, diminish the resistance of this modification. We have, on the other hand, succeeded in obtaining bars of granular selenium through which the current of one Leclanché cell causes a very considerable deflection of the needle. This form is, we find, in its electric resistance almost unaffected by light. Between these two forms of granular selenium-the apparently non-conducting and the comparatively highly conducting-there is another of intermediate resistance. This modification is highly sensitive to light; its conductivity when in the form of flat bars increasing in direct sunlight 75 to 100 per cent, and in artificial light in ratios ranging from 10 to 50 per cent. Our own ex Solid vitreous selenium cannot, according to Berzelius,* be rendered electrical by friction, but, on the contrary, Bondsdorff states that when rubbed in very dry air it has this property. Knox found that fused selenium conducted the current of a battery of sixty pairs. Hittorf§ found that granular selenium at normal temperatures con ducted sufficient of the current of one Grove's element to deflect the astatic needle of a galvanometer having 200 convolutions 17°, and that when the selenium was heated to 210° in a small crucible the needle marked 80°. But when the temperature reached 217° (the point of fusion of granular selenium), the needle went back suddenly to 20°. The action of light as probably effecting some change in the allotropy of selenium was not wholly unsuspected prior to Mr. Smith's observations. Gmelin mentions ex- One form of granular selenium, as Hittorf first showed, posure to sunlight as a favourable condition for the pre- has its resistance considerably diminished by heat: indeed, cipitation of selenium from dilute solutions of selenious he says that could it be heated red hot its conductivity acid by sulphurous acid; and Hittorf, while noting the would not be inferior to that of the metals. likelihood of such an influence, was unable to detect it, periments confirm the diminution of conductivity by heat, and was obliged to attribute the observed change of amor- but we have found, in at least one modification which phous into crystalline selenium, while drying in sunlight, we have produced, a body which so far conforms to the entirely to the effect of heat. In pursuing the line of re- metallic type as to have its resistance strikingly increased search we have marked out for ourselves, we have been by heat. We have made bars of selenium which when obliged to repeat much already published work, which placed in the circuit of a battery and galvanometer, have with improved means of experiment has lost somewhat of shown a deflection of 48°, while upon completing the cirits significance. We have thus encountered several ap-cuit of a nitric acid battery, the current of which heated parently contradictory statements, some of which our ex- a spiral platinum wire surrounding the bar of selenium, perience has either failed to verify, or has placed in a new the needle gradually fell to 15°, as the temperature of the light; and we have been convinced that the properties of bar became greater. this remarkable substance are but imperfectly understood, and still present a wide field for investigation. Vitreous selenium is, we should say at the outset, apparently an absolute non-conductor of electricity. We have been unable to obtain any deflection of the very sensitive astatic needle of a high resistance galvanometer, when the thinnest films of selenium, of the continuity of which we could assure ourselves, are interposed in the circuit of ten Leclanché elements. The difficulty of producing very thin films of absolute continuity disposes us nevertheless to state our belief as to the complete non-conductivity of vitreous selenium with some reservation. For the present we refrain from comment upon these results; and although we have been engaged during many months in this investigation, we defer details of our exFeriments, and especially of the conditions under which the different electrically resisting and light-sensitive forms of selenium are obtained, until with larger experience we may hope to bring before the Academy results tending more closely to the solution of the questions we have proposed to ourselves. As might be expected from this character, selenium in ON the vitreous form becomes electric by friction. So easily indeed have we invariably obtained this result that we find it difficult to understand the contrary conclusion of Berzelius. Hittorf, as has been stated, found that when vitreous selenium is converted into the granular form its electrical resistance diminishes directly with its temperature, but that when 217° is attained the resistance is suddenly and largely augmented. In repeating this experiment we have obtained results concordant with those of Hittorf, who appears, however, not to have exceeded the temperature of 217. Going beyond this point, we find that the resistance diminishes up to the point of complete fusion of the selenium, being at its maximum at 250°. We have obtained also the remarkable result that when the vitreous selenium resulting from the rapid cooling of the completely fused granular form is quickly heated, it begins to conduct the current at a temperature between 165° and 175°, and that its resistance diminishes, not only up to PYROLOGY, OR ANALYSIS AND SYNTHESIS By W. A. ROSS. (Continued from vol. xxxii., p. 252.) (7). IT seems incredible, and yet is undoubtedly a fact, that there are English chemists-accomplished ones too-and even public analysts almost entirely ignorant of the analytical use of the blowpipe. To foreigners, especially Germans, it seems strange that we have not in England any systematic training, by means of lectures and practical teaching, in blowpipe analysis, on the pattern of the famous Freiberg University. Such a course might surely be instituted by a rich Government like ours without much difficulty, in Jermyn Street, under the direction of one of the old Freiberg students, as, for instance, Her Majesty's Inspector of Mines, within whose superintendence the students might make periodical visits to the most characteristic mines, which that gentleman knows so well so that this reproach may be taken from us. It The term "pyrological" ("pyrologischen Versuche") has been adopted by Professor Richter, of Freiberg, in a letter to the writer. |