gotten, that mineralogy is one of the natural history sciences, and therefore deals with natural objects not only as we find them in museums, but investigates their relations and their past history; while it studies their properties by applying to them all the known resources of the laboratory, it also studies through them the operation of the laws of nature, both now and in the distant ages of the past. In mineralogy, as in botany and zoology, it is in general impossible to reproduce past conditions; they have gone for ever. But there is this great difference between mineralogy and the biological sciences. Whilst they trace the history of changing species in the light of modern experiments and observations, the only record of their past, and that a secondary and an imperfect one, is supplied by fossils: the organisms themselves have disappeared. On the other hand, in the case of crystallised minerals, we generally have the actual object surviving through the ages. The liquid carbon dioxide imprisoned in a quartz crystal, or the tiny zircon surrounded by its halo in mica, have been there for untold centuries and survive to tell the tale of their own history. The same is true of the radioactive mineral, which by its present composition gives a clue to past changes; the structure of the crystal remains as a permanent record of its original nature. We have in our hands the very object that existed millions of years ago, and not merely a cast of it, and we can continue on it the processes that nature has begun. The forces of crystallisation that brought together its component elements have held them together in the same relative positions, of which the unchanged form is the guarantee. This circumstance gives a peculiar interest to the study of minerals; it is one which impresses a feeling of awe and fascination akin to that aroused by an ancient manuscript or a work of art which survives as a permanent record of the artistic life and thought of bygone ages, and, further, it invests them with a unique value for the decipherment of the past. There is one other aspect of mineralogy which I wish to emphasise. I have directed attention to the fatal sort of specialisation and exclusion to which the natural history school of thought tended; but let me repeat that mineralogy is, nevertheless, a branch of natural history, and that the proper study of minerals, as of all natural objects, is in itself a safeguard against specialisation. The true mineralogist must bring to this pursuit the resources of all the sciences; one who is merely concerned with the physics or chemistry or crystallography of minerals makes only a one-sided study of the subject. The more intense and concentrated the work of the true mineralogist, the less confined will it be: in fact, the more he specialises in mineralogy, the less of a specialist does he become. For this reason, the natural history subjects have a very particular educational value; they deserve to hold their own for educational purposes, and not to be broken up into specialised sections. They have a particular value, not only as a stimulating introduction to chemistry or physics or biology as taught in schools to beginners, but also as a means of maintaining interest in those sciences at a later stage. In young children, the instinct for collecting is strong, and equally strong is the desire to know something about what they collect; by many of them, the approach to science is most easily made through an interest in natural history. At the later stage, many older boys and girls on the literary side in the upper forms of schools, whose main interest is outside science and who realise that it will not play a large part in their lives, boys and girls to whom prolonged teaching of chemistry, physics, and biology (especially if they are separated by watertight compartments) will be uninteresting, and therefore largely unprofitable might be made to retain their hold on the principles of all these fundamental sciences through the study of natural history. We shall surely do well to utilise any interest that brings home to ordinary students the knowledge and conviction that science is not merely laboratory work, but that its principles are to be found in operation everywhere, though they may have to be explored in the laboratory. Those who specialise in language or history or literature must acquire this conviction by experience, just as those who specialise in science must learn by experience that all great literature is the expression of ideas, and that even scientific ideas cannot be expressed by an illiterate use of language. A judicious employment of natural history will, I believe, do much to establish science in the place that it should occupy in our educational system. When I accepted the invitation of the Council to deliver this lecture, I welcomed the opportunity it would give me of recalling to the Chemical Society this aspect of my favourite pursuit. And, let me add, even for scientific workers in their later years a taste for natural history in any of its branches is worth maintaining as a very real corrective to narrow specialisation. If an illustration be required, I cannot do better than remind you again of the wide scientific interests and achievements of the remarkable man whose name this lecture bears. Mineralogy and botany appealed to his wide and catholic taste; his love for minerals and plants was an abiding possession to him, not only because he loved them as beautiful things, but because he was a man deeply interested in natural objects and in the secrets of science that are revealed by them. To those whose work lies in laboratories, such intercourse with nature is easily maintained, for there are many pathways that lead from experimental science to natural history. Like Antaeus, they can always strengthen themselves by contact with Mother Earth, feeling with Wordsworth: "that Nature never did betray The heart that loved her; 'tis her privilege XXXII. The Synthesis of Ammonia at High Temperatures. Part II. By EDWARD BRADFORD MAXTED. IN a recent paper (this vol., p. 168), the equilibrium between nitrogen, hydrogen, and ammonia at high temperatures was discussed from a thermodynamical point of view, and evidence was brought forward showing that in all probability the ammonia content of such a gas mixture in equilibrium, after decreasing with increasing temperature, eventually passes through a minimum and finally rises once more. On the basis of this theoretical indication of the possibility of a thermal synthesis of ammonia, it was shown experimentally that considerable yields of ammonia may be obtained by cooling a mixture of nitrogen and hydrogen extremely rapidly from the temperature of the oxy-hydrogen flame to that of the room. In continuation of the above work, and especially in view of the known fact that a mixture of nitrogen and hydrogen may be completely converted into ammonia by sparking in a eudiometer over dilute acid (Donkin, Proc. Roy. Soc., 1873, 21, 281), an investigation was begun of the yields of ammonia obtainable from a rapidly cooled, high-tension arc, also by sparking, the present paper being a summary of the results obtained by induction discharge modified in such a way as to constitute a small, high-tension arc burning within a capillary tube, through which the mixture of nitrogen and hydrogen was passed. By confining the action to a capillary tube, in the manner described, the gas to be treated may be brought uniformly into contact with the discharge, and it was found easily possible to obtain at atmospheric pressure yields of ammonia amounting to 1.5 per cent. by volume of the gas-mixture taken for treatment. Induction sparks, as such, were found to exert a comparatively feeble action on the synthesis, energetic formation of ammonia only taking place when the electrodes were brought sufficiently close together to transform the ordinary spark discharge into a small, high-tension arc, accompanied by a visible and apparently continuous flame of high temperature. EXPERIMENTAL. The apparatus employed for the series of experiments about to be described consisted of a capillary glass tube having an internal diameter of 0.65 mm., an external diameter of 5 mm., and a length of about 10 cm. Platinum wire electrodes, 0.25 mm. thick, were sealed into the capillary tube in such a way as to leave a spark-gap of the size required, whilst the passage of gas through the tube was effected by means of fused-on glass side-tubes. It was found inadvisable, on account of frequent fractures, to employ a heavy discharge, the most satisfactory conditions for the investigation being obtained with a "two-inch" coil, supplied with a primary current of from 3 to 3.5 amperes from the laboratory 220-volt main. Pure hydrogen and nitrogen for the synthesis were mixed in the proportion of three to one in a large gasholder and compressed into a cylinder for convenience in use. This mixed gas was passed at carefully determined rates through the capillary spark-gap, the ammonia formed being absorbed in dilute acid and estimated by means of Nessler's solution, The first point to be investigated was the influence of the size of the spark-gap on the yield of ammonia, this gap being varied from 10 to 0.5 mm. while the rate of passage of the gas was first kept constant at 40 c.c. per hour, and, secondly, varied in such a way that the time of contact was kept at 0·0015 second for sparkgaps of various sizes. Table I summarises the results obtained by the first of these two methods, namely, with a constant rate of flow, table II those obtained by the second, that is, with a constant time of contact with the spark-gap, the primary current being in every case 3.5 amperes at 220 volts. TABLE I. Rate of Flow of Hydrogen-Nitrogen Mixture 40 c.c. per hour. It will be seen that as the size of the spark-gap is decreased, the flame loses its well-defined spark-like character and becomes a small but intensely hot high-tension arc, this flame, probably by virtue of its high temperature combined with the rapid cooling effect afforded by the relatively cold walls of the glass capillary tube, possessing the power of inducing the combination of nitrogen and hydrogen to an extent obtainable otherwise only by the action of a catalyst under a high pressure. The rate of flow of the nitrogen-hydrogen mixture through the capillary tube was measured by allowing the gas issuing from the reaction tube to pass through a small, specially constructed gas-washing bottle, in which the ammonia was absorbed by very dilute sulphuric acid, the number of bubbles which formed per minute being counted and the volume of a bubble being known. The estimation of ammonia in the resulting solution was carried out by means of Nessler's reagent. It is difficult to estimate the temperature of the small hightension arc formed, which, however, for the purpose of obtaining rough comparative figures for the various times of contact, has been assumed to be at approximately 3000°, but the effect of increasing temperature on the yield of ammonia formed is clearly seen. This rise in yield with increasing temperature at high temperatures (in contradistinction from the decrease obtained at moderate temperatures) agrees with the results already reported. When working with small spark-gaps, a considerable deposit of platinum, removed from the electrodes by volatilisation and other |
