that the proofs given by Professor Forbes were already sufficient and complete. I cannot, indeed, conceive any explanation of the facts known as to glacier movement which can consist with the entire absence of minute cracks (whether capillary or not) penetrating that part of the ice which is in course of gradual motion. True it is that under pressure these cracks will be closed up again, and therefore those parts of the ice not actually undergoing the process which causes the motion may well be free from fissures. Pending further observations, which I trust Mr. Huxley may be induced to make, I shall venture to adhere to the belief that the irregular network of fissures which pervades the surface of the ice when exposed to air and warmth, represents a structure already existing in the ice before it came to the surface; and that former fissures, though possibly closed so as to show no trace under the microscope, may yet be surfaces of easy melting, which on the first application of heat are recalled into existence. Air-bubbles in the ice.-In common, as I suppose, with many readers of Mr. Huxley's very interesting observations, the absence of more precise details as to the circumstances under which they were made is to me a matter of regret. It is allowable to suppose that his visits to the glaciers were generally made in the middle of the day, and during fine weather, and in that case I feel some doubt whether the ice within 8 or 10 inches of the surface is in the same conditions of temperature as that of the interior of the glacier. I would submit that the hour of the day, the state of the weather, and of the thermometer, at the time and previous to the observations, and also a statement of the precautions taken to prevent the pieces of ice taken out from undergoing changes by exposure to sun and air, would be a valuable addition to the interesting particulars for which we are indebted to Mr. Huxley. Such details seem necessary for satisfactory proof of Mr. Huxley's conclusion, that the entire ice of the glacier is pervaded by cells containing both water and air, a fact which seems, however, no way inconsistent with rational views of the conditions of temperature in the interior of glaciers. Dirt-streaks. I believe that there is no material inconsistency between what I have said as to the dirt-bands of M. Agassiz, and Mr. Huxley's observations on the same phænomenon, for which I gladly adopt the name "dirt-streaks." I would observe, however, that the illustration given at pl. 5 of the 'Travels in the Alps,' to which Mr. Huxley refers his readers, is far less accurate than most of those contained in the same work. A glance at the plate is sufficient to show that the lines represented on the glacier cannot possibly represent the same thing in the foreground, sixty or eighty yards from the eye, and in the middle distance more than half a mile from the point whence the sketch is taken. It is true that something of the structure observed by Mr. Huxley may be traced for some distance along the side of the glacier above the great moraine; but neither on the Brenva, nor on any other glacier, have I ever seen anything like a system of continuous lines or streaks distinctly visible at a distance, and over a wide breadth of glacier, as represented in this illustration. One word more as to the so-called lenticular structure. When Mr. Huxley speaks of veins of ice 9 or 10 inches in thickness and apparently lenticular, which I suppose means that the exposed section thins out on either side, I venture to question whether such veins form any part of the real veined structure, and whether they are not instances of what is by no means rare,-the fillingup of small crevasses with ice generally differing in colour and appearance from the surrounding mass. I do not recollect to have attended to the length of the veins, or the exposed sections of them on the glacier of the Brenva, but I can only say that if 9 feet be the greatest length that can there be traced, the structure of that glacier must differ from all those of the same class that I have ever been able to examine. I LVII. On the Influence of Light upon Chlorine. To the Editors of the Philosophical Magazine and Journal. GENTLEMEN, WISH to make a few observations upon a communication of Dr. Draper's, published in the last (November) Number of the Philosophical Magazine, in which he refers to the researches on the chemical action of light, recently published by Professor Bunsen and myself, in a manner calculated to mislead many of your readers as to the results we obtained, and the conclusions which must follow therefrom. In 1843, Dr. Draper observed that when a mixture of chlorine and hydrogen is exposed to light from various sources, a certain time elapses before combination begins, and that, when commenced, the action continually increases, until at length a permanently constant maximum has arrived. In our experiments we have observed the same fact, and have devoted one complete section of our research to the minute consideration of these remarkable relations. We differ widely, however, from Dr. Draper as to the cause of this phænomenon; and it is the experimental evidence upon which we rest our explanation which I think it advisable here briefly to state, On first thoughts one would naturally conclude that the cause of this peculiar phænomenon was to be sought in some allotropic change which one or both gases undergo on exposure to light, in which state the combining powers are heightened, and the subsequent combination on insolation thus rendered possible. This is, in fact, the explanation given by Dr. Draper. He supposes that during the first insolation it is the chlorine which undergoes a change of properties; and he convinced himself by various experiments that this is the case, and that the chlorine retains this condition of heightened chemical activity when preserved in the dark for a long period, extending in some cases over several weeks. We have with great care frequently repeated these experiments, avoiding the errors to which they were subject, and we have not in any one case succeeded in corroborating Dr. Draper's statements. We found, that to whatever kind or amount of light the gas had been subjected, it rapidly returned to its inactive condition on exclusion of light; that on standing in the dark for about half an hour, the gas did not possess any properties different from those which it possessed before it had been exposed to light. Hence Dr. Draper's position, as regards a permanent allotropic condition of either gas, becomes untenable. That Dr. Draper arrived at these conclusions by help of the experiments mentioned in his former papers in your Journal, will not surprise us when we become acquainted with the difficulty of obtaining by any means accurate results in this subject, and the impossibility of so doing by methods so imperfect as those which Dr. Draper employed. 100,000 We have shown, in our second paper read before the Royal Society, that the presence of a trace of foreign gas, not exceeding dth of the total volume, is sufficient to alter in the most marked degree the photo-chemical sensibility of the mixture. Dr. Draper derives the proof of the existence of a permanent allotropic modification of chlorine from (amongst others) experiments in which that gas was collected over a solution of chloride of sodium, then exposed to the sun, and afterwards mixed with its own volume of hydrogen, and the rate observed at which the mixture combined on subsequent insolation, compared with a similar mixture not previously exposed. The limits of error introduced by this and the other modes of experimentation adopted by Dr. Draper to determine this question, extend, as we have shown in our paper, even beyond the differences in the action arising from the phænomenon itself, and hence his conclusions cannot be relied upon. In order to determine whether either gas undergoes on exposure any change in properties, whether permanent or not, we passed (with all due precautions) the gases, evolved electrolyti cally, separately through long tubes which could be exposed to the direct solar rays; and on observing the photo-chemical properties of the mixed chlorine and hydrogen (by gas-light), first, when the separated gases had been strongly insolated, and secondly, when they had been carefully protected from light, we found that in the one case the combination did not take place more rapidly than it did in the second; conclusively proving that the previous insolation of the separated gases, and hence any allotropic modification of either gas, cannot account for this slow action on the first exposure to light. Dr. Draper's objection, that we did not expose the gases for a sufficient time to solar action, is not valid, inasmuch as the direct solar light to which they were separately exposed was several thousand times as intense as the small gas-flame which in four minutes effected the maximum action. The phænomenon in question depends upon a totally different cause. It belongs to a most important, but hitherto disregarded phase of general chemical action. In all chemical combinations and decompositions a certain time must elapse before the full action is set up. In no case does chemical action commence instantly on bringing the bodies under the requisite conditions; and in many instances the length of time which elapses before the full action occurs is very considerable, so that the progressive phases of the combination can be distinctly traced. The phænomenon which we have considered is a striking example of this general condition of chemical action, of this increase of the chemical activity with the time under which the forces act. This general mode of action has been named by us Chemical Induction. We have shown in our paper on Photo-chemical Induction, that precisely similar relations are observable in the photographic phænomena, and also in pure chemical actions; our investigations form, in fact, starting-points for the consideration of time as affecting chemical action, a subject which I am at present engaged in following out. It is almost superfluous for me to mention, regarding Dr. Draper's experiment on the solar decomposition of chloride of silver, that before one can believe that the black substance insoluble in nitric acid is either a modification of the metal or "a something which is not silver," we must know that it is not a subchloride, particularly as Wöhler states that suboxide of silver receives a metallic lustre on burnishing. I am yours truly, HENRY E. ROSCOE. LVIII. Methods in the Integral Calculus. By the Rev. ROBERT CARMICHAEL, A.M., Fellow of Trinity College, Dublin*. 1. A N interesting exemplification of the reciprocal aid which geometry and analysis lend to each other is afforded by the fact, that there is a large class of differential equations whose integration may be much facilitated by the employment of geometrical considerations, more especially those associated with the transformation to polar coordinates. It is obvious, too, that where differential equations admit of geometrical interpretations, such interpretations are more likely to be suggested in this way than where their integration is conducted by methods purely analytical. Thus, as a simple example, if it be proposed to integrate the equation xd+ydy=m(dexdy), by transformation to polar coordinates we get at once rdr=mr2d0, or dr=mrd0; whence, instantly, the curve denoted by the given equation is the logarithmic spiral r=Ceme The immediate geometrical interpretation of the given equation is easily seen by dividing both sides of its transformed type by ds. In fact, then, we get and, if Q be the intercept between the point on the curve and the foot of the perpendicular from the origin on the tangent (P), this equation is at once equivalent to Q=mP; or the given differential equation represents, as is known, the plane curve for which the perpendicular from the origin on the tangent is in a constant proportion to the intercept between the corresponding point of contact and the foot of the perpendicular. The curve which satisfies this condition is, as we have seen, the logarithmic spiral. Thus the immediate geometrical interpretation of the given equation, as well as the more remote, are with equal facility ob * Communicated by the Author. |