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The photographic news
- Bandzählung
- 27.1883
- Erscheinungsdatum
- 1883
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- Englisch
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- Hochschule für Grafik und Buchkunst Leipzig
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- Hochschule für Grafik und Buchkunst Leipzig
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- http://digital.slub-dresden.de/id1780948042-18830000
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- Bandzählung
- No. 1285, April 20, 1883
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The photographic news
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- Titelblatt Titelblatt I
- Register Index III
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and comparing the two. We ought to see a cone of rays coming out large enough to cover the whole surface, and if there be only a bright pencil of light in the centre of the lens, we are perhaps making too near to the top edge of the lime ; or, in the case of an interchangeable jet, we have not carried the point round far enough to be at right angles, and hence the flame is striking the lime on one side. These are comparatively small matters, but they affect the result; and the beauty of dissolving views is much lessened by any inequality in the illumination. The blow-through jet is often spoken of disparagingly as giving a poor light compared with the oxy hydrogen ; but few are aware how much may be effected by studying the proper con ditions. The two gases should be made to mix as perfectly as possible, and the area of ignition should be large to compensate for its lower intensity. If these points are attended to you will be able to lecture to an audience of 400 people, and the pictures will be distinctly seen at the end of the room. The construction of a good blow-through jet requires as much care as that of a jet for the mixed gases. In my own practice I have quite discarded the common form, and have adopted the one first introduced, I believe, by Mr, Young, of Manchester, namely, a circular aperture of one-eighth of an inch for the hydrogen, and a stream of oxygen blowing through it from behind. Mr. Young’s aperture for the oxygen was not more than one-thirtieth of an inch in diameter ; but his object was to economise that gas, which is not so necessary now that chlorate of potash can be bought at a low rate. The orifice for the oxygen may, therefore, be enlarged with advantage to one- twenty-fifth or one-twentieth of an inch. Mr. E. G. Wood, of Cheapside, London, has patented a blow- through jet, which seems to me to be a good commercial form. It has an aperture of one-eighth of an inch for the hydrogen, and three-sixteenths or one-quarter of an inch behind it, and one of one-twentieth of an inch for the oxygen. The effect of this “ dome ” arrangement is that the oxygen gathers up nearly all the hydrogen, and a small blue flame is seen burning at the out let very much like the flame of the oxy hydrogen or mixed gases. Mr. Wood includes the dome in his specification, but I doubt whether he could establish his claim to priority, and perhaps it would not be necessary for him to attempt to do so ; for, if his jets were all as well made as the one he sent to me, the demand for them would, probably, be as great as he could supply. I notice, inside the dome, an arrangement of three very small screws to keep the oxygen tip in position, so that it may blow exactly through the centre of the hydrogen holes. This is very important, as the jet would certainly give less light if the stream of oxygen were not in the middle. The mixture of the gases is not absolutely perfect either in this or in any other blow-through jet which I have examined. The oxygen is in excess in the inner part of the flame, and the hydrogen on the outside, so that when you look at the lime spot you see a dark nucleus. This nucleus disappeared on removing the cylinder to a greater distance—say, to three-sixteenths or a quarter of an inch from the nozzle of the jet. On one point I am at issue with Mr. Wood. He has slanted the lime cylinder in his blow-through jet in order to preserve an angle of 45° for the incidence of the flame. This, I think, is unnecessary, as a much smaller angle than 45° will answer for the blow-through process. One of the most powerful jets in my possession has an angle of 20°, and the only difference you need make is to approximate the lime a little more closely, in consequence of the smallness of the angle. Mr. W. H. Oakley has constructed a variety of jets for me after .different models, and I am satisfied by repeated trials that an upright cylinder of lime answers well. Complaints have been made of a snapping noise like the ex plosion of a small percussion cap in using blow-through jets in a double lantern. The explanation is that there exists for the moment an explosive mixture in the small chamber behind the hydrogen orifice. I presume the defect might be cured by altering the gas passages of the dissolving tap ; but, short of that, a partial remedy will be to dissolve slowly, to keep the bye-pass hydrogen at a maximum, and not to weight the oxygen bag too heavily. A half-hundred weight ought to be sufficient, even for a large bag, and it is not often necessary to increase the pressure as the lecture proceeds, since the pipes become heated, and this compensates in a measure for the comparative emptiness of the bag. I now proceed to make a few remarks upon the ethoxo lime light of Mr. Broughton, which is likely to come into more general use. No fluid, however volatile, will, I think, entirely supersede coal gas unless it can be shown to give a better light; but when gas cannot be obtained, ether is the best substitute for it which I have tried. Not only is no heat required to volatilise it, b I here is an actual depression of temperature during the volatilisation, and the condensed moisture of the room can be seen streaming down the sides of the tank as the vapour rises. The following mode of working the process is not intended to give the maximum light of which it is capable, but will be found simple for rooms holding 200 persons :—Supposing the lenses to have a focal length of six inches, place the dissolving lantern at a distance of eighteen and a-half feet from the screen ; this will give a disc of twelve feet diameter with four-inch condensers. Fill an eight-foot bag with oxygen, and load it with one and-a- half hundredweight if you use the tube containing pumice, or with one hundredweight if without it. I recommend all who make the oxygen in the lecture room with Chadwick’s generator to use the pumice safety tube, since the pressure is sometimes greater than they are aware of. In my own practice, however, with elastic bags and moderate pressure I have discarded the pumice, as I find it impedes the flow of the oxygen, and necessitates heavier weights on the bag. Neither do I think that there ought to be any danger of explosion, provided the tank be kept full of ether. I am bound to say, however, that Mr. Broughton himself does not agree with me in this opinion. Taking into account the haste and confusion of a lecture room, and the possibility of the weights slipping off, he advises the constant use of the safety tube. This advice is no doubt prudent, and in any case the oxygen bag should be placed under the charge of a careful attendant, whose duty it should be to see that it does not touch the wall, and that it is not interfered with during the lecture. In addition to this precaution a small back-pressure valve may be placed on the nozzle of the bag, and I have found none better than Chadwick’s valve, which inter feres very little with the onward flow of the gas, and has only one fault, namely, that the oiled silk is apt occasionally to stick to the brass. A sharp suck or blow through with the breath will, however, usually suffice to release it, and this point should be attended to before the lecture begins. The true state of the case as regards the danger of using ether vapour appears to be this:—Ether vapour, mixed with air in any proportion, burns quickly when light is applied. Neither does it form an explosive mixture with oxygen when the oxygen is saturated with the vapour. But if the ether vapour be present in only small quantity—about one volume to ten of oxygen—it explodes violently on contact with flame. Now, the other tank is so made that the oxygen gas passes backwards and forwards through the liquid much in the same way that coal gas does in the lime purifying chamber ; hence with a full tank it can hardly fail to saturate itself by the time it reaches the surface. If, however, the tank were used down nearly dry, it is obvious that a point would by-and-bye be reached when the oxygen would take up too little of the ether to give security. And the same thing might happen if a weak ether of sp. gr. 750 were used instead of ether of - 720, because in that case the tank would eventually contain little else than spirits of wine. In order to be sure of the amount of ether contained in the tank, I fill it by weight, and not by measure. The tank now before you- weighs two pounds and six ounces, and if ether be poured into it until it weighs three pounds, you have ten ounces by weight, and about thirteen by measure. After the lecture is over you weigh it again, and find, perhaps that it has lost about four and a-half ounces by weight—some six ounces nearly by measure. You leave what remains of the ether in the tank with the taps closed, and fill up again to three pounds by weight before the next lecture. The vapour of ether has a slightly solvent action on india- rubber, so that the tubing used to convey it must be new and strong. I have had fracture of the india-rubber on two occasions from using old and brittle tubing. A short tube is to be pre ferred to a longer one, and as the tanks lately made by Mr. Broughton have a safety chamber holding fourteen fluid ounces on the side nearest to the bag, the tank may be placed on the table close to the lantern without any fear of suction backwards. The jets for the ethexo light may be either of the blow-through or the mixed gases kind; but the latter are more economical, and perhaps, all things considered, safer in this process. The size of the orifice of the nipple should be about one-thirtieth or one-twenty-fifth of an inch, the ordinary size of one-twentieth being rather too large for dissolving when the vapour of ether
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