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The photographic news
- Bandzählung
- 27.1883
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- 1883
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- Hochschule für Grafik und Buchkunst Leipzig
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- Bandzählung
- No. 1280, March 16, 1883
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The photographic news
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- Titelblatt Titelblatt I
- Register Index III
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MARcH 16, 1883.] THE PHOTOGRAPHIC NEWS. 165 gelose, and other solid colloidal hydrates, all of which are strictly insoluble in cold water, are themselves permeable when in mass, as water is, by the more highly-diffusive substances. But such jellies greatly resist the passage of the less diffusive substances, and cut off entirely other colloid substances like themselves that may be in solution. In this repeat, they resemble animal mem branes A mere film of the colloidal septum pro ¬ duces this separating effect.” And at page 717 :—“ The purifica tion of many colloid substances may be effected with great advantage by placing them on the dialyser. Accompanying crystalloids are eliminated and the colloid left behind in a state of purity. The purification of soluble colloids can rarely be effected by any other known means, and dialysis is evidently the appropriate mode of preparing such bodies free from crystalloids.” Again, on page 718 :—“ Caramel of sugar, purified by repeated precipitation with alcohol and afterwards by dialysis, contains the proportion of carbon in the highest of the caramelic bodies of Gelis ; it forms a tremulous jelly when concentrated, and appears decidedly colloidal. Like all other colloids, it has a soluble and an insoluble modification.” Although no one disputed the quantities I had the pleasure of laying before you in my short communication on the 8th of last month, yet several, from their questions, did not seem convinced that it was impossible to remove decomposed gelatine by dialys ing emulsion, or by simply washing it, when set, in threads. In order, therefore, to convince those few belonging to this Associa tion, and any outsiders who may be of this opinion, I have been making a few more experiments in the same direction. Mr. A. J. Brown thought that blotting-paper coated with gelatine was not a fair septum to use. In order, therefore, to remove any objection that could be made on that head, I tried to use thin films of gelatine supported by threads tied across the ends of the glass tubes I employed; but, unfortunately, after the gelatine had been allowed to soak for some time (twelve hours) in water it detached itself from the threads and ends of the glass tubes used. In order to overcome this difficulty of the swelling of the gelatine I added two drops of a 10 per cent, solution of chrome alum to the gelatine, and used muslin as the support for the film. The muslin was then cemented to the end of the glass tube, using the same mixture of gelatine and chrome alum as above ; the whole was then allowed to dry. When this septum was soaked in water it swelled but very little. To get rid of the solvent action of the ammonia contained in Mr. A. L. Henderson’s “leucine,” I evaporated 10 c.c. of it to dryness over a water bath, redissolved it in water, and then placed it in the dialyser. The amount of solid matter dried at 100° C. was ’2390 gramme. The glass tube with septum at the bottom containing the “ leucine ” was placed in 100 c.c. of distilled water. At the end of twenty-four hours this water was replaced by another 100 c.c., and the dialysing allowed to go on for forty-six hours longer. At the end of that time the dialates were evaporated to dryness over a water bath, and the amount of solid matter amounted to -0750 gramme. This number compares very closely indeed with that which I obtained in my first experiment, given in my previous paper, where I found -0378 gramme as the weight of solid matter in the dialate when 5 c.c. of “leucine” were employed. On evaporating the contents of the dialyser, I obtained • 1650 as the weight of colloid left behind. This, added to the weight of solid matter found in the dialate, gives a total of '2400 gramme, giving a difference of only '001 gramme between the actual quantity taken, and the quantities found in the different vessels. This, I think, proves conclusively that the increase in weight which took place in my previous experiments was due to the action of the gelatine of which the septum was made. In order to test the effects of other septa of a different nature on the dialysing properties of “ leucine,” I stretched a piece of vegetable parchment over the end of a glass tube, using the same kind of cementation as before. Again: 10 c.c. of “leucine” were evaporated to dryness in order to get rid of the ammonia ; the residue amounted to '2400 gramme. The dialysing was allowed to go on for about the same time and in the same manner as in the case where muslin coated witn gelatine was used, and at the expiration of the time, both dialates were evaporated ; in this case, '0770 gramme was passed through—a nunber which, I think, you will acknowledge is practically the same as those obtained when septa of blotting-paper and muslin, covered with gelatine, were used. I was asked by Mr. Henderson if I had tried the effect of absolute alcohol on his “ leucine,” so as to ascertain whether any thing was thrown down by it. At the time he asked the question I had not made the experiment, but since then I have. If five parts of absolute alcohol be added to one part of “ leucine ” a white cloud is formed. On heating the mixture in a test tube to the boiling point the cloudiness does not disappear, but by adding cold water after the mixture had cooled to the extent of thirty per cent, the cloudiness was entirely removed. This, I think, points to the fact that the cloudiness was due not to the precipitation of undecomposed gelatine, but to the strong alcohol throwing down the iodide of potassium. All photo graphers who have practised the wet process know this salt is very much more soluble in weak than in absolute alcohol. In order to cause my last experiments to resemble as nearly as possible what takes place in the ordinary washing of gelatino- bromide emulsion, I took two portions of gelatine, each weigh ing one gramme. These were soaked in 24 c.c. of distilled water, so as to make a jelly of the same consistency as is usually employed in emulsion's. To one portion I added *8650 gramme of colloid, obtained from gelatine that had been acted on by caustic potash. The other was left just as it was, without any addition. Both were then allowed to set for twenty hours. At the end of that time each jelly was passed once through a metallic syringe, the bottom of which had teen perforated with holes 188 of an inch in diameter, and allowed to soak for fifteen minutes in 50 c.c. of distilled water. This was poured away, and another 50 c.c. poured on, and the gelatine allowed to soak for another fifteen minutes. Both washing waters were then evaporated in a basin over a bath, and the residues at 100° C. amounted to '4330 gramme and *0220 gramme respectively, so that in half-an-bour 100 c.c. of water was extracted from the gelatine pure and simple ’0220 grammes of soluble matter, and the same volume of water extracted ■4330 gramme of solid matter, when *8650 gramme of a colloid had been added to it. In this way 47’5 per cent, of the whole of the colloid added was removed; this does not include the soluble matter that the gelatine itself gives up. The washing water in each case was passed through a filter paper once, in order to remove any small pieces of insoluble gelatine that would otherwise have been carried over if the washing water had been simply decanted from the beaker into the evaporating dish. The percentage of decomposed gelatine in one case was very much larger than can ever happen in the case of emulsion pre pared with ordinary care. At the same time I must say that I was very much astonished at the very large quantity removed; but this is undoubtedly due to the fine state into which the jelly was divided, and therefore may have come not from the interior of the small pieces of gelatine, but from the gelatine where fresh surfaces had been exposed, due to the subdivision. The last experiment I made was in the same direction, only using “ leucine ” instead of gelatine decomposed by caustic potash. In this case I again took one gramme of gelatine, and added to it the residue from 10 c.c. of “leucine.” On washing this in exactly the same manner as I washed the other jellies, filtering the washing water and evaporating, I obtained a residue of •1260 gramme. Of this residue ’022 gramme is due to soluble matter in the gelatine, and '0760 gramme is due to crystalloids in the “ leucine ” as proved by the dialysing experiments, so that ■0980gramme must be deducted from the total amount extracted; it leaves '0280 gramme of “ leucine " extracted, but the whole of the colloid in 10 c.c. is .1760 gramme, therefore 15'9 per cent, of the "leucine" was extracted. Thus it would seem that the less the per centage of soluble colloid that there is in the jelly, the more difficult it is to get rid of it, and what is got rid of I am still of opinion is due to the large surface exposed to the action of the water, and not to the passage of the colloid from the interior of the jelly into the water. One thing astonishes me, and that is the very gr eat difference in the per centage removed when a large quantity of soluble colloid is added, and when only a small quantity is present. GELATINE PLATES FOR TRANSPARENCIES. BY ANDREW B. STEWART.* The late exhibition of transparencies by the members of this Society has had several results—some profitable, some unprofit able, and some which may be either the one or the other. The profitable ones may be said to be the attraction of new members, the discovery of unsuspected talent, and the encouragement of those in whom the discovery has been made to persevere. The chief of the unprofitable ones, in my case at least, is the wasting * Read before the Edinburgh Photographic Society.
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