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294 THE PHOTOGRAPHIC NEWS. [May 8, 1885. offered may, nay does, exercise an influence: hence it is that when gelatine is present, the more likely theory would be that the molecules become heterogeneously mixed, and a molecule of silver nitrate seizes upon its nearest neigh bour, whichever that neighbour may happen to be, and thus we obtain a number of such molecules of two or more compounds indiscriminately suspended within the walls of our viscous medium-gelatine. Against this supposition we are confounded with the practical difficulty experienced when attempting to suspend certain of the organic silver salts, whereby for some unexplained reason (probably in creased gravity) the salts quickly precipitate, and this is further augmented by the application of heat, which also tends to reduce the silver to the metallic state. To any one meditating a few experiments with the organic salts of silver, we should recommend the following as an introductory step, tending as it does to somewhat modify many generally accepted ideas in regard to the behaviour of these salts in the light, and pave the way for further investigation. Take five two-ounce test tubes, or beakers (wine glasses will do equally well), and place half an ounce of distilled water in each, adding to the water some silver nitrate—say one gramme to the first, and half a gramme to each of the other four. When this is dis solved, weigh out sufficient of the following five salts to throw down the silver in each glass, leaving just a trace of haloid in excess. Into the first vessel place sodium chloride ; the second, potassium or ammonium citrate ; the third, potassium oxa late ; the fourth, double tartrate of potassium and sodium (Rochelle salt); the fifth, sodic tungstate. There are others that might be added, but these will suffice for illus tration of an instructive experiment. Let the operation be conducted in yellow light, such as that given by “golden fabric,” and wash the precipitate by decantation ; namely, when the precipitate has settled, pour off as much of the water as may be conveniently done without losing the precipitate, refill the vessel with water, and again pour off as much as possible. Take a half-plate—opal by pre ference—and place it on a very hot piece of slate, plate glass, or other slab for the purpose of keeping it warm enough to liquefy the gelatine used. Have ready a little moderately strong solution of gelatine in a liquid state ; beginning with the first beaker or other vessel, take some of each precipitate, and place each portion on the opal glass in the same order they are taken. Repeat this in another row across the glass, adding to each quantity of the precipitate a drop or two of hot gelatine solution, and well mix with separate strips of glass or stirring rods. Then form a third row consisting of portions from the second, third, fourth, and fifth vessels, to each of which has been added a larger quantity out of the first, as well as a few drops of warm gelatine solution. Let these all be mixed up well as before, and the experi menter will then have three rows, the first showing plain silver chloride, citrate, oxalate, tartrate, and tungstate; the second presenting the same salts mixed with an organic substance—gelatine ; and the third intending to demon strate the effect of mixing silver chloride with each of four other silver salts that are affected by the action of light. In the fourth and last row of four portions let the chloride (first vessel) be mixed with the others as near as possible in the proportion of four to one. Take the opal plate so prepared into a bright light and watch the printing action carefully, noting the order in which each portion begins to discolour, and the ultimate density presented by them all after an exposure of, say, half an hour. The organic salts will soon be placed in their proper order of sensitiveness and density. In our second article, contained in the Photographic News of March 6th last,* mention is made of a number of silver salts upon which we had experimented some months before. Among them silver tartrate appears, * Page 149, vol. xxix. and as that substance has been recently formulated for use in printing, it is well that we should refer more in detail to our experiments on the subject. The great difficulty in the employment of silver tartrate is its refusal to emulsify by ordinary means. If a tartrate salt, such as neutral potassium tartrate (soluble), or the double tartrate of potassium and sodium, is mixed with gelatine, and silver added, a very slight precipitate is produced, which, long before setting point, has all settled at the bottom, leaving almost a clear jelly. This jelly contains silver, as shown by the colour indicator, but cannot be said to be an emulsion as we understand one in modern photo graphy ; whether little or much excess is employed, the results are always the same. The acid in the gelatine causes cream of tartar to be deposited in a small quantity, and this, mixed with a little silver tartrate, may be removed from the bottom of the mixture when set. Vigorous printing being a characteristic of tartrate of silver in gelatine, we were anxious to o‘ ercome its dis advantages, and were a long time before we could obtain a proper white emulsion of this kind ; but we succeeded at last in the manner which we propose to explain. If the salt in question be precipitated in water, the p.p. so formed is too coarse for our purpose, but still it can be suspended in gelatine, and an emulsion may be thus formed. Heat ing the precipitate changes it to a brown colour, and pro duces the effect seen in mirror manufacturing, the bottom of the beaker or other glass vessel representing a mirror of metallic silver. As in other organic preparations of silver, the heat must be kept down to the lowest degree at which the gelatine can be liquefied, and as very little heat is capable of depositing the silver, this plan becomes necessarily useless. Our experience led us to suspect the potassium salt as being the cause of the inability to pro duce a satisfactory emulsion, so we tried another salt alone, and with success. In 40 c.c. of distilled water we dissolved •8 gramme of tartaric acid. This we neutralized with '6 gramme of sodium bicarbonate. (Some care must be observed in this experiment that the carbonic acid liberated is not mistaken, by its effect upon litmus paper, for free tartaric acid. To obviate this tendency in some measure, the liquid should be violently agitated and heated up once or twice, when most of the CO, will escape. The solution should then be made just faintly acid.) We then added 14 grammes of hard gelatine (previously soaked in water), and melted at as low a temperature as possible to secure perfect solution : into this we placed 1 gramme of silver nitrate, stirred vigorously, and at once produced a white emulsion. The mixing vessel was quickly transferred to a basin of cold water, and the stirring continued for a few minutes, when setting commenced, and a satisfactory result was so far obtained. Silver tartrate emulsion, formed as above, is available for mixing with a silver chloride emulsion, in a proportion suited to taste or requirements; one part to five of chloride, or one part to four of chloride answers well. We have no doubt that in cases where the gelatin® refuses to hold the silver salt in supension in the form of a white precipitate, some organic compound, not hitherto chemically defined, is produced in conjunction with the gelatine itself; otherwise, how is it the precipitate, if pre pared in water, can be made to form an emulsion by the addition of gelatine and the application of heat. And we are further inclined to the opinion that this change is brought about chiefly through the intervention of a potassium salt, the same effect being sometimes noticeable when using potassium citrate in excess. It is quite probable that rapid stirring during setting tends to produce a whiter emulsion (a sine qua non for either opal or paper) than would be obtained if the mixture were only slightly agitated; however, this is a matter which can bo definitely tested. That certain white compounds are obtained in this manner is a fact of which we are well
