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86 THE PHOTOGRAPHIC NEWS. [Feb. 21,1862. which entirely evaporates without leaving any residue. Turpentine leaves a residue, which will not only rapidly turn yellow, but which some chemists have thought might exercise a detrimental effect on the permanency of the print. Mr. Hardwich, speaking on this subject in a paper on the fading of positive prints, some years ago, remarked : “ Tur pentine, commonly used as the solvent [of wax], is liable to contain a principle possessing oxidyzing properties, as may be shown by agitating commercial oil of turpentine with dilute solution of sulphate of indigo, the blue colour of which it often quickly bleaches.” There are, however, various solvents which are innocuous, and evaporate without leaving any residue. Pure benzole, chloroform, or essential cils. The essence of spike, lavender, or rosemary, the cheapest of the essential oils, we have used with complete satisfaction. There is one class of pictures to which the application of wax will be of the greatest value: we refer to the developed prints produced by the solar camera. The chief defect in these prints arises from their being sunk too much into the texture of the paper, a consequent want of brilliancy result ing. Many continental photographers varnish their enlarged prints, and in pictures of this magnitude the effect is not so offensive as in smaller prints; but we apprehend that wax, whilst answering the same end, would be still more free from objection. Some time ago we called the attention of our readers to the advantages of Clausel’s encaustic paste for this purpose. Our attention had been called to the preparation by Mr. Rejlander, who generally uses it upon his prints. It is prepared by Bailey and Son, of Wolverhampton ; and. if we remember rightly, the recipe was imparted to Mr. Rejlander by the late Prince Consort. It consists of white wax, oglio cotta, (which is, we believe, an Italian preparation of poppy oil inspissated by heat) and an essential oil. This is easy to use and effective when used. Messrs. Newman, of Soho Square, we believe, manufacture an enamel or encaustic paste of a similar kind. The best mode of applying the preparation of wax is to nib a thin coating all over the surface with a piece of clean cloth or the fingers; and then with a piece of woollen fabric, which is quite clean, polish well, crossing and re crossing the strokes, until a fine rich surface is obtained, giving richness and depth, but without the offensive glaze of varnish. Touching out of any slight defects should be done first, and after waxing the inequalities of surface will be imperceptible. IODIDE AND BROMIDE OF SILVER: RESHARCIIES into their PIOTOGENIC Properties according to the Circumstances under which they are obtained. BY M. E. REYNAUD. Most of the photographic processes known at the present day, are based upon the property possessed by certain salts of silver being sensitive to the action of light. Among these salts, chemists have recognized that those most applicable to photography, on account of their greater sensitiveness, are the iodide, the bromide, and the chlo ride of silver. Photographers have naturally directed their researches upon these salts, and consequently to those salts which they form by double decomposition in the photo graphic film. We are therefore led to study the various properties of the iodide, bromide, and chloride of silver, according as these salts are obtained by the decomposition of one or the other of the soluble iodides, bromides, or chlo rides, and also directly by iodine, bromine, or chlorine. For the present, I shall limit myself to the study of the salts employed in the wet collodion process, commencing with iodide oj silver ; this salt being the basis of sensitized photographic collodion. We know that iodide of silver is obtained by the double decomposition of an iodide and of the nitrate of silver form ing the sensitizing solution. We have therefore to examine the different qualities of the iodide of silver, according as it is formed under one or the other of the following con ditions :— 1st.—According as the collodion contains a solution of iodide of potassium, or of iodide of ammonium, or of cad mium, &c., or of some of these iodides together; or also of an excess of iodine. 2nd.—According whether the sensitizing silver solution be neutral, acid, or alkaline. Let us suppose for a moment the silver solution to be slightly acid, as is the case usually in practice; and com mence our study with the examination of the photogenic properties of iodide of potassium. It is a well-known fact that the iodide of silver resulting from the decomposition of iodide of potassium, receives the luminous impression very slowly. A collodion which con tains only this iodide is in fact one of the slowest; but it possesses some special qualities which it is worth while to examine. First, let us mention its property of giving very dense blacks in the negatives, a quality which renders it so valuable in copying old engravings, the yellow paper of which acts but very feebly upon the sensitive film ; and also, generally, for everything requiring great contrasts in the negative, always excluding the representation of animated objects on account of the slowness of photogenic action before spoken of. Another quality of collodion iodized with iodide of potas sium is an extreme delicacy of detail, which is due to the slowness with which the silver reduced on the glass plate during the development of the image, attaches itself to the impressed portions. This second property gives it the preference in obtaining very small negatives, such as are afterwards amplified in the solar camera. I propose to return to this subject, and examine in detail the molecular state of the silver reduced upon the negatives, according to the products and processes employed. These researches will offer much interest for the solution of the problem of amplifying images. Iodide of cadmium, with regard to rapidity, possesses qualities opposite to those of the preceding iodide ; for the iodide of silver formed by it is one of the quickest in re ceiving the luminous impression ; but, on the other hand, it possesses the serious defect of giving fogged pictures: this defect is corrected by the addition of bromide of cadmium ; but a surer means of avoiding fogging without altering the sensibility, consists in adding to the collodion a few drops of chloride of iodine, or a very slight proportion (say about 1 or 2 per 100) of chloride of zinc. The use of a very acid silver solution will produce the same result. Without the addition of the bromide, this extremely rapid collodion will have the defect of being nearly insensitive to the green, yellow, and red rays. I shall now speak of iodide of ammonium. This iodide possesses mixed qualities, which admits of its making a very good collodion, employed either alone, or preferably, with a small addition of bromide of ammonium. Although this collodion is not so sensitive as that with iodide of cadmium, it is preferable to the latter on account of the artistic perfection of the images it yields.*. Let us, however, notice one of its defects, which consists in the great facility with which it forms spots in the sensi tive film upon being immersed in the sensitizing silver solu tion. Certain researches upon this subject have proved to me that this effect is caused by the great facility with which the iodide of ammonium is decomposed into iodide of silver at the time of sensitizing; the liquid veins of the silver bath in motion impress themselves on the film, and it is particularly that part of the film resting on the edge of the bath that is spotted thus, which arises from the veins of the * I must mention in this place a remarkable coincidence existing between the sensitiveness of the three salts studied above, and their solubility in alcohol. We perceive, in fact, that iodide of potassium, the least sensitive among them, is also the least soluble iu this liquid, while the iodide of cad mium, which possesses very great sensitiveness, dissolves in it with remark able facility.