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[September 18, 1891. THE PHOTOGRAPHIC NEWS. 658 100-00 100-00 but, as Schiitzenberger points out, these represent only * Journal of the Chemical Society. 40-36 44-71 4-19 10-74 Platinum Chlorine Water Carbon monoxide 42-01 45-89 12-08 values:— Platinum Chlorine Carbon monoxide Now the water present bears no molecular ratio to the other constituents. Calculating, therefore, the percentage composition of the anhydrous substance, we obtain the gradually. The theoretical objection to an under-cor rected lens might legitimately have been pointed out by a scientific critic if one had appeared ; its practical bearing, however, in the construction referred to, when usually a small amount of positive spherical aberration is introduced, the slight shifting forward of the plane of focus in the object is rather advantageous than otherwise for the desired end to be attained. If excessive aberration be introduced in the construction referred to, the positive chromatic aberration would, of course, assert itself to a noticeable extent. The final portion of Mr. Burton’s letter refers to an entirely different subject, viz., that of the definition given by an aplanatic lens with its powerful discriminating focus, as compared with that of the eye. Has Mr. Burton ever been down a mine or a deep well in the daytime and seen the stars on looking up ? If he has, he will perhaps acknowledge that the less amount of diffused light that reaches the eye, the more discriminating power it has, and also that much more diffused light, under ordinary circumstances, enters the eye in viewing any scene than would enter a well-stopped-down and shaded lens in connection with a camera. To give another example, if a lens has a “ flare spot,” is the discriminating power of the lens over that portion of the plate the same as in the remainder? As to definition over one plane, cited in the last paragraph of Mr. Burton’s letter, by inference, excluding the question of diffused light, per sonally I do see finer definition in the reproduction of a black and white subject in a photographic reduction than I do of the subject itself when viewed under the same angle. I have always understood that the investigations of authorities in the subject uphold this view. changes to a bright canary-yellow. It was obtained in two different experiments, but was only homogeneous in the first. From the method of formation, it seemed pro bable that the product was a combination of platinum, chlorine, and carbon monoxide, since it was not formed till the latter gas had been passed over the platinum chloride for a considerable time. It absorbed water from the air very slowly. An analysis of the substance was made in the following manner. The water absorbed from the air was determined by heating the substance in an air-bath to successively rising temperatures till its weight was constant. It proved to be anhydrous at 105°. The platinum and chlorine were determined by reducing the compound by pure magnesium and acetic acid. The reduced platinum was collected and weighed. The chlorine was precipitated by silver nitrate in large excess of nitric acid, and the precipitate treated and weighed in the usual manner. The carbon monoxide was found by difference. The percentage composition thus obtained was VOLATILE PLATINUM COMPOUNDS* BY W. BULLINGER, BRACKENBURY SCHOLAR OF BALLIOL COLLEGE, OXFORD. The object of this work was to determine, if possible, the vapour density of some compound of platinum. In order to obtain a suitable substance for this purpose, and to find the best conditions under which the trials could be made, some miscellaneous phenomena were observed, worthy of note, but apparently unconnected with the subject. The only volatile compounds of platinum hitherto known are those described by Schiitzenberger (Ann. Chim. Phys. [4], 21, 350; Annalen, Suppl. 8, 242) in the year 1872. They were obtained by passing alternately streams of chlorine and carbon monoxide over platinum sponge heated at 250° in a combustion tube by means of a gas fur nace. The volatilised substance was collected in a suitable receiver. In this manner Schiitzenberger obtained a mixture of three volatile compounds, namely, carbonyl chloroplat inite, CO,PtClg, dicarbonyl chloroplatinite, C,O,PtCl,, and sesquicarbonyl chloroplatinite, C,O,,Pt,Cl,. Now, if we follow out the instructions given by Schiitzenberger, substituting a small furnace for the oil bath in which he heated the tube, we obtain another yellow substance, which remains behind after the volatile compounds have been driven from the heated part of the tube. This compound occupies the same position as the platinum originally did, nor can any amount of heating volatilise it. When hot it is brick-red, and on cooling 42-12) . , 46-66 the 10.001 theoretical 11 "32 J which are the values calculated for a compound having the formula PtCl,,2C0C1,, or PtClg,C202. The com pound would, therefore, appear to be a combination of platinum dichloride with phosgene. It was thought possible that this substance had resulted from the action of a small amount of phosgene on the platinum dichloride. An attempt was, therefore, made to obtain it by passing a stream of phosgene over the heated chloride, but without success. Properties of the New Compound. —It is a yellow, crys talline solid, stable in air, but slightly deliquescent. It dissolves easily in water without blackening, and, on evaporation, separates from its solution in yellow crystals apparently unaltered. In alcohol it is only slightly soluble, and carbon tetrachloride dissolves scarcely any. When strongly heated, it is decomposed with evolution of chlorine and phosgene. Thus, it has none of the pro perties characteristic of Schiitzenberger’s compounds. It may perhaps be called phosgeneplatinum dichloride. Specimens of carbonyl chloroplatinite, CO,PtCl , and dicarbonyl chloroplatinite, C,O,Pt,Cl, were next pre pared in the pure state. The former was obtained by bubbling carbon monoxide for over an hour through the melted crude product; the latter, by a similar method, substituting carbon dioxide for the monoxide. Schiitzenberger says that the action of water on these compounds is very complex (loc. cit.). This, perhaps, results from a secondary action which takes place between the hydrogen chloride and the undecomposed substance. The equations given for the action of water are :— (1.) CO,PtC1, + OH, = co, + Pt + 2HC1, (2.) C,O,PtCl, + OH, = CO, + CO 4- Pt + 2HC1;