TRANSACTIONS OF SECTION 15. 595 



Plates were prepared with this emulsion: in one case the silver chloride was modi- 

 fied in molecular structure by boiling the gelatinous solution, and in the other it 

 was prepared by the cold method. Sucli plates were exposed to the spectrum and 

 developed with a solution of hydrochloric and ammonium carbonate, and in other 

 cases the development was with ferrous citrate. 



The unmodified silver chloride, when examined by transmitted light, had a 

 very faint canary colour ; the absorption-spectrum of this was photographed, and 

 showed that aU the ultra-violet rays as far as II were not off", and that there was a 

 slight weakening of the violet and indigo rays. On the principle of work going hand 

 in hand with absorption, it was surmised that the rays most entirely absorbed 

 should give the maximum efiect when silver chloride was used as a receiving medium 

 for the spectrum. This was proved to be the case, the limit of maximum eflect 

 being about II, the sensitiveness rapidly falling to ' h,' and being still less at G. 

 Dr. Eder had examined the spectrum also, and found the maximum intensity in the 

 violet. This discrepancy may be accounted for by the fact that the author's experi- 

 ments were carried out in July, when the solar spectrum is very rich in ultra-violet 

 rays; whilst Dr. Eder's were carried out with sunlight in the winter, when those 

 rays were almost absent, or the spectroscope itself may be in fault. 



Experiments with the electric light gave the same results with the author as 

 those obtained with sunlight. 



There is a practical bearing of these experiments in photographic printing on 

 albumenised paper. In the process both albuminate and chloride of silver are the 

 media acted upon by light ; the spectrum on albuminate of silver has its maximum 

 in the blue near G. If, then, we have light which possesses strongly the ultra-violet 

 rays as well as the blue rays, we arriv-e at the fact that both the chloride and the 

 albuminate will be strongly acted upon. If, on the other hand, we have a winter 

 light, which is deficient in ultra-violet rays, we shall have the albuminate of silver 

 much more acted upon than the chloride. Any practical worker in photography 

 will have noticed that the appearance of silver prints of equal depth at the two 

 seasons have very different qualities in toning, and this no doubt arises from the 

 fact that the proportions of darkened chloride to albuminate vary veiy considerably 

 in the two cases; the print residting from light possessing much of the ultra-violet, 

 being more vigorous than that produced, say, in winter. The author has in other 

 papers shown that in slow printing a certain amoimt of oxidation of the altered 

 chloride of silver is an accompaniment, and that this produces flatness of image 

 when it is toned with gold ; a still further cause for the same defect, the author 

 believes, may be found in what he has already stated. 



The author has ' printed out ' the spectrum on chloride of silver, and finds that 

 the same position of maximum as that it possesses when the image is developed 

 holds good. 



The author then proceeds to point out that, in actenometers based on the use of 

 silver chloride, the rays measured chiefly lie in the ultra-violet region ; and he pro- 

 poses to try the substitution of bromized paper for the same purpose, and to com- 

 pare them with the results obtained by the chloride paper. 



4. Some Remarks on Crystallogeny. By Professor J. P. CoOKE. 



6. Oh the Action of Zinc and Magnesium on Acidified Solutions of Ferric 

 Sulphate. By Professor T. E. Thorpe, Ph.D., F.B.S. 



The extent of reduction of the ferric salt may vary with the strength of the 

 solution, with its temperature, with the amount of free acid present, and lastly 

 with the specific nature of the metal employed. The author has studied the condi- 

 tions under which the hydrogen does work as a reducing agent. Experiments 

 were made on dilute solutions of ferric sulphate, containing known quantities of 

 free acid. The author finds : (1) that the amount of reduction produced by a given 

 weight of zinc in dissolving increases with the temperiiture ; (2) that it is also 

 aflPected, although to a less degree, by the initial surface of metal exposed. Whilst 

 the extent of reduction, as also the rapidity of solution, increase with the tempera- 



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