1878.] Capt. Abney on the Acceleration of Oxidation, 8fc. 451 



II. " On the Acceleration of Oxidation by the Least Refrangible 

 End of the Spectrum." Note II. By Captain Abney, R.E., 

 F.R.S. Received June 8, 1878. 



In my first note on this subject it was stated that further experi- 

 ments would be undertaken, in which sensitive films would be exposed 

 to the action of the spectrum in atmospheres free from oxygen. These 

 have been carried out by means of apparatus specially designed for the 

 purpose, hydrogen and nitrogen being the atmospheres employed, 

 and in some cases hydrogen vacua. In every case the experiments 

 were confirmatory of what was previously surmised, the image show- 

 ing no signs of oxidation, and there is evidence to show that the 

 limit of sensibility of the compounds used is lowered towards the least 

 refrangible end of the spectrum. 



Exposure of films in solutions which readily combine with oxygen, 

 and at the same time with the halogens, have given most remarkable 

 results. For instance, silver bromide which by its colour should have 

 proved sensitive to the red end, yet when exposed in the usual manner 

 was insensitive below B in the spectrum, proved sensitive when exposed 

 in sodium sulphite (Na 2 S0 3 ), and arrived at M the lowest limit 

 (about W.L. 12,000) which I have as yet photographed. In the 

 same solution, to quote another experiment, silver iodide proved 

 sensitive to a point between a and A. The experiments were carried 

 out in duplicate. In one the plate was immersed in the solution, 

 and in another the salt was dissolved when possible in glycerine and 

 applied to the film. Both methods answered equally well, but for 

 some purposes the latter is more convenient. 



My experiments also prove, that what is technically known as solari- 

 zation is due to the oxidation of the image, accelerated by light generally, 

 be it the more or less refrangible end of the spectrum. This oxidation 

 causes the formation of a compound which is undevelopable, as already 

 has been pointed out. It has thus been found impossible to produce 

 solarization in solutions which have oxygen absorbents. We may 

 therefore conclude that the whole spectrum exercises a reducing action 

 on the sensitive salt, and that this reduced compound is again capable 

 of being oxidized by it. 



The relative power of the two actions seems to vary according to 

 the part of the spectrum. This subject is still under consideration. 



In my first note I also mentioned that photography in natural colours 

 probably depended on the same action. My surmise is confirmed to a 

 great extent. If silver sub-chloride or silver sub-bromide be produced 

 chemically, we have a dark compound formed which, if exposed to the 

 action of the spectrum whilst in an oxidizing solution (such as hy- 

 drogen peroxide), rapidly takes the colour of the rays acting upon it, 



