Joi.Y — Radio-Therapy : its Scientific Basis and its Teaching. 503 



stress set up by electronic segregation accumulates to the point of rupture, 

 and there is a hurried return of the electron to its starting-point under the 

 electric stress. It is noteworthy, however, that tlie recovery of the plate is 

 not complete. Successive reversals give less and less distinctive results. 

 This would be explained if a certain amount of ionisation in the virgin 

 emulsion was neutralized by the electronic discharge. Describing the 

 jihenomenon of reversal in general terms, we may regard the change as 

 referable to an excessive segregation of negative ions, leading to tlie over- 

 balancing and destruction of the ionic system formed by the moderate 

 exposure of the film. 



The latent image may also be induced by friction, pressure, or, generally, 

 by meclianical irritation of the film. The action of the mechanical stimulus 

 is probably to induce directly the separation of electrons ; i.e. to promote 

 the negative ionisation. A latent image so formed can be reversed by 

 radiation. 



Tlie reversal of the latent image may, in some cases at least, be effected 

 by the infra-red and longer heat waves. This might have been anticipated 

 from the Uieory of the latent image given above, for we would expect a feeble 

 displacement of the electrons by the long waves, under which effect they 

 would recombiue. 



We have now to consider tlie formation of the latent image by chemical 

 means. 



If tlie latent image is essentially tlie result of a partial reduction of the 

 silver halide due to a concentration of negative ions, we should expect 

 that the introduction of negative hydroxyl ions should assist in its creation. 

 The developers and sensitizers, in point of fact, generally create the latent 

 image, and act most effectively when alkaline, e.g., pyrogallol, gallic acid, 

 tannin, aqueous solutions of nicotine, and — it is stated' — alkaline solutions 

 of lactose and glucose. These substances, acting as developers, must, 

 whatever stages may intervene, ultimately neutralize the charge upon the 

 positive silver ion in the emulsion, setting it free as metallic silver. The 

 effects are, in short, proba.bly continuous with those of radiation in creating 

 the latent image. Dilute solution of ammonium hydrate alone will give 

 the latent image. Some substances act as sensitizers, or owe their special 

 efficiency as developers to their active absorption of the halogen. It appears 

 certain that this action occurs in the cases of all strong developers and 

 sensitizers ; the halogen being sometimes, however, taken up by the gelatine. 



If a solution of gallic acid, especially if rendered alkaline, is poured on 



' Meldola: "Chemistry of Pliotography," Macmillan, 1S91, p. 190. 



