JoLY — Radio- Therapy : its Scientific Basis and its TeacJiing. 501 



important point lias frequently been brought out that the healthy cell behaves 

 as a less sensitive system. This, of course, is at the basis of radioactive 

 treatment. It is not improbable that a dosage which will do no more than 

 stimulate mitosis in a healthy cell will suffice to destroy the less stable cancer 

 cell. The latter is, indeed, so unstable towards the ionising effects of the 

 rays that a very small dose will arrest development, and even cause the 

 destruction of the cell. A tube guaranteed to contain five milligrams of 

 pure radium bromide was several times applied to cases of cancer in this 

 city, the tube being screened with thin sheet-lead and applied externally. 

 It was subsequently found that tlie tube contained but 0"8 mgrms. of 

 radium element. This was, therefore, a very small dosage. All the results 

 obtained were, however, beneficial. The whole subject is probably in its 

 first stages of investigation in spite of the work which has been done. 



II. 



Failing the guidance which investigation will assuredly one day give 

 us, it is interesting and, possibly, important to discuss the cell as a photo- 

 sensitive molecular system, and in so far comparable with another photo- 

 sensitive molecular system, the study of which is less difficult to pursue.' 



Of all photo-sensitive systems with which we are acquainted the 

 photographic film is at once the most accessible to observation and the 

 best understood ; although in this, no more than in any other case, is our 

 knowledge complete, or our views always capable of actual demonstration. 

 We know it to consist of haloginised molecules emulsified in an organic 

 colloid, the relations of salt and colloid being probably complex, and such 

 that they react one upon the other in responding to the photo-electric effect. 

 Certain features in common with the cell will be recognized in this state- 

 ment. It is, indeed, possible that we might apply it word for word as a 

 general description of the activity of the cell as a photo-sensitive system. 



What is the photograph ? It is an effect of photo-electric activity. This 

 activity, which operates during exposure, generates the latent image. This 

 is subsequently acted on by the developer, and the negative produced. 



In this process we start with a halide of silver, loosely combined with 

 the complex molecule of the gelatine, or in a state of solid solution, the 

 instability of the silver halide being increased by its immersion in the gelatine 

 according to principles which have been pointed out by Sir J. J. Thomson, 

 in the case of ordinary solutions in water.' We end up with separated metallic 



' Joly : Proc. Roy. Soc, Ser. B, vol. Ixxxviii, 19U. 

 U. J. Thomson: Phil. Mag., vol. xxxvi, 1893, p. 320. 



4 K 2 



