182 Scientific Proceedings, Royal Dublin Society. 



that the bacteria did not further encroach on the sterile patch 

 during this and subsequent incubation, although the radium was 

 no longer exercising any direct action on the culture. 



Fig. 1, Plate XVIII., shows a smear-culture of B.prodigiosus. 

 It was exposed to the radiations for one day at a distance of 4*5 mm. 

 from the radium bromide, and was subsequently incubated in the 

 absence of the radium for two days at 22° 0. This culture, even 

 during the day of exposure, had perceptibly developed. The 

 margin of the sterile patch is peculiarly sharply defined. The 

 colonies on the margin developed but little pigment ; while those 

 some few millimetres removed from the patch produced colouring 

 matter very abundantly, as though the more feeble action of the 

 radiations acting at a greater distance stimulated the development 

 of the pigment. 



These experiments amply illustrate the inhibitory action of the 

 radiations from radium bromide; and it is needless to quote others, 

 many of which were made on similar lines on these same bacteria 

 and also on B. anthracis and on B. typhosus. They all gave 

 similar results. 



It next became of interest to know whether the organisms were 

 actually killed in the sterile patches caused by the radiations, or 

 if their development was arrested only. The fact that, even 

 after the removal of the radium, colonies did not develop in the 

 sterile patch, would seem to indicate either that the action of the 

 radiations on the bacteria had rendered them incapable of develop- 

 ment — i.e. killed them — or that some change had taken place in the 

 medium, rendering it unfit for their development. 



To test this point, inoculations were made from each of the 

 sterile patches, either into broth-tubes or on to agar-plates. In 

 almost every case the development of the inoculation showed that 

 all the organisms at least in the patch were not killed, and that 

 the action of the radiations was, in some cases at least, only inhibi- 

 tory, while a change in the medium was probably responsible for the 

 subsequent failure of development. 



The latter deduction — viz., that the medium had undergone 

 some change which had rendered it unfit for the development of 

 the bacteria — is borne out by the fact that fresh bacteria inoculated 

 into the sterile patch also failed to develop. Fig. 2, Plate XVII., 



