464 Prof. ej. C. Bose. On Electric Touch and the MolecAdar 



Electric Reversal. 



It now remains to be proved that the " radiation product " exhibits 

 a change of sign of electric touch. The sensitiveness of certain sub- 

 stances belonging to each of these two classes is very great. On the 

 other hand, in the transition from one class to the other, substances 

 are met with the sensitiveness of which is rather feeble. The experi- 

 mental verification of the hypotheses mentioned above seemed at first 

 very difiicult, as the reversed action was likely to be masked by 

 the stronger normal action of the still unconverted portion of the 

 substance. It however occurred to me that if slightly sensitive sub- 

 stances were taken, then the direct and reversed actions were likely 

 to be obtained with less difiiculty. For this reason I took for my 

 first experiments arsenic, which is moderately negative. It is however 

 possible, though the adjustments are difficult, to exhibit the reversed 

 actions even with strongly sensitive substances, and as a type of such 

 actions that of iron will be taken as an example. 



Ohsermtions with Arsenic Receiver. — E^rperimerd I. — A receiver was 

 made with freshly powdered arsenic ; the critical distance was foiuid 

 to be 25 cm. ; that is to say, when the radiator was moved from 1 to 

 25 cm. there was always produced an increase of resistance, while 

 beyond this distance there was a diminution of resistance ; the critical 

 distance, 25 cm., may therefore be taken as an approximate measure of 

 the negative character of the specimen. As has been said before, if 

 through any cause the substance becomes more negative, the critical 

 distance will be increased; but if the substance tends towards the 

 positive direction by becoming less negative, then the critical distance 

 will be decreased. The receiver was now continuously subjected to 

 radiation for ten minutes. After this it was found that the receiver 

 gave a diminution or positive reaction, even when the radiator was 

 brought close to the receiver. The action of radiation has thus 

 reversed the sign of electric touch. 



Experiment II. — Any arbitrary length of exposiu^e labours mider the 

 defect that what is observed is the final eff'ect, the intermediate efi'ects 

 not being taken into account. In order to observe the intermediate 

 eftects, a very laborious series of observations is necessary. The experi- 

 ment was therefore modified in the f ollo^Wng manner : — A fresh receiver 

 was subjected to radiation, and observations at intervals of fifteen 

 seconds were taken to test the nature of reaction of the sensitive sub- 

 stance. The first action of radiation on the fresh specimen was a great 

 increase of resistance — so very great that the current was reduced to 

 zero ; it was no longer possible to make any further observation with- 

 out re-establishing the current. This was done by a very gradual 

 increase of pressure, eff'ected by means of a fine micrometer screw which 

 moved the compressing electrode in a perfectly parallel manner. There 



