1903.] Oxidising Action of tlie Rays from Radium. 201 



NaCl; KC1; KN0 3 ; Pb(N0 3 ) 2 ; Ba(N0 3 )->j Bad, accelerate 

 K 2 S0 4 j CaC0 3 ; BaS0 4 ; MgC0 3 retard. 



It is remarkable that the salts which were tried should so group 

 themselves that those with univalent acids accelerate, those with 

 bivalent acids retard. 



The influence of salts appears to be purely a case of surface action. 

 If the salt first be heated for a few minutes with two changes of the 

 solvent (benzene), it entirely loses its power.* 



Probably owing to obscure catalytic action of this kind one finds 

 that in certain apparently clean test-tubes the very sensitive solution 

 of iodof orm in chloroform changes to purple even in the dark. The 

 catalysing power of such a tube, however, is very rapidly exhausted. 



Probably also owing to catalytic action some samples of iodoform 

 decompose when dissolved in chloroform even in complete darkness. 

 The impurity which brings about this apparently spontaneous change 

 can be distilled off by suspending the sample in water and boiling for 

 -a considerable time. The first distillate condenses as a red liquid, 

 when this ceases to come over the distillate will be found to be 

 approximately or quite stable. The impurity can be got rid of more 

 effectively by recrystallising from ethylic alcohol. 



The chemical feature on which we wish to lay most stress is that for 

 the liberation of iodine oxygen is needed it is in all probability due to 

 an oxidation, and, like many oxidation processes, it is carried on, under 

 ordinary circumstances and at ordinary temperatures, only in the 

 presence of light. 



The reaction, on the one hand, is a delicate test for the presence 

 of oxygen, on the other, a convenient method for measuring the 

 chemical activity of various rays. We found the trace of oxygen 

 <which remains after C0 2 has been bubbled through the chloroform, 

 and over the iodoform for one hour, sufficient to produce a decisive 

 -change of tint. 



The beautiful purple colour which the liberated iodine makes in 

 solvents other than alcohol, lends itself readily to measurements which 

 may be made by choosing some solution of iodine in chloroform as 

 a standard colour and matching the fluids under examination with it. 



The reaction, when once started, continues for a time in absolute 

 darkness and then ceases. Thus, if light be allowed to play upon a 

 tube so as to produce, say, a faint purple tint, and the tube then be 

 removed to the dark, the faint purple tint will deepen to a certain 

 extent. On renewed exposure to light the action recommences. 



* According to Wurster ('Ber. d. Deut. Chem. Oesellschaft,' vol. 19, p. 3201), 

 finely powdered bodies in general occlude " active " oxygen. The two carbonates, 

 however, were the most finely powdered of the salts used. 



