814 Mr. A. B. Wood on the 



recoils in the atomic form, and not as a molecule of Th D 

 chloride. 



Assuming these conclusions to be correct, then when Th D 

 recoils from Th C chloride we must have a liberation of free 

 chlorine. It is interesting to speculate as to what happens to 

 this chlorine. Does it escape from the surface, or recombine 

 with other free molecules of the active deposit ? 



Case III. Th D in equilibrium with Th B and C. 



(i.) Undissolved in Acid. 



In these experiments the untreated active deposit was 

 heated for 4 minutes at various temperatures, and its sub- 

 sequent activity measured by /3 rays. Up to a temperature 

 of about 500° C. the curve of decay after heating was exactly 

 exponential with a period of 10'6 hours. Above this tem- 

 perature, however, an initial rise or " break " in the expo- 

 nential curve was obtained, indicating that some Th D had 

 been removed in the furnace (see fig. 3.A). The results are 

 quite consistent with previous observations by Barratt and 

 Wood. 



(ii.) Dissolved in pure Nitric Acid. 

 The active deposit dissolved in nitric acid was dried and 

 afterwards heated to different temperatures. The results 

 obtained were exactly the same as those just mentioned for 

 the active deposit untreated with acid. 



(iii.) Dissolved in pure Hydrochloric Acid. 

 When the chloride of the active deposit was heated the 

 results were remarkably inconsistent. On some occasions 

 initial rises in the /3-ray curve of activity after heating were 

 observed at temperatures above 300° C, but these rises 

 indicated varying amounts of ThD volatilized at any par- 

 ticular temperature. In nearly every case, however, it was 

 observed that some Th D was volatilized at temperatures 

 between 300° C. and 500° C. In view- of the results obtained 

 in I. this seems to imply that, at any rate, some portion of the 

 Th D in the active deposit exists as " chloride,'' even after 

 the Th D chloride originally present in the active deposit 

 has had time to decay. The amount of Th I) chloride present, 

 however, seems to vary considerably. It would appear then 

 as if some free chlorine recombined with the recoiling Th D, 

 but the experiment is obviously not conclusive. The formation 

 of chlorine compounds accounts to some extent for the varia- 

 tions in the amount of Th D volatilized from the chloride of 



