surface to the Antarctic. The question must be left open, but seems to be worthy 

 of experimental investigation when the next opportunity is afforded. 



In the paper embodying the results obtained on the '' Terra Nova " during the 

 voyage from England to New Zealand, the suggestion was made that observations on 

 the relation between pressure and ionisation in a perfectly screened chamber might 

 throw some light on the origin of the residual ionisation. Observations of this nature 

 have been since made by Downey,* but they do not supply a complete satisfactory 

 answer on this point. Downey made observations at different pressures on the 

 residual ionisation, the metal chamber being placed on a pier running into the 

 Mississippi River, the water being 8j feet deep underneath the chamber. A linear 

 relation between ionisation and pressure was obtained. This is interpreted by Downey 

 as proof that the residual ionisation is caused by a very hard radiation (the possibility 

 of a spontaneous ionisation or breakdown of the gas being apparently excluded). 



There seems little doubt, however, that the " screening " conditions were not 

 very satisfactory. A lower ionisation was obtained in the laboratory than over the 

 water, which suggests a value of not less than 10 pairs of ions formed per c.c. per 

 second over the water at atmospheric pressure. This argues either a significant 

 amount of radioactive substance in the metal, in the enclosed air, in the river waters, 

 or on the pierf. Downey also states that the departure from linearity of this curve 

 is no more than 1 3 per cent, of the highest value, though the individual curves given 

 are not identical and the variations noted from hour to hour in the laboratory were 

 considerable, arguing significant changes in the amount of radioactive material lying 

 on the ground and deposited on the walls of the laboratory building. In these 

 circumstances, it is obviously unsafe to make too wide a generalisation. 



It should not be forgotten that the lowest values are obtained after filtered fresh 

 air is blown into the chamber, and that the value rises steadily and slowly to a maximum 

 for a few days after this treatment. It is therefore difficult to escape the conclusion 

 that radioactive products in significant amounts are suspended in the air within or on 

 the surface of the cylinder some days after the introduction of fresh air, and this seems a 

 clear indication that the zinc cylinder with which our observations were made contained 

 a radioactive substance, notwithstanding the low ionisation values observed. This 

 radioactive substance may well be the cause of the residual ionisation, since the 

 increase to an equilibrium value with lapse of time is quite comparable in value with 

 the minimum value observed by us for the residual ionisation. 



* ' Phys. llev.,' November, 1920. 



f The " Terra Nova " contributed about two pairs of ions to the total ionisation when lying in 

 Lytti'lton Harbour. 



46 



