708 Prof. J. E. Ives on the Absorption of 



In no case was I able to detect, within the limits of 

 accuracy of the experiment, any absorption of the waves. 



Some of the typical results are given in Table I. The 

 pressure is given in centimetres of mercury, and the gal- 

 vanometer deflexion in centimetres. For every value of the 

 pressure three independent readings of the galvanometer 

 deflexion were taken when the waves were sent through the 

 tubes. The mean of the three readings is the value given in 

 the table. 



The two thermoelectric junctions, as already stated, were 

 iso connected to the galvanometer that they produced nearly 

 equal and opposite deflexions. The deflexion due to one 

 junction alone was equal to about 2*5 cm. The deflexion 

 due to R L was positive and slightly greater than that due to 

 R 2 , which was negative. Consequently, if any absorption of 

 the waves had taken place in T 2 , it would have been indicated 

 by an increase of the differential deflexion of the galvano- 

 meter. It is evident, I think, from the table that a constant 

 increase or decrease of the deflexion of *10 cm. could have 

 been easily detected. Therefore the absorption, if it exists, 

 must be less in all the cases examined than 4 per cent, of the 

 whole radiation passing through the tube in a length of 

 44*5 cm., or less than one-tenth of one per cent, for a length 

 of one centimetre, and we would probably be safe in saying 

 that it must be less than half of this. 



The variations which can be observed in the individual 

 deflexions are probably due, in the main, to slight changes 

 of temperature upsetting the balance between the two ther- 

 moelectric currents. It was for this reason that, after 

 trying other arrangements, I finally placed the two tubes 

 side by side, bringing the two receivers as close together as 

 possible. As is well known, one of the difficulties in work 

 with short electric waves is to obtain a receiver whose 

 readings will be constant. The present experiment, in which 

 the readings represent very small differences between two 

 readings, presents unusual difficulties. 



When the radium was used in the presence of air at 

 different pressures, it was put in three watch-glasses placed 

 at equal distances along the tube, so that it would be in 

 direct contact with the air. In this way its ionizing effect 

 was greatest. About one gram of radium chloride was used, 

 having an activity of about 7000. 



Before beginning the experiments on the effect of varying 

 the density of air in the presence of radium, some preliminary 

 experiments were made in the endeavour to detect some 

 effect due to the presence of radium when the air was at 



