Secrion III, 1912 [129] Trans. R.S. C. 
On the Absorption of Rontgen Rays in Air. 
By Dr. A. S. Eve, F.R.S.C., and F. H. Day, M.Sc., McGill University. 
(Read May 16, 1912). 
An account of some work done by the writers in January, 1912, 
has already been published in the Philosophical Magazine for April, 
1912. The following paper gives an account of some further experi- 
ments, made in May, 1912, confirming and extending the results 
previously obtained. 
The number of ions, I, produced in unit volume in a second in an 
electroscope at a distance r from a source of Rontgen rays is equal 
K  -Xr a ; 
to —e , where K is a constant theoretically equal to the number of 
T2 
ions per cm*. per second at one centimetre from the source, and À is 
the coefficient of absorption of the Rôntgen rays by air. The writers 
suggest that K-and À are the fundamental constants of the radiation 
from a Rontgen ray tube. By integration over a hemisphere of un- 
limited radius it is easy to show that the total number N of ions produced 
per second is equal to 27K/X. 
The observations were made with an aluminium vessel placed over 
the long axial rod of a Wulf electrometer. The vessel was 30 cms. high 
and 18 ems. in diameter, and the walls were 0-6 mm. thick. Measure- 
ments were made on the grass-covered College Campus at stations 20, 
40, 60, 80, 100, 120, 140 metres from the source, consisting of bulb (B), 
placed outside a window of the Physics Lecture theatre, about 5 metres 
above the ground level. 
0 
The values obtained for A, in em, for the same bulb were 
Range January, 1912 May, 1912 
20 — 40 -00040 -00033 
40 — 60 -00029 “00028 
OO SO -00010 
60 — 100 -00013 
The difference in the two cases might be due to variations in the 
hardness of the bulb, or to changes in atmospheric conditions. The 
absorption may well be less in clear weather than when mist or smoke 
is present. Sufficient observations have not been made to settle this 
point. In both cases the results are given for primary rays only, as a 
