408 Prof. W. H. Bragg on the Consequences of 



differing therefore in every imaginable way from ft rays, an 

 explanation would surely be hopeless. 



Considering all this evidence for and against the existence 

 of a direct ionization of a gas by X and 7 rays, I would 

 conclude that the entity hypothesis leads us to expect that 

 there is no such effect, that many experiments fall in readily 

 with this view, and that others are quite likely to show a 

 like agreement when obvious defects have been removed. 



Let us therefore accept this simplification, provisionally at 

 least ; and let us go on to consider a second problem of the 

 ionization-chamber which may then be taken in hand with 

 some success : the problem of the chamber of any form made 

 w holly of any one substance. 



Suppose a block of any material to be crossed by a stream 

 of 7 rays, and let us try to estimate so far as we can the 

 whole length of track cohered by ft rays in any element of 

 volume in a second, irrespective of direction. The number 

 will in the first place depend on the strength of the 7 radiation 

 in the neighbourhood of that element of volume : after 

 allowing for that, it will depend on two things only, (a) the 

 number of ft rays originated in eaeh unit weight of the 

 substance, i. e. the absorption coefficient of the 7 rays by 

 the substance, (b) the v r eight of material traversed by each 

 ft ray before it disappears. If different ft particles traverse 

 different amounts of material, the average is to be taken : 

 we may call such average the average range, or briefly the 

 range. The important thing to observe is that the range 

 need not be all in one straight line : the ft particle may 

 make any number of twistings and turnings during its total 

 path, and the range is the length of the path if it were 

 straightened out, or rather the weight of material which the 

 particle traverses. The deflexion oval and the scattering 

 which the oval represents do not come into consideration 

 at all. Let us say that k is the absorption coefficient of the 

 7 rays and d the range, then the sum of the tracks of ft rays in 

 a unit volume is directly proportional to Ikd, I being the 

 intensity of the 7 rays. It may be of some service to give an 

 analogy. If k points were taken at random in each square 

 centimetre of a sheet of paper, and a line of length d were 

 drawn from each point, then the quantity of ink used and 

 the quantity of ink on each square centimetre w^ould be just 

 the same, on the average, whether the lines were straight or 

 curved or made up of any number of short pieces so as to be 

 zigzag in form. The ordinary coefficient of absorption of 

 ft rays is a compound of d, and of the dimensions of the 

 deflexion oval. We are here dealing with a much simpler 



