83 



having this speed is about one millimetre in air at atmos- 

 pheric pressure. Since the X-rays do not appear to pro- 

 duce cathode rays of any speed from the air molecules, which 

 they traverse, or from the molecules of any gas consisting 

 of atoms of small weight, and since they produce much ioni- 

 sation in some way or other, we may conclude fairly that 

 they produc-e slow-speed ions themselves. Thus, whether they 

 act directly or indirectly through cathode rays, the result is 

 the same. The principal effect api^ears to be due rather to 

 secondary than primary. As Sagnac remarks (Ann. Chim. 

 Phys., 23, p. 196) : "The transformation of X-rays, by in- 

 creasing the activity at any point, permits the detection 

 there of very penetrating X-rays, which would otherwise 

 have passed unperceived." 



In the case of the y rays, such evidence as we have is 

 also in favour of the existence of slow-speed ions, as the 

 result of their action. It is known that /? rays of high speed 

 originate where they strike the molecules of a solid body (Eve, 

 Phil. Mag., December, 1904); such an action may, therefore, 

 be expected in the case of gas molecules also. It is possible, 

 however, that there may be a differential effect in respect to 

 heavy and light atoms, as in the case of the X-rays. The /? 

 rays will produce S rays in their turn; and if, as is probably 

 the case the y rays are themselves able to ionise, the product 

 will consist of 8 rays, a conclusion which may be safely 

 adopted from the analogies of the cathode rays on the one 

 hand and the X-rays and ultra-violet light on the other. As 

 in the case of the hard X-rays, the existence of y rays is often 

 made clear by the secondary effects which they produce, as 

 has been shown by Becquerel. 



To sum up what has been said, the ionisation which 

 we measure in the ionisation chamber is almost wholly due 

 to the emission of slow-speed electrons from the atoms of 

 the gas contained in the chamber, or of the chamber walls : 

 and this is true for all forms of radiation. 



Moreover, there is some evidence to show that the speed 

 of the 8 rays is almost independent of the cause and man- 

 ner of their production. As has already been said, Fucht- 

 bauer found the velocity of the 8 rays, caused by canal rays, to 

 be about 3*3 x 10^, and the same in the case of cathode rays. 

 Logeman found the velocity of the 8 rays, emitted from a 

 plate struck by a rays, to be such that they were deflected by 

 a weak magnetic field. Ewers found (Phys. Zeit., March, 

 1906) the 8 rays of polonium to possess a speed of 3*25 x lO^. 

 With these may be compared Lenard's estimate, viz., 

 108, of the speed with which the ions leave a plate 

 struck by ultraviolet light. It seems probable that we 



