88 



for actinium (Jahrbucli der Rad. imd Elek., Bd. iii., Heft 2, 

 p. 159). In experiments of this kind the radiating material 

 is spread evenly on a level surface, and sheets of absorbing 

 material are placed upon it. The ionisation produced in the 

 space above the sheets is compared with the thickness of the 

 sheets; and the two variables are found to be connected toge- 

 ther more or less exactly by an exponential law. There is 

 some difficulty whether such measurements give more nearly 

 the number or the energy of the stream of particles which 

 emerges from the plate, as Rutherford (Radio-activity, 2nd 

 Ed., p. 134), and Thomson (Conduction through Gases, 2nd 

 Ed., p. 375), have pointed out. The point was also discussed 

 in my address to Section A of the Aus. Ass. for the Adv. of 

 Science, Dunedin, 1904, p. 69. There is also an uncertainty 

 due to the application of a formula to radiation from an 

 assemblage of points which is really only applicable to a plane 

 wave, or a stream moving normally to the plate. If a point 

 source of radiation is placed below an absorbing plate of thick- 

 ness d, and there is a true co-efficient of absorption A, the 

 fraction that emerges from the further side of the plate is not 

 e — \d ; much of the radiation passes obliquely through the 

 plate and is absorbed to a greater degree than that which 

 passes normally. This has often been pointed out, e.g., by 

 N. R. Campbell (Phil. Mag., April, 1905, p. 541), who also 

 gives some figures from which the proper curve of absorption 

 may be drawn. I am not aware, however, that it has been 

 noticed that the form of the absorption curve, which is far 

 from an exponential curve for a thin radiating layer, ap 

 proximates much more closely to it for a thick radiating layer. 

 And it is interesting to find that the experimental curves 

 which are most nearly exponential are those for which the 

 layers of radio-active material were thick compared to the 

 penetration of the rays under investigation. As examples, we 

 may take those of uranium and actinium already mentioned. 

 On the other hand, the curve which H. W. Schmidt (Ann. d. 

 Phys., Bd. 21, 1906, p. 651) has obtained for the/:? rays of 

 RaC, the radio-active material being deposited in a very thin 

 layer on metal foil, shows just about the amount of depar- 

 ture from the exponential form which is to be expected if 

 the absorption is truly exponential, and there is only ^nc 

 absorption co-efficient, not two, as Schmidt has suggested. 



The following figures give the proportional amount of 

 the original radiation which passes through a plate of thick- 

 ness w/A, where A is the absorption co-efficient: (1) for a 

 thin layer; (2) for a thick layer. The figures are also given, 

 for the sake of comparison, for the case of a plane wave, or a 

 pencil of rays passing through the plate normally : — 



