1911] on a Fourth State oj Matter 7 



enter at once on a discussion as to their nature. It is well known 

 that they have the most extraordinary powers of penetration, and are 

 unaffected by electric or magnetic fields. They have one property 

 which alone, as I think, brings them within our experience ; that is 

 to say, the power of exciting (3 rays from the atoms over which they 

 pass. Were it not for this, they would still be unknown. When we 

 examine this production of f3 rays, we find that in the first place 

 their speed depends on the quality of the y rays which cause them, 

 and not on the nature of the atoms in which they arise ; in the 

 second, that the /? rays to a large degree continue the line of motion 

 of the y rays as if the latter pushed them out of the atoms ; and, 

 lastly, that the number of the (3 rays depends on the intensity of the 

 7 rays. It is these facts which suggest the simple theory I have 

 already described. The y ray is some minute thing which moves 

 ^long in a straight line without change of form or nature, which 

 penetrates atoms with far greater ease than the a or y8 particle, which 

 is not electrified, and which sooner or later disappears inside an atom, 

 handing on a large share of its energy to a. f3 particle which takes its 

 jilace. The absorption of y rays is simply the measure of their 

 ■disappearance in giving rise to /3 rays, one y ray producing one /3 ray, 

 and no more. 



We find the same sort of scattering in the case of y rays as in that 

 of f3 rays. Of a stream of rays directed against a plate which it can 

 penetrate easily, we find that a few are turned completely back, a very 

 much larger number are only slightly turned out of their path, and 

 the rest go on. The scattered rays are very similar to the original 

 rays : there is no need to suppose that the original ray disappears 

 to be replaced by a secondary, any more than there is to suppose that 

 a and /? rays disappear and are replaced by others in similar cases. 

 When therefore a y ray enters an atom three possibilities await it. 

 The first is a negative one, it may go through the atom untouched, 

 and this must happen in the majority of cases ; the second chance is 

 that of deflection, and the third that of conversion into a^ray, using 

 the word conversion in a general sense without going into details as 

 to the nature of the process. 



Now we may consider our y ray problem. Suppose a stream of 

 these rays passing over a block of any substance such as aluminium 

 or zinc or lead. When they are really penetrating rays they are 

 equally absorbed by equal weights of these materials, which means 

 that in equal weights equal numbers of (3 rays spring into existence. 

 If these /8 rays were able to move through equal weights of the metals, 

 we should find in each metal the same " density" of f3 rays ; and the 

 important point is that this is independent of whether the rays are 

 straight or crooked in their paths. If ten lines of given length were 

 begun in every square centimetre of a sheet of paper, the ink used in 

 drawing them would be independent of the straightness of the lines 

 but proportional to their length. Now if we make a cavity in each 



