Eotial Institxttton of cSrmt iritaiH: '*' ^*^^ ^^^^ 



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WEEKLY EVENING MEETING, ^""^^^ . "^ /x 



■ Friday, January 27, 1911. X.^^- ',^''f^'' 



Sir James Crichton Browne, M.D. LL.D. D.Sc. F.R.S?,"^'" 

 Treasurer and Yice-President, in the Chair. 



Professor William H. Bragg, M.A. F.R.S. 



Radioactivity as a Kinetic Theory of a Fourth State of Matter. 



There are many points of resemblance between the movements of 

 the molecules of a gas and the movements of those corpuscular radia- 

 tions with which we have become acquainted in following up the 

 discovery of radioactivity. In both cases we find that things of ex- 

 tremely minute dimensions are darting to and fro with great velocity ; 

 and in both cases the path of any one individual is made up of straight 

 portions of various lengths, during which it is moving uniformly and 

 free from external influence, and of encounters of short duration with 

 other individuals, during which energy is exchanged and directions 

 of motion are altered. There is even a resemblance in the universality 

 of each movement. The motion of molecules is a fundamental fact 

 throughout the whole of our atmosphere, and indeed in all material 

 bodies ; the motion of the radiant particles emitted by radioactive 

 substances is also widely distributed, and of great importance. Taking 

 Eve's estimate of the usual ionisation of the air we can calculate that 

 in this room, in every second, some thousands of a and /S particles 

 ^nter into existence, complete their paths through all the atoms they 

 meet, and sink into obscurity ; some of them, viz. the a particles, as 

 atoms of helium. Of these last some move through a range in air 

 of just over 4 cm., an equal number have a range of just under 5 cm,, 

 and again an equal number move through 7 cm. ; and the speed is so 

 great that the life of each a particle as such is completed in about a 

 thousand-millionth of a second. They leave their mark behind them 

 in the ionisation of the air through which they have passed, and in 

 the heat into which their energy has been commuted. The former 

 effect is easily detected by the sensitive measuring instruments which 

 we now possess ; the latter is too small to measure, and must be greatly 

 increased by the aid of radium itself before it can be investigated. 

 But on a large scale which takes into account the distribution of 

 radioactive material through the earth, the sea and the air, the effects 

 are of first rate importance to the physical conditions of our earth. 



If we compare the movements a little more closely, we find differ- 

 ences as interesting as the resemblances. The motions which the 

 kinetic theory of gases considers are those of the molecules of which 



Vol. XX. (No. 105) b 



