INAUGUKAL ADDRESS. 15 



Lord Kelvin. The energy was supposed to come from without, and the 

 radio-active substances were merely transforming- agents. But this 

 view became discredited chiefly, I think, because it was found impos- 

 sible to stay the radio-active action by surrounding the radium with 

 screens which might be expected to ward off the action of the external 

 agent. That objection can hardly be held to be final now. The gamma 

 rays, though turned aside by all substances generally in proportion to 

 their mass, yet are specially affected by heavy atoms, with an exces- 

 sive transformation of energy. May there conceivably be a veiy 

 penetrating radiation which is practically not to be observed by ordi- 

 nary means, but acts especially upon the radio-active atoms 1 The idea 

 has its fascination, but there are many difficulties in the way of its 

 acceptance. 



We have noM considered, very briefly, the circumstances of the 

 origin and progress of these new radiations. We see that they are 

 ejected from certain atoms, of the disintegration of which they are the 

 accompaniment; that they have no regard for physical conditions; 

 that they move with tremendous speeds, and that they penetrate 

 atoms with ease. Let us go on to consider, as far as we can, wdiat 

 happens to them in the end. Again, only a little is known, but that 

 little is very interesting. 



Take the alpha particles first. Their special peculiarity may, 

 l^erhaps, be best expressed by saying that something happens to them 

 before they have made more than, perhaps, one collision causing 

 deflection, perhaps none at all; so that after this event they are lost 

 to view. Pi'ior to it, their tracks through gases are abundantly clear 

 on account of the clouds of delta particles which they leave behind. 

 In consequence, the particles move straight through the gas for some 

 distance, and then seem to disappear. This distance I have called the 

 range; and I have shown that each radio-active substance sends out 

 alpha particles of special range, so that it is possible to distinguish 

 the different substances by the ranges of their particles. I find it 

 possible to measure the range, which varies from about 3 to 8 centi- 

 metres in the known cases, with a precision of about ^ per cent. The 

 ranges in different gases depend for some obscure reason nearly on 

 the square roots of the atomic weights ; and it is to be observed that 

 the range in a gas (or solid) containing a complex molecule is to be 

 calculated from the knowledge of the ranges in gases containing 

 simpler molecules formed of the same atoms. This, again, illustrates 

 tlie absence of dependence of radio-active effects upon chemical con- 

 ditions. With the aid of Dr. Rennie and Dr. Cooke, of the Adelaide 

 University, I have spent some time in the attempt to verify this 



