16 SCIENCE PROGRESS 



of a helium atom at an initial speed of many thousands of miles 

 per second, the remainder of the atom forming an atom of a 

 new substance, which is very generally found to be itself 

 radioactive and to undergo the same process in its turn. 

 Besides the projection of helium atoms generally termed the 

 a radiation, there is also an emission of what are called /3 rays 

 and 7 rays ; and it is by reference to the phenomena attending 

 the passage of these various types of radiation through matter 

 that I propose to illustrate the three ideas just stated. Ruther- 

 ford's conception implies that one kind of matter is continually 

 being derived from another, the latter disappearing to have its 

 place taken by the former. Since different kinds of matter have 

 different chemical and physical properties, it must therefore 

 be possible by processes such as are already in common use 

 to separate any newly formed product from the substance of 

 its origin and to watch a fresh crop come into existence. 

 Again, since the new product is in itself radioactive and may 

 disintegrate at any moment after its formation, it can never 

 accumulate to an unlimited extent. If, for example, a given 

 quantity of radium be laken, the first product, which Rutherford 

 called the emanation, goes on perceptibly growing in quantity 

 for several days but at the end of a week or so a balance is 

 reached. The number of births, so to speak, is practically a 

 constant quantity all the time, for the number of radium atoms 

 breaking down in a week is so small a fraction of the number 

 of radium atoms present that no perceptible change takes place 

 in the latter. But the number of deaths !among the emanation 

 atoms is growing, being equal to the product of two things, 

 the one the number of emanation atoms in existence, the other 

 their mortality. Each radioactive substance has its own 

 mortality. We know nothing whatever of the reason for this ; 

 we do not indeed understand in the least why the break-up 

 takes place at all. We can only say that it is a matter of 

 chance. No sooner is a radioactive atom born than it stands 

 the risk of dying; moreover, as experiment shows, the 

 chance that it may die at any moment is quite independent of 

 the time it has been in existence. There is no senile decay 

 nor any process which tends to make a given atom at any time 

 more likely to break down than any of its fellows, however it 

 may differ from them in age. It was at one time held strongly 

 that the process of disintegration was the outcome of a long- 



