CONSTITUTION OF MATTER RUTHERFOED. 191 



radius were onl}^ one eighteen-hundredth of that of the negative 

 electron, viz, about 10'^'' cm. 



There is no evidence to contradict this point of view, and its sim- 

 plicity has much to commend it. In viewing the essential differences 

 exhibited by positive and negative electricity in connection with 

 matter and the obvious asymmetry of the distribution of the two 

 electricities in the atom, one is driven to the conclusion that there is 

 a fundamental distinction between positive and negative electricity. 

 Since the unit of positive charge is identical in magnitude with the 

 unit of negative charge, the only possible difference is the mass of 

 the two units, and this on modern views is mainly dependent on the 

 dimensions or degree of concentration of the electricity in these 

 fundamental entities. 



If we take the view that the hydrogen nucleus is the positive elec- 

 tron, it is to be anticipated that the nuclei of all atoms are l^uilt up 

 of positive and negative electrons, the positive electricity being al- 

 ways in excess, so that the nucleus shows a resultant positive charge. 

 The mass of the atom will depend mainlj'' on the numl^er of the 

 massive positive electrons in the nucleus, although it will be affected 

 to a slight extent by the number of the lighter negative electrons 

 involved in the structure of the whole atom. The mass of the atom 

 will no doubt be influenced also by the distribution of the positive 

 and negative electrons in the nucleus, for these must be packed so 

 closely together that their field must interact. As Lorentz has shown, 

 the mass of a number of closely packed electrons is not necessarily 

 the same as the sum of individual masses of the component electrons. 

 Taking such factors into account, we should not necessarily expect 

 the mass of all atoms to be nearly an integi'al multiple of the mass 

 of the hydrogen atom, although it is known that in a number of cases 

 such a relation appears to hold fairly closely. 



The appearance of a helium atom in such a fundamental process 

 as the transformation of radioactive atoms indicates that helium is 

 one of the units, possibly secondary, of which the nuclei of the heavy 

 atoms are built up. In course of its successive transformations a 

 uranium atom loses eight helium atoms, a thorium atom six, and an 

 atom of actinium five. The probability that helium is one of the 

 units of atomic structure not only in the case of radioactive atoms 

 but for ordinary atoms is strengthened by the fact that the atomic 

 weights of a number of elements differ by about four units. 



The fact that the helium nucleus survives the intense disturbance 

 resulting in its violent ejection from a radioactive atom suggests 

 that it is a very stable configuration. On the views discussed it is 

 natural to suppose that the helium nucleus, of atomic weight about 

 four, is made up of four positive electrons united with two negative 

 electrons. No doubt it is difficult to understand why such a system 



