1 32 THE POPULAR SCIENCE MONTHLY 



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 or- 

 dinary 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 should hold to- 

 gether, but it must be remembered that we have no information as to 

 the nature of magnitude of the forces existing at such minute distances 

 as are involved in the structure of the nucleus. 



We have so far assumed without proof that while the nucleus of an 

 atom carries a resultant positive charge, negative electrons are also 

 present. The main evidence on this point comes from a study of the 

 radioactive elements. A substance which breaks up with the emission 

 of swift electrons (beta rays), but no alpha particles, suffers disintegra- 

 tion according to the same laws and gives rise to a new element in the 

 same way as when an alpha particle is lost. It seems necessary to sup- 

 pose from a number of lines of evidence that a transformation which 

 is accompanied by the emission of primary beta particles must have its 

 origin in the ejection of a negative electron from the nucleus itself, or 

 from a point very close to the nucleus. 



There are no means at present of deciding definitely the relative 

 number of positive and negative units composing the nucleus, except 

 possibly from a consideration of the atomic weight of the atom in terms 

 of hydrogen. It is, however, premature to discuss such questions until 

 more information is obtained as to the structure of the nucleus and 

 the effect of concentration and distribution of the component electrical 

 charges on its apparent mass. 



Charge Carried by the Nucleus 

 "We are now in a position to consider a very important question, viz., 

 the magnitude of the positive charge carried by the atomic nucleus. 

 Since an atom is electrically neutral, the negative charge carried by 

 the exterior distribution of electrons in the structure of the atom must 

 be equal and opposite to the resultant positive charge carried by the 

 nucleus. The electrical charge is most conveniently expressed in terms 

 of the number of the fundamental units of charge in the nucleus. 

 Since the charge carried by the electron is one unit, the charge on 

 the nucleus of the atom may be expressed numerically by the number 



