166 CHEMICAL DISCOVERY AND INVENTION 



positive mass is a kind of shell or skeleton in which the electrons 

 are imbedded, or whether the positive charge is concentrated at 

 the centre while the electrons are distributed round it. In either 

 case the number of electrons is not indefinitely large, as it has 

 been shown by Sir J. J. Thomson that only a minute fraction of 

 the mass of an atom consists of electrons, which in number are 

 about three times the atomic weight of the element. The positive 

 mass is supposed to be, at least in part, made up of helium 

 atoms, associated with hydrogen, especially among the elements 

 of high atomic weight. 



Since the electrons are all negative they must repel one 

 another, and the helium and hydrogen particles being all positive 

 also repel one another. The electrons must therefore be dis- 

 tributed in such a manner throughout the structure that electrical 

 neutrality is preserved. In any of the more complicated cases 

 the structure must be unstable, and under the action of forces 

 the nature of which is unknown disruption occurs, a helium 

 atom escapes and with it two electrons, giving rise to the 

 phenomena of radio-activity. On the other hand, the stability 

 and chemical indifference of helium and its allies are probably 

 due to simplicity of structure and the presence in the atom of 

 the least possible number of electrons. 



Crookes' speculations concerning the genesis of the elements 

 have already been recounted and some criticisms formulated. 

 It seems to the write? that for many reasons the assumption 

 that all the elements from first to last were evolved from one 

 protyl by a process essentially the same throughout is untenable. 

 In every atom it seems to have been established that there are 

 two principles associated respectively with positive and negative 

 electricity. Though the properties of the elements are in some 

 way connected with the masses of the atoms of which they consist 

 it seems improbable that atomic mass alone determines properties. 

 There are too many anomalies in the periodic scheme itself. 

 There can be little doubt that their properties are dependent in 

 no small degree on the arrangement of the constituent particles, 

 positive and negative, within the atom. Like properties indicate 

 a like constitution, and the conclusion seems irresistible that in 

 certain cases the analogy between a family of elements and a 

 homologous series of carbon compounds expresses the physical 

 fact. 



Take the alkali metals for example, the formula a+nd, in 



