414 HELL SVSrEAf TF.CHMCAL JOURNAL 



The aljiha-particle is siii)[K)seci, like tlie electnm, to be imuii smaller 

 than an atom; partly because it can go through a thin sheet of metal, 

 chiefl>' because of evidence to be expounded in the next paragraph. 



Collisions between alpha-particles and other jiarticles of similar 

 mass are occasionally observed; the mass of the struck particle can 

 be deduced from the directions in which it and the alpha-particle fly 

 off after the impact, assuming only that conservation of momentum 

 and conservation of kinetic energv- prevail during the impact. In 

 this way it is possible to determine the masses of tiny particles (pre- 

 sumably atoms) of hvdrogen, helium, oxvgen and nitrogen (perhaps 

 eventually of other elements) in terms of the mass of the alpha- 

 particle, which is determined from its charge-mass ratio and its 

 charge, which are determined directly. If all the properties of the 

 elements could be explained In- atoms possessing no features except 

 charge and mass, all the fuuncj.iiinns of science nii.ulu be laid dnwii 

 already. 



The alpha-particle is one of liie most \aluabk- and jjowirlul instru- 

 ments in the physicist's equipment. It is a sort of luper-microscope, 

 penetrating and revealing the arrangements of sv'stems so minute 

 that microscopic objects are universes compared with them. Ruther- 

 ford's development of the technique of using the ali)lia-pariicle is to 

 be ranked among his greatest works. 



Positively-charged particles with masses as low as that of the 

 electron have never been observed; the least massive of the knnwii 

 positively-charged particles has 1,840 times the mass of ilu' eitciron. 



|). lui. Nil i.i-;ar .\i(im-M()1)I-;i, 



Since we ha\e met with positively-charged particles which are 

 accepted as atoms deprived of one or more of their electrons, and 

 since these incomplete atoms are much greater in mass than the 

 electrons, it is natural to suppose that the completed atom consists 

 of a positively-charged particle or nucleus in which almost its entire 

 mass is concentrated, and one or more electrons which compensate 

 the charge of the positive particle but add little to the mass of the 

 atom. If we further suppose that the dimensions of the electrons 

 and of the positi\el\-charged particle are small in comparison with 

 the distance between them, we invent the nuclear atom-model.^ 



The flirect evidence for the nuclear atom-model consists of a very 



' ('(itniiiDiily known as tlif Uiiihirford atoni-niodi'l, after the physicist who in- 

 vrnteil il and diMovcred most of thri-vidi-nre for il; ixcasionally as Nagaoka's, after 

 another physicist who suKRcsted it; occasionally as the Satiirnian model, as some 

 have siipiHisetl that the electrons lie in Mat rings around the nucleus like the rings of 

 .Siiturn around that planet. 



