1917] on Recent Developments of Molecular Physics 85 



It appears, then, that we are brought to the contemplation of a 

 universe in which the ultimate motion is of a discontinuous nature. 

 The supposed continuity of nature must be only an illusion ; motion 

 when seen on a large scale is continuous, but is resolved into discon- 

 tinuity when we imagine it viewed under a sufficiently high-powered 

 microscope. Such revolutionary conceptions will be made mentally 

 more palatable to us if we can find direct evidence of the " jumps " 

 in question, and a good deal of such evidence exists. 



Perhaps the most striking, although not the most direct, evidence 

 is provided by Bohr's theoiy of line-spectra. We know from the 

 researches of Rutherford that the hydrogen atom consists of two 

 constituents — a positive electron and a negative electron — circling 

 round one another. Bohr assumes that only a limited number of 

 orbits are possible ; the two electrons may circle at distances such 

 that the motion possesses one, two, or any integral number of the 

 quanta of energy corresponding to the frequency of rotation, but at 

 no other distances. Sudden drops from one of these distances to 

 another can take place, and when this happens the energy set free 

 leaves the atom in the form of one quantum of monochromatic light, 

 the frequencies of these bundles of light giving the line-spectrum. 

 The line-spectrum has always defied interpretation in terms of the 

 old mechanics ; indeed, the old mechanics made it impossible that a 

 line-spectrum could occur at all. Bohr's theory has given a brilliant 

 explanation ; his theory predicted the position of the lines exactly, 

 and further predicted the existence of other lines in the infra-red 

 which were not known to him when he published his theory, but 

 were subsequently discovered by Lyman. The theory also predicted 

 a large part of the helium spectrum, and a comparison of this spectrum 

 with that of hydrogen enabled Fowler to determine the mass of the 

 electron to an accuracy at least equal to that of the best of previous 

 determinations. 



Einstein has supposed that when the quantum of energy is set 

 free in the form of radiation from radiating matter it does not spread 

 out in space, but remains as a compact bundle of energy. We must, 

 on this view, think of radiation not as waves spreading out in a sea 

 of ether, but perhaps rather as fishes swimming out into a sea of — 

 we do not know what. If this is so, we might be able to obtain very 

 direct evidence of the existence of these fishes by spreading nets to 

 catch them. Suppor.e we spread nets of varying meshes, say two, 

 four, six, eight inches, and set free what the quantum theory tells us 

 ought to be five-inch fishes, and what the old mechanics tells us 

 ought to be waves in our sea. Suppose, we find that the six-inch and 

 eight-inch nets are unafi'ected, while the two and four-inch nets show 

 holes in each case of five inches. Suppose further that millions of 

 what the quantum theory pronounces to be one-inch fishes have no 

 effect, while even the smallest number of what the quantum theory 

 calls three and five-inch fishes are found to make holes of the corre- 



