CONTEMPORARY ADVANCES IN PHYSICS 669 



the electrons may adhere to atoms or molecules.^ One sees that there 

 are several items of knowledge about the effects of the gas upon the elec- 

 tron-stream, which cannot be discovered by studying the inverse 

 effects of the electron-stream upon the gas, the formation of ions or of 

 excited atoms; it is necessary to observe the stream itself. Even our 

 knowledge of the latter effects may be improved by examining the 

 former. These are the purposes of the experiments which I shall now 

 describe. 



The first (and the most) of these experiments may seem rather para- 

 doxical, in view of what I have just been saying; for they are experi- 

 ments not on electrons of the stream, but on the absence of electrons 

 from the stream. One sends a beam of these corpuscles (or, it may be, 

 a beam of protons or of once-ionized potassium atoms) across a 

 stratum of gas, measures the number which go in and the number in the 

 emerging beam, and puts down the value of the difference as the num- 

 ber which "vanish from the beam." "Vanish" is a good word in this 

 connection; it is not meant, of course, that the missing corpuscles and 

 their charges literally cease to be; it is meant simply that they do not 

 belong among those which go straight through with undiminished speed 

 and undeflected path, as though the gas were not there. I will say that 

 they have been "intercepted," for this is a word which does not imply 

 any choice among the varied possibilities of stoppage, adhesion, and 

 deflection with or without loss of energy. Experiments on intercep- 

 tion of fast electrons — up to 30,000 equivalent volts — ^were first per- 

 formed at the beginning of this century; but the earliest accurate 

 work on slow electrons — say 50 equivalent volts and downward — ^is 

 only ten years old. 



The results of these experiments are very striking; but of course they 

 yield only a small part of what is wanted. We want to know what 

 becomes of the "vanished" electrons, which way they have gone and 

 with what residual speed — the total number and the distribution-in- 

 direction of those which have been scattered without loss of energy; the 

 total number and the distribution-in-energy and the distribution-in- 

 direction of those which have ionized or excited the atoms which they 

 struck; and the number of those which have stuck to atoms, if such 

 there be. 



Such information, as anyone would foresee, is harder to acquire. Of 

 the distribution-in-direction of the scattered electrons, nothing was 

 known four years ago ; and what in this last quadrennium has been ex- 



- Some gases being monatomic and others not, it is necessary to say "atoms or 

 molecules" when making general statements, if one wishes to be exact; but in the 

 following pages I shall often use either word by itself, even when the statement in 

 which the term occurs is meant to apply to gases of both kinds. 



