282 MODERN SCIENCE READER 



granules thus produced exhibited the looked-for Brownian 

 movements, and the free path was longer in air than it had 

 been for granules of the same size in water. Perrin's cal- 

 culations have, on the other hand, been questioned by 

 Duclaux. But we appear to be justified in assuming that 

 the observer, watching the movements of colloidal particles, 

 sees movements similar to those which we ascribe to the 

 invisible molecules of gases. 



Another demonstration of luminous effects, ascribed to 

 single particles, was given by Crookes in London and 

 Regener in Berlin, seven or eight years ago, and thus be- 

 fore the above-mentioned experiments on colloids in which 

 molecules are supposed to be concerned. When the a rays 

 of radium are allowed to fall on a screen or fluorescent zinc 

 sulphide, each particle seems to produce a flash of light 

 like a tiny spark, and brilliant scintillations are observed. 

 Still more instructive is the demonstration of single a 

 particles, which Rutherford and Geiger gave in the Royal 

 Institution two years ago. A tube containing radium 

 bromide was held in front of the window of a long tube, 

 several feet away from the window, so that only a few a 

 particles perhaps not more than one per second would 

 find their way to the electrometer at the far end of the 

 tube. A sudden jerk of the electrometer indicated that a 

 particle had struck and the number of particles shot out 

 per second were actually counted by counting the jerks. 

 Each a particle is supposed to represent a charged atom of 

 helium, which turns into helium gas on losing its electric 

 charge. Dewar has carefully determined how much helium 

 is produced by a given weight of radium per second, and 

 by putting that figure together with his count of the number 

 of a particles discharged per second, Rutherford arrived 

 at the conclusion that 1 cubic centimeter of helium is 

 formed by 2.56 X 10 19 particles a most remarkable con- 

 firmation of the N. 



Another exemplification of the visibility of molecules or, 

 at any rate, of the discontinuity of an apparently homogen- 



