September 20, 1901.] 



SCIENCE. 



437 



netized when the force ceases to act. Pro- 

 fessor Ewing has imitated all the minute 

 details of these complicated properties by 

 an arrangement of small isolated compass 

 needles to represent the molecules. It may 

 fairly be said that as far as this particular 

 set of phenomena is concerned a most in- 

 structive working model based on the molec- 

 ular theory has not only been imagined 

 but constructed. 



The next illustration is no less striking. 

 We may liken a crowd of molecules to a 

 fog ; but while the fog is admitted by every- 

 body to be made up of separate globules of 

 water, the critics of scientific method are 

 sometimes apt to regard the molecules as 

 mere fictions of the imagination. If, how- 

 ever, we could throw the molecules of a 

 highly rarefied gas into such a state that 

 vapor condensed on them, so that each be- 

 came the center of a water-drop, till the 

 host of invisible molecules was, as it were, 

 magnified by accretion into a visible mist, 

 surely no stronger proof of their reality 

 could be desired. Yet there is every reason 

 to believe that something very like this has 

 been accomplished by Mr. C. T. R. Wilson 

 and Professor J. J. Thomson. 



It is known that it is comparatively diffi- 

 cult to produce a fog in damp air if the 

 mixture consists of air and water-vapor 

 alone. The presence of particles of very 

 fine dust facilitates the process. It is evi- 

 dent that the vapor condenses on the dust 

 particles and that a nucleus of some kind is 

 necessary on which each drop may form. 

 But electrified particles also act as nuclei ; 

 for if a highly charged body from which 

 electricity is escaping be placed near a 

 steam jet, the steam condenses ; and a cloud 

 is also formed in dust-free air more easily 

 than would otherwise be the case if elec- 

 tricity is discharged into it. 



Again, according to accepted theory, 

 when a current of electricity flows through 

 a gas some of the atoms are divided into 



parts which carry positive and negative 

 charges as they move in opposite directions, 

 and unless this breaking-up occurs a gas 

 does not conduct electricity. But a gas 

 can be made a conductor merely by allow- 

 ing the Rontgen rays or the radiation given 

 off by uranium to fall upon it. A careful 

 study of the facts shows that it is probable 

 that some of the atoms have been broken up 

 by the radiation, and that their oppositely 

 electrified parts are scattered among their 

 unaltered fellows. Such a gas is said to be 

 ionized. 



Thus by these two distinct lines of argu- 

 ment we come to the conclusions : 1st, that 

 the presence of electrified particles pro- 

 motes the formation of mist, and 2d, that 

 in an ionized gas such electrified particles 

 are provided by the breaking-up of atoms. 



The two conclusions will mutually sup- 

 port each other if it can be shown that a 

 mist is easily formed in ionized air. This 

 was tested by Mr. Wilson, who showed 

 that in such air mist is formed as though 

 nuclei were present, and thus in the cloud 

 we have visible evidence of the presence of 

 the divided atoms. If then we cannot 

 handle the individual molecules we have 

 at least some reason to believe that a 

 method is known of seizing individuals, or 

 parts of individuals, which are in a special 

 state, and of wrapping other matter round 

 them till each one is the center of a discrete 

 particle of a visible fog. 



I have purposely chosen this illustration, 

 because the explanation is based on a 

 theory — that of ionization — which is at 

 present subjected to hostile criticism. It 

 assumes that an electrical current is noth- 

 ing more than the movement of charges of 

 electricity. But magnets placed near to an 

 electric current tend to set themselves at 

 right angles to its direction ; a fact on which 

 the construction of telegraphic instruments 

 is based. Hence if the theory be true, a 

 similar effect ought to be produced by a 



