A MODEL OF NATURE. 183 



When !i pioco of iron is niiijifnetizod its Ix^hiivior is dirt'orent accord- 

 ing as tho magnetic force applied to it is weak, moderate, or strong. 

 When a (certain limit is passed the iron behaves as a nonmagnetic sub- 

 stance to all further addition of magnetic force. With strong forces 

 it does and with very weak forces it does not remain magnetized when 

 the force ceases to act. Professor 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 fairl}^ be 

 said that as far as this particular set of phenomena is concerned, a 

 most instructive working model based on the molecular theory has not 

 only been imagined l)ut 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 everbod}^ to be 

 made up of separate glol)ules of water, the critics of scientific method 

 are sometimes apt to regard the molecules as mere fictions of the 

 imagination. If, however, we could throw the molecules of a highl}" 

 rarefied gas into such a state that vapor condensed on them, so that 

 each became the center of a water drop, till the host of invisible mole- 

 cules was, as it were, magnified b}^ accretion into a visible mist, surely 

 no stronger proof of their reality could ])e desired. Yet there is 

 every reason to believe that something ^'ery like this has been accom- 

 plished by Mr. C. T. R. Wilson and Prof. J. J. Thomson. 



It is known that it is comparatively difficult 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 

 evident 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 l)ody 

 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 electricity 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 direc- 

 tions, and unless this breaking up occurs a gas does not conduct elec- 

 tricity. But a gas can be made a conductor merely by allowing the 

 Rontgen rays or the radiation given ott' ])y uranium to fall upon it. A 

 careful study of the facts shows that it is probable that some of the 

 atoms have ])een 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 argument we come to the conclu- 

 sions: First, that the presence of electrified particles promotes the 

 formation of mist, and, second, that in an ionized gas such electrified 

 particles are provided by the breaking up of atoms. 



