ADDRESS. 17 



explained, and in large part discovered, by the aid of the atomic theory ; 

 and, though there are outstanding difficulties, they are, for the most part, 

 related to the nature of the atoms and molecules, and do not affect the 

 question as to whether they exist. 



The fact that different kinds of light all travel at the same speed in 

 interplanetary space, while they move at different rates in matter, is 

 explained if matter is coarse-grained. But to attempt to sum up all 

 this evidence would be to recite a text-book on physics. It must suffice 

 to say that it is enormous in extent and varied in character, and that the 

 atomic theory imparts a unity to all the physical sciences which has been 

 attained in no other way. 



I must, however, give a couple of instances of the wonderful success 

 which has been achieved in the explanation of physical phenomena by the 

 theory we are considering, and I select them because they are in harmony 

 with the line of argument I have been pursuing. 



When a piece of iron is magnetised its behaviour is different according 

 as the magnetic force applied to it is weak, moderate, or strong. When 

 a certain limit is passed the iron behaves as a non-magnetic substance to 

 all further additions of magnetic force. With strong forces it does and 

 with very weak forces it does not remain magnetised 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 fairly 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 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 everybody 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, however, we could throw the molecules of a highly rarefied gas into 

 such a state that vapour condensed on them, so that each became the 

 centre 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 difficult to produce a fog in damp 

 air if the mixture consists of air and water-vapour alone. The presence 

 of particles of very fine dust facilitates the process. It is evident that the 

 vapour condenses on the dust particles and that a nucleus of some kind is 

 necessary on which each drop may form. But electriBed 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 electricity is discharged into it. 



1901. c 



