president's address — SECTION B. 66 



In trying to realise these things, I have asked you to 

 magnify the dimensions of the atom in the structure of 

 matter : this has ah'eady heen done, as you well know, by 

 Sir Wilham Thomson ; but, as a better estimate of the size 

 of the atom will be gained, his calculation will, I think, bear 

 repetition. " Imagine," he says, "a globe of water or glass, 

 as large as a football,* to be magnified up to the size of the 

 earth, each constituent molecule being magnified in the same 

 proportion. The magnified structure would be more coarse- 

 grained than a heap of small shot, but probably less coarse- 

 grained than a heap of footballs.'' 



A glassful of pure water, then, consists of an enormous, 

 an unthinkable number of molecules, each consisting of a 

 complete system of atoms arranged by law in a definite and 

 determinate order, like the orbs we see 



" In the sacred glory of the azure night." 



It follows that in this mass of water there must be intra- 

 molecular spaces, and that the molecules are everywhere 

 throughout the hquid jostling, colliding, and sliding over one 

 another. It also follows that, if we can communicate mot on 

 from without, the mere gliding of molecule over mclecule 

 will give way to sharp collision, impact succeeding impact 

 until the crowd of molecules is in a state of great internal 

 commotion. Such an eiFect is brought about by the mere 

 application of heat, and the energy of this motion is capable 

 of being accurately measured. 



This internal bombardment of molecules may proceed 

 further, until cohesion and surface tension is overcome and 

 the gaseous state is arrived at. 



Regarding matter as being thus constituted, what should 

 happen when we throw a few grains of salt into water ? 

 Clearly, it must be that as soon as the molecules of sodium 

 chloride come within range of the water molecules fi'iction 

 and collisions ensue, the nearest salt molecules being driven 

 into intra-molecnlar space. The amount of heat necessary to 

 undo the crystalline structure — in other words, the motion 

 acquired by the sodium chloride on coming into solution — is 

 abstracted from the water molecules ; that is to say, work is 

 done by the water molecules, hence a reduction of temperature 

 in the liquid. Had we substituted potassium iodide with its 

 greater molecular weight instead of the sodium chloride, the 



* Or, say a globe of 16 centimeters diameter. 



