1914] on X-Rays and Cr7Stalline Structure 207 



modes in which thej tend to come apart under the action of solvents 

 and other agents. By showing how atoms arrange and disarrange 

 themselves under innumerable variations of circumstances we mu'st 

 gain knowledge of the nature and play of the forces that bind the 

 atoms together. 



There is yet a third direction in which enquiry may be made, 

 though as yet we are only at the beginning of it. In the section 

 just considered we have thought of the atoms as at rest. But they 

 are actually in motion, and the position of an atom to which we have 

 referred so frequently must be an average position about which it is 

 in constant movement. Since the atoms are never exactly in their 

 places, the precision of the joint action on which the reflection effect 

 depends suffers materially. The effect is greater the higher the 

 order of the spectrum. When the crystal under examination is con- 

 tained within a suitable electric furnace and the atoms vibrate more 

 violently through the rise of temperature, the intensities of all orders 

 diminish, but those of higher order much more than those of lower. 

 The effect was foreseen by the Dutch physicist Debije, and the amount 

 of it was actually calculated by him on certain assumptions. I have 

 found experimental results in general accord with his formula. In 

 passing it may be mentioned that as the crystal expands with rise of 

 temperature the spacing between the planes increases and the angles 

 of reflection diminish, an effect readily observed in practice. 



This part of the work gives information respecting the movements 

 of the atoms from their places, the preceding respecting their average 

 positions. It is sure, like the other, to be of much assistance in the 

 enquiry as to atomic and molecular forces, and as to the degree to 

 which thermal energy is locked up in the atomic motions. 



This brief sketch of the progress of the new science in certain 

 directions is all that is possible in the short time of a single lecture : 

 but it may serve to give some idea of its fascination and possibilities. 



[W. H. B.] 



