Sir William Bragg 41 



is harder than iron. This is the age of alloys, and one 

 important modern alloy is a mixture of aluminium and 

 copper. A very small amount of the former metal added 

 to copper hardens it greatly, and the X-rays show us that 

 when aluminium is added the structure of the copper 

 crystals remains the same, but that here and there an 

 aluminium atom takes the place of a copper atom. This 

 prevents slipping and causes the hardness. But there 

 must not be more than ten per cent, of aluminium in the 

 mixture. If more than ten per cent, is used the copper 

 crystals are broken up altogether and a new structure is 

 formed. 



All this may seem a little technical, but it is difficult to 

 put it more simply, and it is important because it is yet 

 another proof of the value of Science to industry. Thanks 

 to the researches of Sir William Bragg and his followers, 

 metal-workers are now able to compound their alloys on 

 a definite scientific basis, instead of working blindly as 

 before. 



We have spoken of the result of the first experiment 

 made by M. Laue — explaining that the pattern shown by 

 the photograph was very beautiful in shape and perfectly 

 symmetrical. Similar results have been obtained in all 

 photographs taken under similar conditions, thus em- 

 phasizing the fact that Nature always tries to arrange 

 things regularly. 



Take an X-ray picture of a section of fine-drawn 

 aluminium wire, no thicker than a hair, and you have a 

 black circle with a white centre. Around this centre 

 there are rays, then two broken circles (broken, however, 

 with perfect regularity), and near the rim two more 

 kindred circles. The whole is like a burnished convex 

 shield of great beauty. 



The X-ray photograph of a section of a thin cord of 

 rubber resembles the sun in eclipse with a dark corona 

 around it ; rock salt gives a dark centre with an intricate 



