EXPLORATION OF THE MINERAL WORLD BY X-RAYS 437 
SECOND EVENING DISCOURSE. 
Mownpay, SEPTEMBER 10, 1934. 
THE EXPLORATION OF THE MINERAL 
WORLD BY X-RAYS 
BY 
PROF. W. L. BRAGG, F.R.S. 
(1) THE mineral world has supplied us with many of the most beautiful 
examples of crystal structure. Crystals grow best when the growth takes 
place in very constant conditions and very slowly, and these conditions are 
fulfilled in nature in a way that cannot be rivalled in the laboratory. The 
beauty of natural crystalline forms has always attracted attention, and some 
of the rare and durable varieties have been prized, as jewel stones, as the 
most valuable of all natural objects. 
Crystalline arrangenient is not confined, however, to such well-developed 
specimens as are displayed in mineral collections. Ruskin, in his Ethics of 
the Dust, draws his moral from the exquisite patterns which would be revealed 
if we could magnify up sufficiently any speck of dirt. The Ethics of the 
Dust is a series of lectures in which the theme is based upon the varieties of 
mineral species and the ordered arrangement of the atoms which compose 
them. Ruskin pictured his listeners gifted with a power of vision which 
enabled them to see the arrangement, and made a series of guesses about its 
nature. Now that this power of vision has become a reality, and we are 
able to study crystal patterns by means of X-rays, it is remarkable to see 
how close to the truth his imagination, unhampered by scientific caution, 
often led him. 
The present is a suitable time to review our knowledge of the structure 
of the mineral world, because all the main types of minerals have been 
analysed. ‘The existence of any well-crystallised mineral has always been 
a challenge to those whose research is the analysis of crystals by X-rays. 
Nature provides us with such excellent material on which to exercise our 
technique. The first crystals to be analysed were minerals, rock salt, 
diamond, fluor, blende, pyrites and calcite. For twenty years the enquiry 
has been pursued, and with the recent analysis of the felspars it may be 
claimed that the main survey has been completed. There are, of course, 
many fascinating points of detail still to be investigated, but we can sum- 
marise the general laws which govern the different structures composing 
the solid crust of the earth. 
(2) We may first enquire how it is that we are able to speak of minerals 
as a limited class of chemical compounds. The number of compounds that 
can be formed from the chemical elements is endless. Yet the number of 
mineral species is restricted, and if we except the rare kinds which are found 
in odd corners where very special conditions have existed, the number is 
quite small. It must be admitted that part of the interest in mineralogy 
has been the interest of the collector. The fun of making a collection 
would be spoilt if nature kept on producing endless new varieties of minerals, 
