PREFATORY NOTE 
I am indebted to the Royal Society of Edinburgh for per- 
mission to reproduce the Frontispiece and Plate I., which 
have already appeared in the Proceedings of the Society 
as Illustrations to papers of my own. 
I desire, also, to thank Sir Herbert Maxwell for valued 
criticism with regard to certain views I have ventured to 
express in this little book, and Mr. H. W. Johnston for 
many kindnesses in facilitating the experimental work in 
which we have been much associated. 
W. L. C. 
CARR BRIDGE, 
August 15, 1907. 
