The pressure of varied work has prevented my forwarding the text of these lectures for publication so early as I could have wished, and I take this occasion of expressing my regret at the delay. The circumstance that thus impels me to preface with a few words their issue affords me also the opportunity of recording how much I am indebted to President Hadley and the authorities of Yale University for tneir kindness during a visit which I shall ever remember with pleasure. To Professor Chittenden, Director of the Sheffield Scientific School of the University, I owe further a debt of gratitude for unstinted assistance open to me from him on all occasions.

C. S. S.