[The following is an account of experiments carried out by the author during the year 1913–14 in collaboration with Professor S. J. M. Auld, D.Sc., Ph.D., F.I.C., etc., of University College, Reading. Dr Auld is now engaged in military duties and the task of presenting this report therefore devolves upon the author who wishes it to be clearly understood that Professor Auld must not be held responsible for any inaccuracies or other faults.]