Lead and the End Product of Thorium. 823 



whence it follows that 



and 



6 '_ /C — V\ 2 



oblique incidence may be treated similarly, 

 using the fact that the angle of reflexion is not equal to the 

 angle of incidence but is given by 



sin 6 _ sin 6' 

 c — v.cosO c + v.cos6 p 



which can be obtained using Huyghens' principle, when the 

 mirror is moving at right angles to its own plane with a 

 velocity v (away from the source), 9 being the angle of 

 incidence, 6' the angle of reflexion. 



The normal pressure on the mirror becomes that given by 

 Mr. Edser, viz.: 



(c.cos# — v) 2 



p = 2e . - „ 2~ • 



* c — v* 



The tangential pressure becomes zero, as for a stationary 



mirror, the change in energy density and momentum in the 



reflected beam being just compensated by the alteration in 



the angle of reflexion due to the motion of the mirror. 



For teaching purposes it seems an advantage to regard the 



pressure as a consequence of momentum in the waves. 



East London College. 



XCI. Lead and the End Product of Thorium. (Part I.) By 

 Arthur Holmes, A.R.C.S.,' B.Sc, F.G.S., Imperial 

 College, London, and Robert W. Lawson, M.Sc. 

 Radium Institute, Vienna*. 



Contents. 

 § 1. Introduction. 



§ 2. Rate of Production of Thorium E. 

 § 3. Method of Estimating Thorium in Minerals. 



(a) Preparation of Solutions. 



(b) Treatment of Zircons. 



(c) Determination of Thorium. Fig. 1. 

 § 4. Experimental Results. Table I. 



§ 5. Discussion of Results. 



§ 6. Association of Lead with Uranium and Thorium 

 in other series of Minerals. 

 («) Introduction. 



Communicated by the Authors. 



