THE 



SOUTH AFRICAN JOURNAL 

 OF SCIENCE: 



COMPRISING THE REPORT OF THE 



SOUTH AFRICAN ASSOCIATION 



FOR THE 



ADVANCEMENT OF SCIENCE. 



19 20. 



BULAWAYO. 



OFFICERS: 1920-1921. 



■:(^ * 



PRESIDENT. 

 Prof. J. E. DuERDEN, M.Sc., Ph.D., A.R.C.S. 



VICE-PRESIDENTS. 



Prof. G. E. Cory, M.A. 



Prof. R. Leslie, M.A., F.S.S. T. R. Sim, D.Sc. 



Prof. J. A. Wilkinson, M.A., F.C.S. 



HON. GENERAL SECRETARIES. 



H. E. Wood, M.Sc F.R.A.S., Union Observatory, Johannesburg. 



C. F. JuRITZ, M.A,, D.Sc, F.I.C., Agricultural Chemical Research Laboratory, Capetown. 



HON. GENERAL TREASURER. 

 J A. FoOTE, F.G.S., F.E.I.S., Commercial High School, Johannesburg. 



HON. LIBRARIAN. 

 Annie Porter, D.Sc, South African Institute for Medical Research, Johannesburg. 



ASSISTANT GENERAL SECRETARY. 

 H. A. G. Jeffreys, O.B.E., Box 6894, Johannesburg. Telegraphic Address: "Science." 



HON. EDITOR OF JOURNAL. 

 Prof, H. B. Fantham, M.A., D.Sc, University College, Johannesburg. 



VOL XVII. APRIL 1921. N o. 2. 



JOHANNESBURG, 

 PUBLISHED BY THE ASSOCIATION. 



192 1. [Price 10«. 



