IS the sum total of the world's wealth increasing or diminishing since the War ? Is mankind as a whole getting richer or poorer ? These are the interesting questions that Sir Arthur Salter will discuss tonight. As an economist of international reputation, who has been Director of the Economic and Financial Section of the League of Nations, Secretary-General of the Reparations Commission, and actively connected with the financial reconstruction of Austria and Hungary, the Dawes plan for Germany and the organization of the World Economic Conference this year, he has an exceptional knowledge of the economic condition of the world.