titanium oxide and 2 per cent' sulphuric acidand 63 per cent water, are slowly added to a solution containing 1050 pounds of barium sulphide, held in a large cylindrical tank, provided with a rotary agitation :capable of producing rapid agitation. The mass isthus v rapidly agitated, and the 2 per cent of sulphuric acid contained in the titanium acid cake reacts with a small portion of the barium sulphide. This reaction may be represented by the following equation TiO H 80 The free sulphuric acid of the titanium acid cake is neutralized by thebarium sul-' phide solution, forming barium sulphate and hydrogen sulphide, as indicated by the above equation. As the sulphuric acid is present only in a small percentage, the major porltiion of the barium sulphide remains as suc very fine colloidal suspension. The barium sulphate produced is also very fine, and the presence of this. very fine barium sulphate in suspension, and also of the very fine colloidal titanium oxide, is believed to be the explanation of the great improvement in the properties of the finished lithopone.