In what has been said before in regard to the production of a mineral froth, I have had in mind the use of some insoluble substance like oil to promote the flotation and froth prodution. In the Potter-Delprat process a froth is presumably produced without and oil, but in that case carbonic acid gas is gas used. It is a significant fact that no method has yet been found of producing a mineral froth by air bubbles without the addition of oil or some other substance. Among other substance that will assit in producing a commercially effective mineral froth are a number of soluble substances, as amyl alcohol, camphor, most of the essential oils, the phenols, and a number of gas-tar derivatives.

The phenomena to be observed in connection with the use of these soluble substances as mineral frothing agents are confusing. What has been said about the oil adhering to the sulphides in the first part of this chapter does not apply in the case of these substances, because they are completely soluble in the proportions used ; being completely soluble in the proportions used, the small amount employed in treating the ore (i Ib. per ton of ore) cannot be thought to adhere preferentially to the sulphides. On the other hand, although these substances are soluble, the solution cannot be used over and over again beyond narrow limits, but the frothing agent must be added continuously. It is also inconceivable that there is any chemical reaction between, say, the soluble essential oil of eucalyptus amygdalena and any of the ingredients of the ore. The froth produced in this case has identically the same appearance as where air, oil, and acid, or acid and carbon dioxide, are the froth constituents, and we are justified in assuming that in the main there is no difference in the forces acting. A complete answer as to just ' What is a froth ? ' would probably clear away all the uncertainty as to just ' What is a flotation process ? '