We must go further afield in the study of molecular forces before we can understand clearly the act of the flotation of particles heavier than water, and consider the molecular phenomenon known as ' adhesion.' Referring again to Fig. 5, at C there are molecules that have uncompensated attractions unless we assume these are compensated by attraction between the liquid and the solid walls of the vessel. But the force of adhesion is also made up in part of the molecular activities of the solid surface molecules, which, we believe, lie much closer together than those of a liquid. The surface activity of a solid would be difficult to measure ; but by analogy we are justified in assuming it is in general greater than the surface activity of a liquid. So that a liquid and a solid in contact should have an attraction for each other in general stronger than the surface tension of the liquid. This state of unbalanced liquid stresses at the bounding surfaces of a liquid also gives rise to a further noticeable condition, as at D, where the corner molecule has not only its upward component, but also one of its lateral components near E abnormally compensated. This will give an illustration of what produces the meniscus, where a liquid, a solid, and a gas come into contact. Adhesion is not, however, limited to solids on liquids, but acts in solid-gas, and probably gas-liquid couples. The mathematics of these forces and the theoretical discussion could be pursued considerably further, but enough has been given to illustrate the terms employed.

The films of gases that are attracted to the surfaces of solid particles adhere strongly in some instances, and are displaced therefrom by a liquid with considerable difficulty. This quality of adhesion varies remarkably with different substances.

With some substances the gas adhesion is strong and the liquid adhesion is weak ; in other words, they are wetted with difficulty. In this class are the metallic sulphides, which, although they have a natural tendency not to adhere strongly to water, do fortunately have a strong natural tendency to adhere to oil. And further, oil has an even stronger tendency to adhere to its gas film, so that a sulphide particle covered with an oil film has an already strong tendency not to adhere to water considerably increased. Quartz and gangue minerals generally, on the other hand, have preference in directly the opposite direction. They have a comparatively feeble adhesiveness to gas films and oil, and a strong adhesiveness to water, and this already strong adhesiveness for water is greatly increased by a slight acidulation of the water. There has been no satisfactory theory yet propounded as to why acid does promote the preferential adhesion of water to gangue particles, and probably also at the same time the preferential adhesion of oil to sulphides, and for the present commercial purpose it does not seem necessary to do more than record this important fact. Bertram Blount, in speaking of sulphide particles, has expressed the idea of selection or preferential oiling as follows : " Before they were oiled at all their surfaces had certain qualities, one of which is a tendency not easily to be touched by and wetted with water. On the other hand, correlated with that property is the power of being easily touched by and wetted by oil. When they were put through this process they exercised both properties, and acquired or annexed a minute quantity of oil, which was presented indifferently to them and to the gangue, only they obtained it and the gangue did not. They came out of the process with that inherent quality enhanced by a minute film of oil, and then they were even less ready to be wetted by water and to sink through it than they were before, but it was only an exaggeration of what was a native quality."* Kenneth A. Mickle, in an excellent paper,f giving the results of experimental work on gas-bubble and oil attachment, points out that gas-bubble adhesion does not take place if the amount of oil is too large that is, if the sulphide particles are too much wetted by oil. His paper is the most valuable contribution that has yet appeared on these subjects ; but, unfortunately, he has used both the words ' cohesion ' and ' adsorption ' where he should have used ' adhesion.'

Thus it has been established that there are :

(1.) Forces acting at the surface of a liquid, the resultant of which tends to prevent rupture of the surface.

(2.) Forces acting at the surfaces of all substances, and especially at the surfaces of sulphide particles, that cause films of gases resisting displacement to adhere to their surfaces.

(3.) Forces acting at the surfaces of sulphides that cause these surfaces to show a preferential adhesion to oil.

(4.) Forces acting at the surfaces of gangue minerals that cause these surfaces to show a preferential adhesion to water, and especially acidified water. We now have the main principles by which to account for the ' floating ' of particles heavier than water, and we shall not worry as to what more these forces are, nor discuss the mathemetical formulas by which they can be resolved into their component parts.

In the simplest case we have a small particle of zinc sulphide, which has been exposed to the air, and upon the surface of which there is adhering a film of air. This particle of blende P, Fig. 6, is then brought into contact with water. The force of adhesion between the particle and the film of air prevents the water from displacing the film of air, and the force of surface tension prevents the particle of blende from penetrating the water surface ; therefore the particle is suspended at the surface thus :

If this particle of blende, P, before being brought into contact with the water, had been coated with a thin film of oil, and if the water had been slightly acidified, then both the adhesiveness of the particle for the gas film and the resistance of the liquid surface to rupture would have been increased, and the act of ' floating ' would have been made triply sure by the natural tendency of the blende and the enhancing action of the oil and acid. If at the same time a particle of quartz gangue had been brought into contact with the acidified water surface, it would have behaved in an entirely different manner. The particle of quartz, having a much feebler adhesiveness for its air film, and having a strong natural tendency to adhere to water, and also having this tendency to adhere to water and become wetted with water increased manifold by the acidulation of the water, it would have quickly penetrated the liquid surface and sunk to the bottom, for the act of wetting implies at the outset penetration of the surface.