All masses of substances, whether solid, liquid, and probably gaseous also, have, acting at their surfaces, a force parallel to the surface that is the resultant of molecular activity within the body of the substance, and that tends to resist rupture of the surface. This force, in the case of a liquid, is known as ' surface tension.' It is difficult to measure it, but it has been measured for many liquid substances. For the surface of plain water in contact with air, the magnitude of this force has been determined to be 81 dynes per square centimetre." In all likelihood the figures will be modified by further experimentation, but for any purpose of the engineer this figure is quite accurate enough.

When a slightly oiled needle floats upon the surface of water, the force that causes it to float is ' surface tension.' This force is strong enough to prevent the needle from rupturing the surface ; hence, if the needle cannot rupture the surface, it lies apparently on top of the water, or ' floats.'

As to the nature of the manifestation M energy or force, which we call ' surface tension,' the textbooks will give enough to clarify and even confuse, but some attempt muoc niad here to render clear what the nature of this force may be. W. R. Ingalls says :* " A body of water, or of any liquid, has greater cohesion in its free surface, i.e., the surface exposed to the air, than elsewhere in its interior, because, whereas the particles in the interior are mutually attracted by adjacent particles in all direcxtions, those which are at the surface have no attractions from the outside from above, let us say to counteract the pull from those in the interior." The state of affairs he describes can be illustrated diagrammatically by imagining the molecules of the liqr : o be o. sufficient size to be represented on paper. In Fig. 5 each circle will represent a molecule. The molecular attraction of each molecule can be resolved into resultants for purposes of illustration, the resultants being placed as shown by arrows in the figure.* Now the molecule at A, for instance, attracts and is attracted by adjoining molecules by resultant forces that are equal, and the molecule is free to move in all directions. The molecule at B, however, having no molecule above it to attract and be attracted is in a somewhat different state from the molecule at A. The molecule at B has its upper one of the six resultant forces uncompensated by a molecule lying above it, so we can assume that this upper resultant is compensated between the molecules on any side of B. This gives the topmost row of molecules a stronger attraction for each other than the molecules adjacent to A have for one another. The attraction of the molecules within the liquid, as, for instance, those at and adjacent to A, is known as the force of cohesion. If this force of cohesion is resolved into the six resultants as above described, and its magnitude is called 6 units, then the magnitude of the molecular attraction acting on a molecule in the top layer, as at B, will be 6 units just the same, but the molecule will not be as free to move in every direction as is the molecule at A. If the stress between any two molecules in the horizontal row containing A is 2, then the stress between any two molecules in the horizontal row containing B will be 2j. Ingalls goes on to say in explanation of this : " The " effect of this is to reduce the mobility of the particles on the " surface. The surface, as it were, is stretched by [like] an elastic " skin, the effect being the same as if the surface layer exerted a " pressure on the interior. The production of this ' surface " tension ' represents energy, i.e., molecular energy which may " be measured." As we have quoted above, from the Encyclopedia Britannica, this surface tension has been determined for water to be 81 dynes per square centimetre, a force that resists penetration from within or without the liquid.