If you remember that this movement only occurs with strong wine, and that it must have been known to every one at the time that these words were written, and used as a test of the strength of wine, because in those days every one drank wine, then you will agree that this explanation of the meaning of that verse is the right one. I would ask you also to consider whether it is not probable that other passages which do not now seem to convey to us any meaning whatever, may not in the same way have referred to the common knowledge and customs of the day, of which at the present time we happen to be ignorant. [Illustration: Fig. 14.] Ether, in the same way, has a skin which is weaker than the skin of water. The very smallest quantity of ether on the surface of water will produce a perceptible effect. For instance, the wire frame which I left some time ago is still resting against the water-skin. The buoyancy of the glass bulb is trying to push it through, but the upward force is just not sufficient. I will however pour a few drops of ether into a glass, and simply pour the vapour upon the surface of the water (not a drop of _liquid_ is passing over), and almost immediately sufficient ether has condensed upon the water to reduce the strength of the skin to such an extent that the frame jumps up out of the water. There is a well-known case in which the difference between the strength of the skins of two liquids may be either a source of vexation or, if we know how to make use of it, an advantage. If you spill grease on your coat you can take it out very well with benzine. Now if you apply benzine to the grease, and then apply fresh benzine to that already there, you have this result--there is then greasy benzine on the coat to which you apply fresh benzine. It so happens that greasy benzine has a stronger skin than pure benzine. The greasy benzine therefore plays at tug-of-war with pure benzine, and being stronger wins and runs away in all directions, and the more you apply benzine the more the greasy benzine runs away carrying the grease with it. But if you follow the directions on the bottle, and first make a ring of clean benzine round the grease-spot, and then apply benzine to the grease, you then have the greasy benzine running away from the pure benzine ring and heaping itself together in the middle, and escaping into the fresh rag that you apply, so that the grease is all of it removed. There is a difference again between hot and cold grease, as you may see, when you get home, if you watch a common candle burning. Close to the flame the grease is hotter than it is near the outside. It has therefore a weaker skin, and so a perpetual circulation is kept up, and the grease runs out on the surface and back again below, carrying little specks of dust which make this movement visible, and making the candle burn regularly.