It might, however, be asserted that this combined experiment exhibits no more than each single experiment; for it appears the colours excited by pressure disappear in proportion as the glasses are less in contact, and the moisture then evaporates with its own colours.
Fourth condition.—Iridescent appearances are observable in almost all bubbles; soap-bubbles are the most commonly known, and the effect in question is thus exhibited in the easiest mode; but it may be observed in wine, beer, in pure spirit, and again, especially, in the froth of chocolate.
As in the above cases we required an infinitely narrow space between two surfaces which are in contact, so we can consider the pellicle of the soap-bubble as an infinitely thin lamina between two elastic bodies; for the appearance in fact takes place between the air within, which distends the bubble, and the atmospheric air.
The bubble when first produced is colourless; then coloured stripes, like those in marble paper, begin to appear: these at length spread over the whole surface, or rather are driven round it as it is distended.
In a single bubble, suffered to hang from the straw or tube, the appearance of colour is difficult to observe, for the quick rotation prevents any accurate observation, and all the colours seem to mix together; yet we can perceive that the colours begin at the orifice of the tube. The solution itself may, however, be blown into carefully, so that only one bubble shall appear. This remains white (colourless) if not much agitated; but if the solution is not too watery, circles appear round the perpendicular axis of the bubble; these being near each other, are commonly composed alternately of green and red. Lastly, several bubbles may be produced together by the same means; in this case the colours appear on the sides where two bubbles have pressed each other flat.