Mr. Seymour expressed a fear that, in their present state of knowledge, they would be scarcely able to understand the explanation he should afford them. “But,” said he, “I believe you know that a ray of light is divisible into seven colours, and that when it passes through certain media, or is reflected from certain surfaces, this division is effected, and the various colours produced(32). The film of the soap bubble is one amongst the latter bodies; but I must refer you, for farther information upon this subject, to Mrs. Marcet’s beautiful account of ‘Refraction and Colours.’” “Now, Tom,” said his father, “fetch your squirt, for we have not yet finished our enquiry into the effects of the air’s pressure.” The squirt was produced; but it was out of repair: for, on attempting to fill it with water, the instrument entirely failed in the performance of its office. “I see the defect,” said Mr. Seymour, “which a little string will easily remedy.” A piece of string was instantly produced from that universal depot, the breeches pocket of a school-boy. Mr. Seymour said he should bind a portion of it around the end of the piston. “What do you mean by the _piston_?” enquired Tom. “The rod which moves up and down in the cylinder, or tube; and, unless its end fit so exactly as to prevent the admission of air, it is clear that the squirt cannot draw any water. It was for the purpose of making this part fit tightly that I wanted the string, and you will now perceive that the instrument is ready for use:--fetch me a vessel of water.” Tom soon produced the water, and, on placing it on the ground, requested that he might be allowed to fill the squirt. This he accordingly effected without difficulty, and, on pressing down the handle, he projected a stream of water to a considerable distance. “I perceive,” said Tom, “that the stream describes a curve, like my ball.” “To be sure; it is under the joint influence of the same forces, viz. that of projection and of gravity. But explain the operation of the squirt.” “As soon as I raised the piston, an empty space was left in the lower part of the cylinder, which I suppose would have remained as a _vacuum_, had not the water rushed into it.” “And why did the water rush into it?” Tom hesitated. “Was it not, think you, owing to the pressure of the atmosphere upon the surface of the water? When you raised the piston, the air above it was also raised, and ultimately driven out by the force of the ascending piston; and since the air could not find any entrance from below as long as the point was under the water, the interior of the squirt would necessarily have remained quite empty, or have been a vacuum, had it not been for the weight of the atmosphere, which, not having any counteracting pressure, drove the water into the tube, and thus filled it; and which, by pressing down the piston, you again expelled with considerable force.”