Page:Various Forces of Matter.djvu/56

44 There it is—the same substance spread upon paper, and there too is the same substance; and here is some more of it [exhibiting a piece of paper as large as the other sheets, but having only very little red colour on it, the greater part being yellow], a little more of it, you will say. Do not be mistaken; there is as much upon the surface of one of these pieces of paper as upon the other. What you see yellow is the same thing as the red body, only the attraction of cohesion is in a certain degree changed; for I will take this red body, and apply heat to it (you may perhaps see a little smoke arise, but that is of no consequence), and if you look at it it will first of all darken—but see, how it is becoming yellow. I have now made it all yellow, and what is more, it will remain so; but if I take any hard substance, and rub the yellow part with it, it will immediately go back again to the red condition. [Exhibiting the experiment.] There it is. You see the red is not put back, but brought back by the change in the substance. Now [warming it over the spirit lamp] here it is becoming yellow again, and that is all because its attraction of cohesion is changed. And what will you say to me