44 COHESION. 



i 



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 attrac- 

 tion 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 experi- 

 ment.] 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 



