28 FARADAY 



in the liquid, and have put some of it upon paper [exhibiting 

 several sheets of paper coated with scarlet biniodide of 

 mercury] ( 8 ). There it is the same substance spread upon 

 paper; and there, too, is the same substance; and here ii 

 some more of it [exhibiting a piece of paper as large a 

 the other sheets, but having only very little red color 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 sur- 

 face 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 yel- 

 low part with it, it will immediately go back again to the 

 red condition [exhibiting the experiment]. There it i&. 

 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 when I tell you that this piece of common 

 charcoal is just the same thing, only differently coalesced, 

 as the diamonds which you wear? (I have put a specimen 

 outside of a piece of straw which was charred in a par- 

 ticular way it is just like black lead.) Now this charred 

 straw, this charcoal, and these diamonds, are all of them the 

 same substance, changed but in their properties as respects 

 the force of cohesion. 



Here is a piece of glass [producing a piece of plate- 

 glass about two inches square]. (I shall want this after- 

 ward to look to and examine its internal condition), and 

 here is some of the same sort of glass differing only in its 

 power of cohesion, because while yet melted it had been 



8 Paper coated with scarlet biniodide of mercury. In order to fix the 

 biniodide on paper, it must be mixed with a little weak gum water, and 

 then spread over the paper, which must be dried without heat. 



Biniodide of mercury is said to be dimorphous; that is, is able to assume 

 two different forms. 



