LECTURE VI 



THE CORRELATION OF THE PHYSICAL FORCES 



WE have frequently seen, during the course of 

 these lectures, that one of those powers or forces 

 of matter, of which I have written the names on 

 that board, has produced results which are due to the action 

 of some other force, Thus you have seen the force of 

 electricity acting in other ways than in attracting; you have 

 also seen it combine matters together or disunite them by 

 means of its action on the chemical force ; and in this case, 

 therefore, you have an instance in which these two powers 

 are related. But we have other and deeper relations than 

 these; we have not merely to see how it is that one power 

 affects another how the force of heat affects chem- 

 ical affinity, and so forth, but we must try and compre- 

 hend what relation they bear to each other, and how these 

 powers may be changed one into the other; and it will 

 to-day require all my care, and your care too, to make this 

 clear to your minds. I shall be obliged to confine myself to 

 one or two instances, because to take in the whole extent 

 of this mutual relation and conversion of forces would sur- 

 pass the human intellect. 



In the first place, then, here is a piece of fine zinc foil, 

 and if I cut it into narrow strips and apply to it the power 

 of heat, admitting the contact of air at the same time, you 

 will find that it burns; and then, seeing that it burns, you 

 will be prepared to say that there is chemical action taking 

 place. You see all I have to do is to hold the piece of zinc 

 at the side of the flame, so as to let it get heated, and yet 

 to allow the air which is flowing into the flame from all 

 sides to have access to it ; there is the piece of zinc burning 

 just like a piece of wood, only brighter. A part of the zinc 

 is going up into the air in the form of that white smoke, and 



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