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Rh —they all fall to the earth—for the force of gravitation never fails. Now, I have here a centre of power which I will not name at present, and when these particles are placed upon it, see what an attraction they have for each other.

Here I have an arch of iron filings (fig. 12)

regularly built up like an iron bridge, because I have put them within a sphere of action which will cause them to attract each other. See!—I could let a mouse run through it, and yet if I try to do the same thing with them here [on the table] they do not attract each other at all. It is that [the magnet] which makes them hold together. Now, just as these iron particles hold D 3