A VOYAGE IN SPACE
pepper some iron filings all over the glass. They do not go into a pattern at once, because even the smooth glass is not quite smooth; but if we tap the glass gently so as to make the filings jump up and down, then while they are in the air they feel the pull of the magnet and get a chance to obey it. He soon arranges them in the curved lines (see Fig. 64). Now if we were to scatter among these iron filings some filings of silver or other non-magnetic substance, the silver would not feel the pull of the magnet. Mr. W. S. Gilbert made a pretty song about the "silver churn" if you remember, in his opera Patience—
"A magnet never by any endeavour
Can attract a silver churn;"
and it is the same with silver filings; if they were scattered on the glass along with the iron filings, you might tap the glass as long as you like and