93 
Suspension of the Magnetic Needle. 
improbable that this increased attraction should be owing to 
any chemical change in the filings ; and as it was so necessary to 
place them right, it appeared to me likely that whilst each 
particle of iron was emitting bubbles of inflammable air, and 
thereby increasing the perpendicular height of the filings, this 
circumstance would prove the only cause of such increased 
attraction. To verify this, I put a quantity of filings into an 
earthen vessel, which covered the bottom an inch deep ; then 
placing them so as to draw the needle two degrees from its 
meridian, I marked exactly where the vessel stood, and taking 
it away, I mixed with the filings a quantity of sand, which 
raised the whole depth of the mixture to two inches ; then re- 
placing the vessel, I found the needle drawn above four de- 
grees from its meridian. Still more to confirm this, I placed 
the filings below 7 instead of above the south pole, and repeated 
both the effervescence >and mixture of sand, and thereby caused 
as great an increase of repulsion, as of attraction by placing 
them above. Each of these experiments was repeated, by 
placing the vessel above and below the north pole also, and 
the results were accordingly varied : but when small nails were 
used instead of the filings, no increased attraction was pro- 
duced, because the nails were too heavy to be raised higher in 
the vessel, although the effervescence was very violent ; whence 
I was fully convinced that this experiment depended only on 
raising the perpendicular height of the column of iron filings. 
EXPERIMENT XV. 
To obviate the suspicion that effervescence might agitate 
the iron filings, and bring a greater number of them to that 
side of the vessel which stands contiguous to the magnetic 
