t Mr. Cavallo’s Magnetic al 
\ 
their attraction towards the magnet became more evident; 
whereas thole pieces of brafs, which naturally had not any 
degree of magnetifm fufficient to affect the needle, nor acquired 
any by hammering, but yet fhewed fome tendency towards 
the magnet when floatin'! upon quickfiiver, never, or very 
feldom, had that tendency inereafed by hammering. 
As in the courfe of thofe experiments it naturally occurred 
to obferve leveral particulars, which may be of ufe to thofe 
perfons who wifh to repeat thele experiments, I lhall now lub- 
join the principal of them. 
It is neceflary firft of all to obferve, that the veflfel wherein 
the quickfiiver is put for the purpofe of examining the mag- 
netifm of divers bodies, mud: beat leaft fix inches in diameter; 
otherwife the fubftances that are fet to float upon the mer- 
cury, will be continually running towards the fides of the 
veflfel, on account of the curvature of the furface of that 
metal, which in narrow vefiels begins from a greater diflance 
from the edge, than in vefiels of a larger diameter. 
It is neceflary likewife to keep the quickfiiver very clean, 
and alfo very pure ; the want of which precautions will render 
the event of the experiments precarious. I have obferve<J a 
very remarkable phenomenon, with refpedt to the furface of 
the mercury that is ufed for this purpofe. It is, that though 
fubftances will float upon it with wonderful nimblenefs, when 
the mercury is firft poured out of the bottle into the open veflfel, 
yet a (hort time after, viz. after having remained for an hour 
or two, and fometimes for a Ihorter time, expofed to the atmo- 
fphere, a piece of brafs or other fubftance will by no means 
float upon it with equal facility ; fo that fome pieces of brafs, 
or grains of platina, which, after firft pouring the quickfiiver 
into the open veflel, were evidently attra&ed by the magnet, 
about 
