*4 Mr. Cavallo’s MagneticaJ 
Having repeated the experiment with different pieces of iron 
and of fteel, the refult was conftantly the fame, viz, whilft 
the iron or fteel remained quite red hot or white hot, the mag- 
net did not attraCt it ; but the attraction began when that degree 
of rednefs, which is clearly perceivable in the daylight, begau 
to difappear ; and it was as ftrong as ever when the iron was 
cooled a little more than when the rednefs quite difappeared in 
the dark. In regard to this limit or maximum of attraction, 
I think 1 have obferved, as well as the nature of the experi- 
ments would permit, a difference between fteel and iron, 
which is, that in the fteel the maximum of attraction follows 
the difappea ranee of the red heat fooner than in iron. 
This experiment is fubjeCt to two fources of miftake, which 
perhaps mifled Father Kircher, and which are neceffary to be 
mentioned for the lake of others who with to repeat this expe- 
riment. The firft is, that when a piece of iron of no great 
extent is red hot, or even white hot, in one place, and below 
a red heat in other parts, the magnet will frequently attraCt it, 
though the red-hot fide be prelented to it. The lecond caufe 
of miftake is, that when a fmall piece of iron or fteel, as a 
common fewing needle, is made red-hot, and is then prefented 
to the magnet, if the magnet touch it, that contaft cools it 
inftantly below the neceffary degree of heat, and of courl'e the 
attraction takes place. It is owing to this laft caufe that I 
have not yet been able to afeertain, whether the attraction be- 
tween the magnet and the iron be quite annihilated, or only 
diminifhed to a great degree, by rendering the iron red or 
white hot ; fo that I can only fay with certainty, that a mag- 
net will not attraCt a certain piece of iron red hot or white 
hot ; whereas it will attraCt another piece of iron at leaft fifty 
times bigger, if it be cold or below a red heat. 
i 
To 
