on tlie ma^netisingAr^uenceqfLhe Violet rays, ^41 
tain experiments, they underwent no such change in many 
^ther trials; and at present no reason can be assigned for this 
difference, for in both cases every other known source of mag- 
netism had been oarefully removed. The summer of 1813, in- 
deed, was not favourable to these kind of experiments, it was 
so bad.” 
About the end of April 1817, when Professor Playfair was 
at Rome, he availed himself of the opportunity which was pre- 
sented to him of witnessing this singular experiment, w hich was 
performed by Dr Carpi, in the absence of Morichini, before 
a party of English and Italian gentlemen. The violet light was 
obtained in the usual manner by means of a common prism, 
and was collected into a focus by a lens of a sufficient size. The 
needle was made of soft iron, and was found, upon trial, to pos- 
sess neither polarity nor any power of attracting iron filings. It 
was fixed horizontally upon a support by means of wax, and in 
such a direction as to cut the magnetic meridian at right angles. 
The focus of violet rays was carried slowly along the needle, pro- 
ceeding from the centre towards one of the extremities, care be- 
ing taken never to go back in the same direction, and never to 
touch the other half of the needle. At the end' of half an hour 
after the needle was exposed to the action of the violet rays, it 
was carefully examined, and it had acquired neither polarity 
nor any force of attraction ; but after continuing the operation 
twenty-five minutes longer, when it was taken off and placed on 
its pivot, it traversed with great alacrity, and settled in the di- 
rection of the magnetical meridian, with the end over which the 
rays had passed turned toward the north. It also attracted and 
suspended a fringe of iron filings. The extremity of the needle 
that was exposed to the action of the violet rays, repelled the 
north pole of a compass needle. This eflect was so distinctly 
marked, as to leave no doubt in the minds of any who were 
present, that the needle had received its magnetism from the 
action of the violet rays 
M. Dhombres Firmas*!*, who resides, we believe, at Alais in 
* An account of this experiment was drawn up at Professor Pictet’s request by 
Mr James Playfair, and published in the Bibliqtheque Univcrselle 1817, vol. vi. 
p. 81. 
f Ann, de Chim, et Phys, Mars 1819, p, 286. 
