Mr. Warington on the Biniodide of Mercury. 209 
the dampness of the weather, a singular difference occurred 
between two needles, one of which was of brass and the other 
magnetic, and both of them five-eighths of an inch in length. 
The brass one invariably placed itself at right angles, while 
the magnet remained uninfluenced, though it was the more 
delicate of the two, as it had an agate socket. This fact, as 
well as what I have just stated of the other magnetic needle, 
showed that there was a struggle between the polarity of the 
needle and the influence of the electric current. It was ob- 
served that whenever there was a powerful stream of electri- 
city employed, it was seen to escape in a beautiful pencil of 
light from the point of the other end of thewire which was kept 
coiled up and unemployed. The machine used in these ex- 
periments is of the plate construction and eighteen inches in 
diameter, but is at present in very indifferent working order. 
- Becquerel mentions in his History of Electricity that 
he thought he had made needles of various substances move 
in a galvanometer when under the influence of a voltaic cur- 
rent, but that he afterwards found that the slight movements 
he had observed were owing to currents of air occasioned by 
the heat evolved. The present experiments, however, are so 
decisive and unequivocal that they cannot be attributed to 
such a cause; still my own convictions are, that they are not 
dependent upon magnetic influence. 
I am, Sir, &c., 
January 31, 1843.- Graves C. Haueuton. 
XXXV. On the Biniodide of Mercury. By Roperr 
Wanineton, Esq., Secretary to the Chemical Society. &c. 
To the Editors of the Philosophical Magazine and Journal. 
GENTLEMEN, 
I SHALL feel obliged by your inserting in an early Num- 
ber of your Journal the following observations in reply 
to a letter on this subject by Mr. Fox Talbot, which ap- 
peared in the Number of the Philosophical Magazine for 
November 1842 (page 336); at the same time I cannot help 
regretting that your pages should be occupied by a matter of 
personal discussion ; but as it would appear, from the contents 
of the letter in question, that I had, either ignorantly or wil- 
fully, made use of the published observations of another, and 
as Mr. Talbot pointedly “ begs to draw my attention” to the 
question of his priority to the ** discovery of one of the most 
curious phenomena in optics,” it obliges me to adopt the 
same public channel of reply for the purpose of clearing my- 
Phil. Mag. S. 3. Vol. 22. No. 144. March 1843. P 
