Dr. Wright on the Behaviour of Mercury as an Electrode. 129 
rock-crystal cut parallel to the natural pyramidal surfaces, to 
place them one above the other in such a manner that their 
principal sections form an angle of 90° with each other, and to 
introduce them between the glass-piece and tourmaline, or be- 
tween the glass-piece and Nicol. A lens then throws these 
bands objectively upon a suspended screen or upon the thermo- 
electric pile. 
It appeared to promise interest to examine also in this field 
the quality of the thermic colours produced by interference. In 
order to obtain the latter, a thin plate of gypsum was introduced 
between two Nicol’s prisms, 85 millims. in length and 42 mil- 
lims. in diameter. The test itself was instituted by means of 
diathermanous substances, such as coloured glasses, placed suc- 
cessively before the pile. Observation proved that equal quan- 
tities of rays of heat, after passing through the polarizing appa- 
ratus and the gypsum, possess in different degrees the power of 
traversing the same diathermanous substance according as the 
principal sections of the polarizing and analysing Nicols cross 
each other at right angles or are parallel; further, that both 
these groups of rays differ from that which corresponds to an 
angle of 45° between the principal sections of the two Nicols, 
and which constitutes the transition from any thermic colour to 
its complementary one. 
XVII. Remarks on the behaviour of Mercury as an Electrode. 
By T. Srrerum1. Wricut, M.D., President of the Royal 
Physical Society, Edinburgh*. 
Cae voltaic movements of mercury have been investigated by 
Davy, Gerbour, Hellwig, Erman, Pfaff, and Runge, and 
especially by Sir John Herschel, who almost exhausted the sub- 
ject in the Bakerian Lecture for 1824. The great majority of my 
observations were similar to those made by the authors above men- 
tioned. I shall therefore not detail them to the Society tonight, 
but merely bring forward those which appear to be new. 
Experiment 1.—An ounce of mercury was poured into a shallow 
vessel containing a quantity of sulphuric acid diluted with water, 
sufficient to cover the surface of the metal. The mercury was 
then connected by a fine copper wire with one of the terminal 
wires of a galvanometer. The other wire of the galvanometer 
was armed with a small plate of amalgamated zinc ; the whole 
consequently formed a voltaic circle of zinc, mercury, and dilute 
acid. On plunging the zinc plate into the acid, the needle was 
* Extracted from a paper communicated to the Royal Society of Edin- 
burgh on the 21st of February, 1859. 
Phil. Mag. 8. 4. Vol. 19. No. 125, Feb. 1860, K, 
