20 
TOMLINSON ON OBSERVING ACCIDENTAL COLOURS. 
the lead to the platinum, po intense as to 
decompose a solution of the iodide of po- 
tassium, when acted upon in the manner 
described at the beginning of the paper, 'bhis 
likewise appears to have been an instance 
of the effect of oxidation. 
An important point to determine is the 
state of the metals and the conductor in a 
simple circuit, before, and at the instant 
when the metallic contact is completed. Dr. 
Paraday conceives it impossible to resist the 
idea that the voltaic current which we have 
seen is dependent upon oxidation, must be 
preceded by a state of tension in the fluid, and 
between the fluid and the zinc, the first con- 
sequence of the affinity of the zinc for the 
oxygen of the water. He endeavoured to 
investigate this by transmitting a ray of po- 
larized light through a solution of sulphate 
of soda across the course of the electric cur- 
rent, and examined it by an analyzing plate, 
but though it penetrated seven inches, not 
the slightest trace of action on the ray could 
be detected, nor was the effect different when 
nitrate of lead was substituted. A beautiful 
experiment proves a state of tension acquired 
by the metals and the electrolyte before the 
electric current is produced, and before the 
metals are brought in contact. He took a 
voltaic apparatus consisting of a single pair 
of large plates, namely, a cylinder of amal- 
gamated zinc and a double cylinder of cop- 
per, and placed them in ajar containing di- 
lute sulphuric acid, so that they could at 
pleasure be placed in metallic communica- 
tion by means of a copper wire, arranged so 
as to deposit the ends into two vessels of 
mercury connected with the two plates. As 
long as the plates were kept separate no 
action occured ; but when connected, a 
spark (contrary to the common idea) was 
elicited, and the solution decomposed. Hence, 
it appears that as the electricity is produced 
by the material action of the zinc and water, 
so these by being brought in contact are 
placed in a state of powerful tension, which, 
although it did notdecomposethe water, caused 
a spark to pass between the zinc and a fit dis- 
charger when the interval was small enough. 
The idea which Berzelius has broached that 
the heat and light of combustion are the con- 
sequences of the action of chemical affinity, 
without the production of an electric current, 
appears to the author to be a mere imagina- 
tion. 
With regard to the direction of the move- 
ment of evolved and combining bodies, it ap- 
pears that if in a voltaic circuit, the activity 
of which is determined by the attraction of 
zinc for the oxygen of water, the zinc move 
from right to left, then any other action in- 
cluded in the circuit being part of an elec- 
trolyte wdll also move in the same direction, 
and as the oxygen of the water by its natu- 
ral affinity for the zinc, moves from left to 
right, so any other body of the same class 
with it, i. e. any union will follow the same 
course. 
These statements of our author correspond 
with hegeneral views of Davy in his Bake- 
rian lecture. 
(To be continued.) 
ON THE ACCIDENTAL COLOURS 
OF CERTAIN SOLUTIONS ON 
MERCURY. 
By Charles Tomlinson, Esq. 
To the Editor of the Records of General 
Science. 
Dear Sir, — In the course of my exj eri- 
mentson Visible Vibration, 1 noticed a ready 
and convenient method of observing acci- 
dental colours without fatiguing the eye, wdiich 
was new to me, and vAill, 1 hope, prove inter- 
esting to some of the readers of your Journal 
Having occasion to diminish somewhat 
the reflecting surface of mercury contained 
in a foot glass, I poured al out an ounce of a 
solution of litmus, which had become slight- 
ly reddened by exposure to the air, upon the 
surface of the mercury, when the upper por- 
tion of the glass above the fluid was reflected 
twice, the low'er reflection by the mercury 
and the upper one by the litmus solution. 
On i)lacingthe finger on the periphery ofttie 
glass, and bringing one eye near to another 
part of the periphery, two reflections of the 
finger were seen ; one the colour of the lit- 
mus, a beautiful purple inclining to red, and 
the other a delicate light green, its acciden- 
tal colour. 
On adding a few drops of nitric acid to 
the litmus solution, the accidental colour was 
of a dark and decided green. 
With mercury and a solution of chromate 
of potash a fine blue accidental colour was 
obtained. 
With muriate of lime the same result was 
obtained with this addition : on looking 
steadfastly into the glass with one eye, the 
other being closed, a variety of white spots 
began to form on the iris, giving the eye an 
unpleasant mouldy sort of appearance. The 
aqueous humour seemed to consist of one 
isolated drop of water, so distinct from any 
other part of the eye, that it seemed as if it 
would have dropped down into the glass ; in 
a short time the transparent membrane co- 
vering the pupil became milky, and the glass 
and fluids indistinct. 1 have repeated this 
experiment with the same results, except 
that the white spots on the iris were not so 
numerous. 
With a deep blue solution on mercury ob- 
tained by indigo in sulphuric acid, the acci- 
dental orange-yellow was obtained. 
These accidental colours are neither modi- 
fied nor changed by the reflection of various 
coloured solids, such as blue, yellow and 
green balls, &c., the accidental colour be- 
longing to the upper fluid and not to the 
object reflected. Jn order to obtain them, 
however, two liquids of different densities 
must be employed in order to obtain two re- 
flections, and for- the lower fluid nothing is 
so convenient as mercury. Indeed, 1 have 
not as yet met with any other fluid that at 
all answers the purpose. 
The effect is very beautiful with litmus so- 
lution and mercury when the flame of a 
candle is employed ; the two reflections have 
the appearence of hollow cones placed above 
