Mr. Warington on the Biniodide of Mercury. 
point of its surface, it instantly 
turns scarlet at the point touched, 
and the scarlet colour is rapidly 
propagated over the whole crystal. 
“4th. The crystal moves, and is 
spontaneously agitated during the 
time it is changing colour. 
** 6th. I added that I thought 
this phenomenon the most evident 
proof we yet possessed of the de- 
pendency of colour upon internal 
2h 
of contact instantly becomes of a 
rich scarlet, and the same colour 
spreads over the whole surface of 
a single crystal, and extends to 
the most remote angle, if a group 
of crystals be the subject of ex- 
periment. 
“ This change of colour is ac- 
companied by a sensible mechani- 
cal motion, so that a small heap of 
the crystals appears asif animated, 
affording an elegant illustration 
of the connexion between colour 
and the mechanical structure of 
bodies.” ; 
molecular arrangement.” 
A comparison of the above extracts will doubtless entirely 
alter Mr. Talbot’s opinion as to these points of his claims to 
originality in regard to the observations of 1836. 
I must now turn to the printed paper in the 9th vol. (S. 3.) 
of the Philosophical Magazine, p.2. Mr. Talbot there admits 
*¢ that chemical writers have observed” these “changes of co- 
lour,” quotes Dr. Inglis’s authority for the retention of the yel- 
low tint for a considerable time, and states, that ‘ wishing to 
examine into the cause of these facts” he instituted certain ex- 
periments, which are then detailed; so, that independent of the 
above paper in 1829, of M. A. Hayes, Mr. Talbot proves in this 
very paper of 1836, that Nos. 1,2, and 3 of the points claimed 
in 1842 to have been “sufficiently established” by him in 
1836 had been previously observed. ‘The next point has re- 
ference to the first part of the claim No. 6. In 1836 Mr. 
Talbot states that “the change of colour is accompanied by 
a visible internal motion in the crystal, like a sinking or giving 
way of successive ranks of particles.” ere is not a syllable 
about lamine which he has now laid claim to, and which 
term I consider implies a very different effect from either 
‘ranks of particles” or “‘rows of molecules:” it is used in 
my paper to represent the plates of the crystal. Again, as 
to the commencement of the change of colour taking place 
** by the appearance of a red streak along one of its sides or 
edges,” and “ the boundary of the red and yellow” being “a 
straight line parallel to two sides of the rhomboid,’ and that 
“its motion is across the crystal from one of these sides to the 
opposite one ;” on these points I cannot agree with Mr. Talbot; 
and he will perceive in A, fig. 1, c, d and e, fig. 2, of my own pa- 
per, that the very reverse is shown, by drawings taken directly 
from the field of the microscope by the camera lucida; so that 
P2 
