[ 'e92 7} 
L.. On changeable Colours and Glories. By THOMAS. 
Younc, M.D. F.R.S. L.S. &e.* 
Changeable Colours. 
‘ 
: Is examining some of the dust of the lycoperdon, I had 
put it with a drop of water on a glass, when I observed a 
purple tinge in the water, which [ thought at first was a 
stain extracted from the powder; but the water viewed se- 
parately was perfectly transparent, and the light transmitted 
directly through the water, when the globuies were present, 
was of a yellowish green. After some consideration, | conjec- 
tured that this appearance of colour must be analogous to that 
of the mixed plates which I had formerly observed, depending 
on the difference of refractive density of the water and the 
globules (Young’s Nat.Phil.); and by substituting fluids of 
different densities for water, I bad the pleasure of finding 
my conjecture confirmed ; for, when the water was saturated 
with salt, the yellow green became nearly blue, and the 
purple redder or browner ; and when olive oil was employed, 
the light directly transmitted was purple, and the obhque 
light greenish: in balsam of Tolu, again, this purple became 
red, and the indirect light afforded a faint blue. In air, too, 
I found that the powder appeared of a bright blue green by 
direct light, and of a purplish hue with a light a little 
oblique; but when the obliquity became a little greater, the 
tint changed to a brownish yellow green, which continued 
afterwards unchanged: this alteration may perhaps be de- 
rived from the admixture of a portion of light coming 
round the particles by a more circuitous route. By com- 
paring the opposite effects of water and olive oil, of the re- 
fractive densities 1°336 and 1*379, the refractive density of 
the particles themselves may be calculated to be 1°62, or 
somewhat less. 
“¢ Grey beaver wool seems of a purplish bue in direct, and 
greenish in oblique light, both in air and in olive oil; its 
grey colour seems to be derived from a mixture of these 
tints; 1n olive oil, the rings of colours which it affords are 
considerably altered in their appearance, the reds becoming 
every where very faint. Lead precipitated from its acetate, 
or silver from its nitrate, by common water, affords a reddish 
direct and a blueish indirect light, and the same seems to be 
true of smoke, and of other bodies consisting of very minute 
particles: but when the indirect light is very powerful, 
smoke sometimes appears reddish in it, as might be ex- 
* ¥rom “ An Introduction to Medical Literature,” 8vo. 1813. 
pected 
