53 o C H R O M 
aiiimal fubjects lie examined, the colours were produced 
by the traiifmifliori of light from a white ground through 
a tranfparent coloured medium. 
The mineral kingdom abounds with coloured fub¬ 
ftances, belonging principally to two dalles, earths and 
nletais. The former, when pure, are all white, and their 
colour arifes flom metallic mixtures. Calcareous earths, 
when indurated, cohftitute marble, and may be tinged 
with various colours by means of metallic folutions, all of 
which are fimilar in their nature to the dyes put upon iilk, 
cotton, or linen, and invariably proceed from the fame 
catife, the tranfmiftion of light through a very thin trarif- 
par'ent medium. Flints are formed from filiceous earths, : 
and owe their colour to the ftate of fire within them ; 
when fufficiently heated, they are rendered white by the 
lofs of the inflammable matter which produced their co¬ 
lour; when impregnated With metals, they form agates, 
cornelians; jafper, and coloured cryftals. The coloured 
genis alfO receive their different hues from metals, and 
may be imitated by glafles tinged with fuch inflammable 
or metallic matter as entered into the original fubftances, 
all exhibiting their various tints in the fame mariner; by 
the tranfiriifiion of light from a reflected white grorind. 
Mr. Delaval obferves, that even the colours of metals 
a're prod'uced in the fame manner. Gold exhibits a white 
light tiriged with yellow; this is grounded oft ail experi¬ 
ment of fir Ifaac Newtori, who lays, that gold in a 'Advice 
light appeaVs of the Fame colour as in the day-light, but 
that on intercepting a due quantity of the yellOw-riiaking 
rays, it Will appear white like filver, which (hews that its 
yell'oWri'efs arifes from an excels of the intercepted rays, 
tinging that whitenefs, with their colo'rir when they are 
let pafs through. A fblution of filver is pellucid and co- 
joufleifs; a folu'tion of gold tranlinits yellow, bu't reflects 
no colour. This metal, when united to glafs, yields no 
colour by reflection, but only by tranfmiftion. All thefe 
circumitaiices feem to indicate, that the yellow colour of 
gold arifes from a jmllow tranfparent matter, which is a 
-conflituerit part of that metal, and that is equally mixed 
with the white particles of the gold, and tranlinits the 
light refloated by them; in like manner as when filver is 
gilt, or foils are made by covering white metals with 
tranfparent colours. But thefe factitious coverings are 
only fnpei ficikl, whereas the yellow matter of gold is dif- 
fufed throughout the whole i'ubftance of the metal, and 
appears to envelop and cover each of the white particles ; 
the yellow matter bears to the white about the fame pro¬ 
portion that the yellow-making rays, which were inter¬ 
cepted, bear to all the other rays coiuprifed in the white 
light of the fun. 
it has been (hewn by fir Ifaac Newton, that when the 
fpaces or interftices of bodies are replenifhed with media 
of different denfities, the bodies are opake ; that thole fu- 
perficies of tranfparent bodies r theft the gee at eft quantity 
of light, which intercede media that differ molt in their 
refractive denfities; and that the reflections of thin 'tranf- 
parerit fubftances are confiderably ftronger than thofe 
made by the fame fubftances of a greater thicknefs. Herice 
the minute portion of air, or of the rarer medium, which 
occupies the pores or interftices of denfe bodies, is a mi- 
inite White fubftance. This is mini fell in the Adiitenefs 
of froth, and of all pellucid colourlefs fubftances, fuch as 
glafs, cryftal, or lalts reduced to powder, or otherwife 
flawed ; for in all thefe inftances a white light is reflected 
from the air or rarer medium, which intercede the parti¬ 
cles of the denfer fubftance, whofe interltices they poflefs. 
Hence alio we fee why white opake fubftances are rendered 
pellucid by being reduced to uniform mafles, whofe com¬ 
ponent parts are every where nearly of the fame denfity; 
for as all pellucid fubftances are rendered opake and white 
by the admixture of pellucid colourlefs media, of confi¬ 
derably different denfities, they are again deprived of 
their'opacity, by extracting thefe media, which keep their 
particles at a diftance from each other: thus froth and 
:fno\V, When relolved into water, lofe their whitenels, and 
a 
A T I C S. 
and affume their former pellucid appearance. In like 
manner the opake white earths are by proper fluxes re¬ 
duced to pellucid colourlefs glafs; becaufe all reflections 
are made at the furfaces of bodies differing in denfity 
from the ambient medium, and in the confines of equally 
denfe media there is no reflection. 
As the calces of metals are capable of reflecting their 
colours by the intervention of air, fo, when mixed with 
oil in making paints, the)' always affume a darker colour, 
becaufe the excefs of the denfity of oil over air forms a 
fenfible difference, when comparatively confidered with 
refpeCt to the fpecinc gravity of the rarer metals. From 
this caufe perceptibly lei’s light is reflected from the moie- 
culte of oil than thofe of air, and confequently the mafs 
appears darker. The cafe is however different with fuch 
paints as are formed of the denfer metals, as vermilion, 
minium, &c. for though oil differs very confiderably from 
ah- in its fpecific denfity, yet it alfo differs very much in 
this refpect from the denfer metallic powders; and the 
moleculse of oil, which divide their particles, aCt upon the 
light fo ftrongly, that the reflection of light occafioned 
by them cannot be diftinguifhed from thofe which are 
Caufed by rarer media. Hence, when we mix vermilion 
Or minium with oil, the colour is not fenfibly changed. 
All thofe earths, which in their natural ftate are of a 
pure White, coriftittite tranfparent colourlefs media when 
vitrified with proper fluxes, or when diffolved in colour¬ 
lefs menftrua ; and the Valine mafles, obtainable from 
thei^' folutions, are tranfparent and colourlefs, while they 
retain the water which is hecefiary to their crystalliza¬ 
tion, and are not flawed or reduced to powder; but after 
their pores and interftices are opened in fuch a manner as 
to admit the air, they become white and opake by the 
admittance of that rare medium. The earthy.particles, 
which form the folid parts of bodies, generally exceed 
each other in denfity ; confequently thefe particles, when 
contiguous to the rare media already mentioned, mu ft re¬ 
flect the rays of light with a force proportionate to their 
denfity. The refleClive power of bodies does not depend 
merely upon their excefs of denfity, but upon their dif¬ 
ference of denfity with refpeft to the furrounding media. 
Tranfparent colourlefs particles, whofe denfity is greatjy 
inferior to that of the media they come between, alfo 
powerfully reflect all forts of rays, and thereby become 
white ; of this kind are the air, or other rare fluids, which 
occupy the interftices of liquors, and in general of all 
denier media, where fuch rare particles find accefs. Hence 
we may conclude, that white opake bodies are conftitu- 
ted by the union or contiguity of two or more traniba- 
rent colOuilels media, differing confiderably from each 
other in'their refleClive powers. Of thele fubftances we 
have examples in frothy eirtulfions, or other imperfect 
combinations of pellucid liquors, as milk, fnow, calcined 
or pulverized fal'ts, glafs or cryftal reduced to powder, 
white earths, paper, linen, and even thofe metals which 
are called White by mineralogifts; for thofe metals do not 
appear white unlefs their furfaces be rough; as in that 
cafe only there are interftices on their furface iufficient to 
admit the air, and thus make a reflection of a white and 
vivid light. The poiilhed furfaces of metallic mirrors re¬ 
flect the incident rays equally and regularly according to 
their leverai angles of incidence, fo that the reflected rays 
do not interfere with each other, but remain feparateand 
unmixed, and therefore diltihftly exhibit their feveral co¬ 
lours. Hence it is Evident, that A'hite furfaces cannot 
aft upon the light as mirrors, becaufe'all the rays which 
are reflected from them are blended in a difoVderly and 
promifeuous manner. 
The foregoing phenomena give us fome infight into the 
nature and caule of opacity, as they clearly fhew, that 
even the rareft tranfparent colourlefs fubftances, when 
their furfaces are adjacent to media differing greatly from 
them in refraftive power, maytherelyy acquire a perfect 
opacity, and may aifume a hue and rdplendence fimilar 
to that of white metals; that the rarer pellucid fubftances 
cannot 
