CHROMATICS. 
529 
glafs, two Inches thick,, he inclofed them in black cloth 
on all tides, except their anterior and farther furfaces. In 
this fituation each of them {hewed a vivid colour when 
light was tranfmitted through them ; but when the pof- 
terior furface was likewife covered with the cloth to pre¬ 
vent the tranfmiflion, no other colour but black was ex¬ 
hibited. From tbefe phenomena he drew the following 
inferences : 1. That the colouring particles do not refleft 
any light. 2. Thar a medium, fuch as is defcribed by fir 
Ifttac Newton, is diffufed over both the anterior and pof- 
terior furfaces of the plates, whereby objects are equally 
and regularly refle&ed, as by a mirror. 
Mr.Delaval next confiders the colouring particles them- 
felves, pure and unmixed with other media. To procure 
malfes made up of fuch particles, feveral tranfparent co¬ 
loured liquors were reduced to a folid confidence by eva¬ 
poration ; by employing a gentle heat the colouring mat¬ 
ter will not be injured, and may have its particles again 
feparated by water or other fluids, and tinging them as 
before. In this ftate alfo the colouring particles reflect no 
light, and therefore appear uniformly black, whatever be 
the fubllance from which they may have been extradited. 
He endeavours to prove by experiments on the colouring 
particles of opake bodies, that thefe colours are produced 
on the above-mentioned principles ; that they leem black 
■when very denfe, but {hew their proper tinge when fpread 
thin upon a white ground. The green of grafs and leaves 
of plants being obtained by digefting them in re&ified 
fpirits of wine, and placed in one of the above-mentioned 
phials, the part in the neck tranfmitted the vivid green, 
but that contiguous to the uncovered fide of the phial 
was black. After the colour had been totally extradited, 
the leaves remained apparently unaltered as to figure or 
texture, but were entirely white, or of a white tinged with 
brown. Red, blue, and purple, flowers were alfo digefted 
with fpirits of wine, all of which yielded their colouring 
matter to the fpirit, and became white when deprived of it. 
From mod of theie flowers the fpirit, however, either ac¬ 
quired no tinge at all, or only a very faint one; but when 
acidulated it became red, and by the addition of an alkali 
became blue, purple, or green, according to the quantity 
of the. alkali, and the nature of the infufion. In theie 
Rates all of them, when viewed by tranfmitted light, or 
poured upon a white paper, (hewed their colours, but uni- 
verfally appeared black by reflection. Other experiments 
were tried with other flowers, but the final refult was the 
fame, no colour by reflection. Linen, cotton, white pa¬ 
per, See. may be tinged of any of thefe colours, by dipping 
them in the infufions ; and the confideration of the man¬ 
ner in which the colours are imparted to linen, affords 
much infight into the manner in which natural colours 
are produced. See the article Chemistry, p.343, &c. 
of this volume. 
It has been already obferved, that when the colouring 
matter of plants is extracted from them, the folid fibrous 
parts, thus diverted of their covering, difplay their natu¬ 
ral whitenefs. White linen, paper, &c. are formed of fuch 
fibrous vegetable matter, which is bleached by dilfolving 
and detaching the heterogeneous colouring particles ; 
when thefe therefore are dyed or painted with vegetable 
colours, it is evident that they do not differ in their man¬ 
ner of aCting on the rays of light from natural vegetable 
bodies; both yield their colours by tranfmittingthrough 
the tranfparent coloured matter the light which is reflec¬ 
ted from the white ground. This white matter everexiits 
without any confiderable mixture in plants while they are 
in a ftate of vegetation, as cotton, white flowers, the pith, 
wood, feeds, roots, and other parts of feveral kinds of ve¬ 
getables. When decayed leaves of trees have been long 
expofed to the atmofphere, their coloured juices are fome- 
times fo perfectly extracted, that their fibres appear white. 
Mr. Delaval has even rendered allies intenfely white, by 
carefully calcining them, and afterwards grinding with a 
fmall proportion of nitre, and expofing them to fuch a de¬ 
cree of heat as would caufe the nitre to deflagrate. The 
Vol. IV. No. 217. 
allies were then digefted with the marine acid, in order to 
diffolve the ferruginous matter diffufed through them, and 
the remainder repeatedly walked in water. Hence it would 
appear, that the earth which forms the fubftance of plants 
is white, and leparable from that fubftance which gives to 
each its peculiar colour ; that whenever it is pure and un- 
mixed, or diffufed through colourlefs media, it (hews its 
native whitenefs, and is the only vegetable matter en¬ 
dowed with a native whitenefs. This white matter may 
be dil'covered by other means befides combuftion : thus 
rofes may be whitened by expofing them to burning ful- 
phur, and the colour may be again rertored by the addi¬ 
tion of an acid either mineral or vegetable. 
Dyed fubftances have their colour deftroyed by the rays 
of the fun.—Thus dyed filk, and other fubftances of that 
kind, when expofed to the fun’s light, are deprived of 
their colour in every part on which the rays are allowed 
toaCt; whilft thofe preferve their colours which are de¬ 
fended from the light. The colours, thus impaired, may 
be reftored, if acids are employed while the injury is re¬ 
cent. All Mr. Delaval’s experiments lhew, that the co-. 
louring matter of plants does not exhibit any colour by 
reflection, but by tranfmiflion only ; that their folid earthy 
fubftance is a white matter, and that it is this part that 
has the property of reflection ; that the colours of vege¬ 
tables are produced by the light reflected from this white, 
and tranfmitted from thence through the coloured coat 
or covering which is formed on its furface by the colour¬ 
ing particles; that whenever the colouring matter is ei¬ 
ther difeharged pr divided by folution into particles too 
minute to exhibit any colour, the folid fubftance itfelf 
difplays that whitenefs which is its diftinguifhing charac- 
teriftic. Having fettled this point, he proceeded to exa¬ 
mine the coloured parts of animal fubftances, and found 
them exaCtly fimilar with regard to the manner in which 
the colour is produced, to the vegetable fubftances al¬ 
ready treated of. The tinCtures and infufions of cochineal 
andkermes yield their colours when light is tranfmitted 
through them, but (hew none by reflection. On diluting 
frefli ox-gall with water, and examining it in the above- 
mentioned phials, the part of it viewed by tranfmitted 
light was yellow ; but the anterior furface in the lower 
part of the phial was black, and reflected no colour. 
Flelh derives its colour entirely from the blood, and when? 
deprived of it the fibres and vefl'els are perfectly white; 
as are likewife the membranes, tendons, and bones, when 
freed from their aqueous and volatile parts. The florid 
red colour of the flelh arifes from the light which is reflec¬ 
ted from the white fibrous fubftance, and tranfmitted back 
through the red tranfparent covering formed by the blood 
on every part of the lurface of the body. 
In like manner the red colour of the (hells of Iobfters 
after boiling, is no more than a mere fuperficial covering 
fpread over the white calcareous earth of which the (hells 
are compofed, and may be removed from the furface by 
feraping or filing. Before the application of heat this fu¬ 
perficial colouring is much denier, infoniuch that in fome 
parts of the Ihell it appears quite black, being too thick 
to admit the paffage of the light to the (hell and back 
again ; but where this tranfparent blue colour of the un¬ 
boiled lobfter is thinner, it conftantly appears like a blue 
film. In like manner the colours of the eggs of certain 
birds are entirely fuperficial, and may be feraped off, 
leaving the white calcareous earth expofed. It is the fame 
with feathers, which owe their colours entirely to a very 
thin layer of fome tranfparent matter upon a white 
ground; this was afeertained by feraping off the fuper¬ 
ficial colours from certain feathers, which were Itrong 
enough to bear the operation, and which feparated the 
coloured layers from the white ground ori which they had 
been naturally fpread. The lateral fibres cannot have 
their colours feparated in this manner; but their texture, 
when viewed by a microfcope, feems to indicate that their 
colours are produced on them by no other means than 
thofe already related. In a word, he found that in all the 
6 T animal 
