D Y I N G. 
144 
wards of 1S00 gallons, when filled with cold water from 
the ciftern, requires no more than half an hour to heat it 
■till it actually boils! By the greateft fire that could be 
made under fuch a copper, with coals, it would hardly 
be poll’ble to make it boil in lefs than an hour. It is 
eafy to perceive that the laving of time which refults 
from the adoption of this new mode of applying heat, 
will be very great; and it is likewife evident that it may 
be increafed, almoft without limitation, merely by aug¬ 
menting the diameter of the (team-tube ; care being taken 
that the boiler be fufficiently large to furnifh the quanti¬ 
ties of (team required. The having of fuel is alfo very 
confiderable; for, from the belt calculation, it amounts 
to near two-thirds of the quantity expended, when each 
copper was heated by a feparate fire. But tliefe favings 
are far from being the only advantages derived from the 
introduction of thefe improvements in the management 
of heat: there is one, of great importance indeed, which 
alone mult be fufficient to recommend the general adop¬ 
tion of them. As the heat communicated by fleam can 
never exceed the mean temperature of boiling water by 
more than a very few degrees, the fubftances expofed to 
it can never be injured by it, as is too often the cafe in 
manufactories where coal only is ufed. 
On the PERMANENCY of DYED COLOURS. 
No objeCt in the art of dying is perhaps of fo much im¬ 
portance as the permanence or duration of colour; for of what 
avail are the moft beautiful tints, whether communicated 
to wool, filk, cotton, or linen, or ever fo elegantly diver- 
fified in figured calicoes, if thofe tints are fugitive, and 
liable to fade or vanilh by mere expofure to light and air ? 
It is therefore highly expedient to inveftigate the nature 
and quality of all colouring matters, and of every mate¬ 
rial employed in the operation of dying, before they are 
committed to the vat. Thefe colouring matters are com¬ 
monly called dyc-Jhjfs, are extracted from animal, vege¬ 
table, and mineral, fubftances, and are found to poll'efs 
very peculiar chemical properties; for befides their fe- 
veral affinities with particular rays of light, they have 
others which render them fufceptible of being aCted 
upon, and modified by a variety of chemical agents. But 
in refpeCt of thefe affinities, colouring matters alfo difter 
eftentially from each other, and mu ft therefore be applied 
in different ways, and with very different means, to pro¬ 
duce permanent colours on all occafions. The art of dy¬ 
ing is founded upon a knowledge of the particular pro¬ 
perties and affinities of thefe matters, not only as far as 
, they relate to the fubftances intended to be dyed, but 
alfo as far as they are connected with the operations of 
other agents, by which they are liable to be aCted upon 
either during the procefs of dying, or afterwards. Many 
fpecies of animal and vegetable colouring matters fuffer 
nearly (imilar changes from the adtion of acids, alkalies, 
and other chemical agents ; from which it may be pre¬ 
fumed, that there is fomething of a common or (imilar 
nature in the conftitution of many of them. But though 
it would be highly ufeful to eftablifti general principles 
and concluftons on this fubjeCt, we are not yet furnilhed 
with the neceffary information requifite for fuch a purpofe. 
Sir Il'aac Newton fuppofed coloured matters to reflect 
the rays of light; fome bodies reflecting the more, others 
the lefs, refrangible rays moft copioufly ; and this he con¬ 
ceived to be the true, and the only reafon of their co¬ 
lours. See the article Colour, vol. iv. p. 789. Mr. 
Delaval, however, has maintained, in the fecond volume 
ef the Memoirs of the Philofophical and Literary So¬ 
ciety of Manchefter, <£ that, in tranfparent coloured fub¬ 
ftances, the colouring matter does not refleCt any light; 
and that when, by intercepting the light which was tranf- 
mitted, it is hindered from pafljng through fubftances, 
they do not vary from their former colour to any other 
colour, but become entirely blackand he inftances a 
confiderable number of coloured liquors, none of them 
•endued with reflective powers, which, whenfeen by tranf- 
mitted light, appeared feverally in their true colours y 
but all of them, when feen by incident light, appeared' 
black: which is alfo the cafe of black cherries, black 
currants, black berries, &c. the juices of which appear 
red when fpread on a white ground, or otherwife viewed 
by tranfmitted, inftead of incident, light ; and he con¬ 
cludes, that bleached linen, &c. “when dyed or painted 
with vegetable colours, do not differ in their manner of 
affting on the rays of light, from natural vegetable bo¬ 
dies ; both yielding their colours by tranfmitting through 
the tranfparent coloured matter the light which is reflect¬ 
ed from the white ground it being apparent, from dif¬ 
ferent experiments, “ that no reflective power refides in 
any of their component parts, except in their white mat¬ 
ter only,” and that “ tranfparent coloured fubftances, 
placed in fituations by which the tranfmiffion cf light 
through them is intercepted, exhibit no colour, but be¬ 
come entirely black.” If the cloth be black, it is clear 
that we cannot dye it any colour whatever ; becaufe, as 
no light is in that cafe refledted, none can be tranfmit¬ 
ted, whatever dye-ftuff we employ. If the cloth were 
red, or blue, or yellow, we could not dye it any colour 
except black; becaufe, as only red, or blue, or yellow, 
rays are reflected, no other could be tranfmitted. The 
art of dying, therefore, according to Mr. Delaval, con- 
fids principally in covering white fubftances, from which 
light is ftrongly reflected, with tranfparent coloured me¬ 
dia, which, according to their feveral colours, tranlmit 
more or lefs copioufly the feveral rays reflected from the 
white fubftances, fince the tranfparent media themfelves 
refleCt no light; and it is evident that if they yielded 
their colours by reflecting, inftead of tranfmitting the 
rays, the whitenefs, or colour of the ground on which 
they are applied, would not in any wife alter or affeCt 
the colours which they exhibit. For a complete view of 
this doctrine of light and colours, fee the article Chro¬ 
matics, vol. iv. p. 528, &c. 
Such is undoubtedly the faCt, fo far as relates to tranf¬ 
parent colouring matters, when applied to wool, filk, &c. 
without the interpofition of any earthy or metallic bafts or 
mordant. But when any fuch opake bafts is interpofed, 
the reflection is doubtlefs made by it, rather than by the 
fubftance of the dyed wool, filk, &c. and more efpecially 
when the bafis or mordant confifts of the white earth of 
alum, or the white oxyd of tin; which by their ftrong 
refleCtive powers greatly augment the luftre of colours. 
There are, moreover, fome opake colouring matters, 
particularly the acetous and other folutions of iron, ufed 
to (tain linen, cotton, &c. which mu ft necefl'arily them¬ 
felves refleCt, inftead of tranfmitting the light by wdiich 
their colours are made perceptible. 
When the rays of light are feparated from each other 
by the prifm, in confequence of their different degrees of 
refrangibility, they- produce a perception of feven dif- 
tinct colours, with their intermediate (hades; which are 
all equally fimple and primitive. There is, however, this 
peculiar property belonging to the red, yellow, and blue, 
colours, whether prifmatic or permanent ; that' they are 
incapable of being produced, like all the reft, by the 
combination of any other colours. Blue and red will 
compofe a purple; blue and yellow, a green ; red and 
yellow, an orange, See. but none of tliefe, by any artifice 
or compofition, will produce either the blue, yellow, or 
red : thefe laft, therefore, are in all cafes fimple or un¬ 
compounded ; but all the others may be, and in reality 
are, fometimes fimple, and fometimes compounded ; not 
only thofe which are merely prifmatic colours, but thofe 
which exift naturally in bodies, or are communicated by 
painting, dying, &c. Iron, as has been already mention¬ 
ed, will, by different degrees of oxydation, produce all 
poflible varieties of colour ; and thefe colours will be all 
fimple or uncompounded ; and fo will the purple of gold, 
the green of copper, ahd the other colours found in the 
feveral oxyds of metals. This is alfo the cafe of the 
violet and purple dyed from logwood; of the green co- 
4 louring 
