on the brain, by various surfaces or media when light falls upon them or
passes through them. Remove the light, and colour ceases to exist. The
colour of a substance does not depend so much on the chemical character
of that substance, but rather and more directly upon the physical
condition of the surface or medium upon which the light falls or through
which it passes. I can illustrate this easily. For example, there is a
bright-red paint known as Crooke's heat-indicating paint. If a piece of
iron coated with this paint be heated to about 150° F., the paint at
once turns chocolate brown, but it is the same chemical substance, for
on cooling we get the colour back again, and this can be repeated any
number of times. Thus we see that it is the peculiar physical structure
of bodies which appear coloured that has a certain effect upon the
light, and hence it must be from the light itself that colour really
emanates. Originally all colour proceeds from the source of light,
though it seems to come to the eye from the apparently coloured objects.
But without some elucidation this statement would appear as an enigma,
since it might be urged that the light of the sun as well as that of
artificial light is white, and not coloured. I hope, however, to show
you that that light is white, because it is so much coloured, so
variously and evenly coloured, though I admit the term "coloured" here
is used in a special sense. White light contains and is made up of all
the differently coloured rainbow rays, which are continually vibrating,
and whose wave-lengths and number of vibrations distinguish them from
each other. We will take some white light from an electric lantern and
throw it on a screen. In a prism of glass we have a simple instrument
for unravelling those rays, and instead of letting them all fall on the
same spot and illumine it with a white light, it causes them to fall
side by side; in fact they all fall apart, and the prism has actually
analysed that light. We get now a coloured band, similar to that of the
rainbow, and this band is called the spectrum (see Fig. 16). If we could
now run all these coloured rays together again, we should simply
reproduce white light. We can do this by catching the coloured band in
another prism, when the light now emerging will be found to be white.
Every part of that spectrum consists of homogeneous light, _i.e._ light
that cannot be further split up. The way in which the white light is so
unravelled by the prism is this: As the light passes through the prism
its different component coloured rays are variously deflected from their
normal course, so that on emerging we have each of these coloured rays
travelling in its own direction, vibrating in its own plane. It is well
to remember that the bending off, or deflection, or refraction, is
towards the thick end of the prism always, and that those of the
coloured rays in that analysed band, the spectrum, most bent away from
the original line of direction of the white light striking the prism,
are said to be the most refrangible rays, and consequently are situated
in the most refrangible end or part of the spectrum, namely, that
farthest from the original direction of the incident white light. These
most refrangible rays are the violet, and we pass on to the least
refrangible end, the red, through bluish-violet, blue, bluish-green,
green, greenish-yellow, yellow, and orange. If you placed a prism say in
the red part of the spectrum, and caught some of those red rays and
allowed them to pass through your prism, and then either looked at the
emerging light or let it fall on a white surface, you would find only
red light would come through, only red rays. That light has been once
analysed, and it cannot be further broken up. There is great diversity
of shades, but only a limited number of primary impressions. Of these
primary impressions there are only four--red, yellow, green, and blue,
together with white and black. White is a collective effect, whilst
black is the antithesis of white and the very negation of colour. The
first four are called primary colours, for no human eye ever detected in
them two different colours, while all of the other colours contain two
or more primary colours. If we mix the following tints of the spectrum,
_i.e._ the following rays of coloured light, we shall produce white
light, red and greenish-yellow, orange and Prussian blue, yellow and
indigo blue, greenish-yellow and violet. All those pairs of colours that
unite to produce white are termed complementary colours. That is, one is
complementary to the other. Thus if in white light you suppress any one
coloured strip of rays, which, mingled uniformly with all the rest of
the spectral rays, produces the white light, then that light no longer
remains white, but is tinged with some particular tint. Whatever colour
is thus suppressed, a particular other tint then pervades the residual
light, and tinges it. That tint which thus makes its appearance is the
one which, with the colour that was suppressed, gave white light, and
the one is complementary to the other. Thus white can always be
compounded of two tints, and these two tints are complementary colours.
But it is important to remark here that I am now speaking of rays of
coloured light proceeding to and striking the eye; for a question like
this might be asked: "You say that blue and yellow are complementary
colours, and together they produce white, but if we mix a yellow and a
blue paint or dye we have as the result a green colour. How is this?"
The cases are entirely different, as I shall proceed to show. In
speaking of the first, the complementary colours, we speak of pure
spectral colours, coloured rays of light; in the latter, of pigment or
dye colours. As we shall see, in the first, we have an addition direct
of coloured lights producing white; in the latter, the green colour,
appearing as the result of the mixture of the blue and yellow pigments,
is obtained by the subtraction of colours; it is due to the absorption,
by the blue and yellow pigments, of all the spectrum, practically,
except the green portion. In the case of coloured objects, we are then
confronted with the fact that these objects appear coloured because of
an absorption by the colouring matter of every part of the rays of light
falling thereupon, except that of the colour of the object, which colour
is thrown off or reflected. This will appear clearer as we proceed. Now
let me point out a further fact and indicate another step which will
show you the value of such knowledge as this if properly applied. I said
that if we selected from the coloured light spectrum, separated from
white light by a prism, say, the orange portion, and boring a hole in
our screen, if we caught that orange light in another prism, it would
emerge as orange light, and suffer no further analysis. It cannot be
resolved into red and yellow, as some might have supposed, it is
monochromatic light, _i.e._ light purely of one colour. But when a
mixture of red and yellow light, which means, of course, a mixture of
rays of greater and less refrangibility respectively than our spectral
orange, the monochromatic orange--is allowed to strike the eye, then we
have again the impression of orange. How are we to distinguish a pure
and monochromatic orange colour from a colour produced by a mixture of
red and yellow? In short, how are we to distinguish whether colours are
homogeneous or mixed? The answer is, that this can only be done by the
prism, apart from chemical analysis or testing of the substances.
[Illustration: FIG. 16.]
The spectroscope is a convenient prism-arrangement, such that the
analytical effect produced by that prism is looked at through a
telescope, and the light that falls on the prism is carefully preserved
from other light by passing it along a tube after only admitting a small
quantity through a regulated slit.
Now all solid and liquid bodies when raised to a white heat give a
continuous spectrum, one like the prismatic band already described, and
one not interrupted by any dark lines or bands. The rays emitted from
the white-hot substance of the sun have to pass, before reaching our
earth, through the sun's atmosphere, and since the light emitted from
any incandescent body is absorbed on passing through the vapour of that
substance, and since the sun is surrounded by such an atmosphere of the
vapours of various metals and substances, hence we have, on examining
the sun's spectrum, instead of coloured bands or lines only, many dark
ones amongst them, which are called Fraunhofer's lines. Ordinary
incandescent vapours from highly heated substances give discontinuous
spectra, _i.e._ spectra in which the rays of coloured light are quite
limited, and they appear in the spectroscope only as lines of the
breadth of the slit. These are called line-spectra, and every chemical
element possesses in the incandescent gaseous state its own
characteristic lines of certain colour and certain refrangibility, by
means of which that element can be recognised. To observe this you place
a Bunsen burner opposite the slit of the spectroscope, and introduce
into its colourless flame on the end of a platinum wire a little of a
volatile salt of the metal or element to be examined. The flame of the
lamp itself is often coloured with a distinctiveness that is sufficient
for a judgment to be made with the aid of the naked eye alone, as to the
metal or element present. Thus soda and its salts give a yellow flame,
which is absolutely yellow or monochromatic, and if you look through
your prism or spectroscope at it, you do not see a coloured rainbow band
or spectrum, as with daylight or gaslight, but only one yellow double
line, just where the yellow would have been if the whole spectrum had
been represented. I think it is now plain that for the sake of
observations and exact discrimination, it is necessary to map out our
spectrum, and accordingly, in one of the tubes, the third, the
spectroscope is provided with a graduated scale, so adjusted that when
we look at the spectrum we also see the graduations of the scale, and so
our spectrum is mapped; the lines marked out and named with the large
and small letters of the alphabet, are certain of the prominent
Fraunhofer's lines (see A, B, C, a, d, etc., Fig. 16). We speak, for
example, of the soda yellow-line as coinciding with D of the spectrum.
These, then, are spectra produced by luminous bodies.
The colouring matters and dyes, their solutions, and the substances dyed
with them, are not, of course, luminous, but they do convert white light
which strikes upon or traverses them into coloured light, and that is
why they, in fact, appear either as coloured substances or solutions.
The explanation of the coloured appearance is that the coloured
substances or solutions have the power to absorb from the white light
that strikes or traverses them, all the rays of the spectrum but those
which are of the colour of the substance or solution in question, these
latter being thrown off or reflected, and so striking the eye of the
observer. Take a solution of Magenta, for example, and place a light
behind it. All the rays of that white light are absorbed except the red
ones, which pass through and are seen. Thus the liquid appears red. If a
dyed piece be taken, the light strikes it, and if a pure red, from that
light all the rays but red are absorbed, and so red light alone is
reflected from its surface. But this is not all with a dyed fabric, for
here the light is not simply reflected light; part of it has traversed
the upper layers of that coloured body, and is then reflected from the
interior, losing a portion of its coloured rays by absorption. This
reflected coloured light is always mixed with a certain amount of white
light reflected from the actual surface of the body before penetrating
its uppermost layer. Thus, if dyed fabrics are examined by the
spectroscope, the same appearances are generally observed as with the
solution of the corresponding colouring matters. An absorption spectrum
is in each case obtained, but the one from the solution is the purer,
for it does not contain the mixed white light reflected from the
surfaces of coloured objects. Let us now take an example. We will take a
cylinder glass full of picric acid in water, and of a yellow colour. Now
when I pass white light through that solution and examine the emerging
light, which looks, to my naked eye, yellow, I find by the spectroscope
that what has taken place is this: the blue part of the spectrum is
totally extinguished as far as G and 2/3 of F. That is all. Then why,
say you, does that liquid look yellow if all the rest of those rays pass
through and enter the eye, namely, the blue-green with a trifle of blue,
the green, yellow, orange, and red? The reason is this: we have already
seen that the colours complementary to, and so producing white light
with red, are green and greenish-blue or bluish-green. Hence these
cancel, so to say, and we only see yellow. We do not see a pure yellow,
then, in picric acid, but yellow with a considerable amount of white.
Here is a piece of scarlet paper. Why does it appear scarlet? Because
from the white light falling upon it, it practically absorbs all the
rays of the spectrum except the red and orange ones, and these it
reflects. If this be so, then, and we take our spectrum band of
perfectly pure colours and pass our strip of scarlet paper along that
variously coloured band of light, we shall be able to test the truth of
several statements I have made as to the nature of colour. I have said
colour is only an impression, and not a reality; and that it does not
exist apart from light. Now, I can show you more, namely, that the
colour of the so-called coloured object is entirely dependent on the
existence in the light of the special coloured rays which it radiates,
and that this scarlet paper depends on the red light of the spectrum for
the existence of its redness. On passing the piece of scarlet paper
along the coloured band of light, it appears red only when in the red
portion of the spectrum, whilst in the other portions, though it is
illumined, yet it has no colour, in fact it looks black. Hence what I
have said is true, and, moreover, that red paper looks red because, as
you see, it absorbs and extinguishes all the rays of the spectrum but
the red ones, and these it radiates. A bright green strip of paper
placed in the red has no colour, and looks black, but transferred to the
pure green portion it radiates that at once, does not absorb it as it
did the red, and so the green shines out finely. I have told you that
sodium salts give to a colourless flame a fine monochromatic or pure
yellow colour. Now, if this be so, and if all the light available in
this world were of such a character, then such a colour as blue would be
unknown. We will now ask ourselves another question, "We have a new blue
colouring matter, and we desire to know if we may expect it to be one of
the greatest possible brilliancy, what spectroscopic conditions ought it
to fulfil?" On examining a solution of it, or rather the light passing
through a solution of it, with the spectroscope, we ought to find that
all the rays of the spectrum lying between and nearly to H and b (Fig.
16), _i.e._ all the bluish-violet, blue, and blue-green rays pass
through it unchanged, unabsorbed, whilst all the rest should be
completely absorbed. In like manner a pure yellow colour would allow all
the rays lying between orange-red and greenish-yellow (Fig. 16) to pass
through unchanged, but would absorb all the other colours of the
spectrum.
Now we come to the, for you, most-important subject of mixtures of
colours and their effects. Let us take the popular case of blue and
yellow producing green. We have seen that the subjective effect of the
mixture of blue and yellow light on the eye is for the latter to lose
sense of colour, since colour disappears, and we get what we term white
light; in strict analogy to this the objective effect of a pure yellow
pigment and a blue is also to destroy colour, and so no colour comes
from the object to the eye; that object appears black. Now the pure blue
colouring matter would not yield a green with the pure yellow colouring
matter, for if you plot off the two absorption spectra as previously
described, on to the spectrum (Fig. 16), you will find that all the rays
would be absorbed by the mixture, and the result would be a black. But,
now, suppose a little less pure yellow were taken, one containing a
little greenish-yellow and a trifle of green, and also a little
orange-red on the other side to red, then whereas to the eye that yellow
might be as good as the first; now, when mixed with a blue, we get a
very respectable green. But, and this is very important, although of the
most brilliant dyes and colours there are probably no two of these that
would so unite to block out all the rays and produce black, yet this
result can easily and practically be arrived at by using three colouring
matters, which must be as different as possible from one another. Thus a
combination of a red, a yellow, and a blue colouring matter, when
concentrated enough, will not let any light pass through it, and can
thus be used for the production of blacks, and this property is made use
of in dyeing. And now we see why a little yellow dye is added to our
coal-tar black. A purplish shade would else be produced; the yellow used
is a colour complementary to that purple, and it absorbs just those blue
and purple rays of the spectrum necessary to illuminate by radiation
that purple, and _vice versâ_; both yellow and purple therefore
disappear. In like manner, had the black been of a greenish shade, I
should have added Croceine Orange, which on the fabric would absorb just
those green and bluish rays of light necessary to radiate from and
illumine that greenish part, and the greenish part would do the like by
the orange rays; the effects would be neutralised, and all would fall
together into black.
THE END.
INDEX
Acetone, 64
Acid, boric. _See_ Boric acid.
" carbolic. _See_ Phenol.
" colours, mordanting, 74
" hydrochloric. _See_ Hydrochloric acid.
" nitric. _See_ Nitric acid.
" sulphuric. _See_ Sulphuric acid.
Acids, distinguishing, from alkalis, 23, 49
" neutralisation of, 50
" properties of, 49
" specific gravities of, 49
Affinity, chemical, 71
Alizarin, 75, 76, 80, 83, 91, 99
" blue, 90
" paste, 91
" pure, 91
" purple, 77
" red, 77
Alkali, manufacture of, by ammonia-soda process, 55
" manufacture of, by electrolytic process, 56
" manufacture of, by Leblanc process, 53
Alkalis, distinguishing, from acids, 23, 49
" neutralisation of, 50
" properties of, 49
" specific gravities of, 49
Alum, cake, 73
Aluminium sulphate, 73
Ammonia, 23, 95
Ammonia-soda process, 55
Aniline, 91
" black, 81
" constitution of, 96
" preparation of, 96
" reaction of 97
" violet 77, 81
Animal fibres. _See_ Fibres.
Annatto, 83, 85, 87
Anthracene, 90
Archil. _See_ Orchil.
Aurin, 91, 98
Azo dyestuffs, 98
Barwood, 99
Basic colours or dyestuffs, mordanting, 76
Bast fibres. _See_ Fibres.
Bastose, 4
Bastose, distinction between, and cellulose, 4
Beaumé hydrometer degrees, 31
Benzene, 90, 96
Bixin, 88
Black-ash process, 54
Blue colour, absorption spectrum of pure, 114
Boilers, incrustations in, 42
Boiling-point, effect of pressure on, 32
" of water, effect of dissolved salts on, 36
" of water, effect of increase of pressure on, 35
Borax, 59
" tests of purity of, 59
Boric acid, 57
Boronitrocalcite, 59
Brasilin, 99
Brazil wood, 99
Camwood, 99
Carbolic acid. _See_ Phenol.
Carminic acid, 76
Carré ice-making machine, 32
Carrotting. _See_ Sécretage.
Carthamic acid, 87
Carthamin, 87
Cellulose, action of cupric-ammonium solutions on, 5
" composition of, 3
" distinction between, and bastose, 4
" properties of pure, 5
Cholesterol, 100
Chrome mordanting, 78
Chrome orange, 84
" yellow, 84
Chroming, over-, 78
Clark's soap test, 43
Coal-tar, 90
" yield of valuable products from, 90
Cochineal, 75, 76, 82, 83, 99
Coerulein, 90
Colour, absorption spectrum of pure blue, 114
" absorption spectrum of pure yellow, 114
" acids, 77
" bases, 77
" nature of, 107
Coloured substances, spectra of, 112
Colours, acid, mordanting of, 74
" basic, 75
" classification of, 79
" complementary, 109
" mixed, spectra of, 115
" pigment, 110
" primary, 110
" spectral, 110
Conditioning establishments, 21
Congo red, 71
Copper salts, dissolving, in iron pans, 39
" wet method of extracting, 38
Corrosion caused by fatty acids, 35
Cotton and woollen goods, separation of mixed, 5
Cotton fibre, action of basic zinc chloride on, 5
" composition of, 3
" dimensions of, 2
" stomata in cuticle of, 2
" structure of, 1
Cotton-silk fibre, 3
" " composition of, 3
Crookes' heat-indicating paint, 107
Cudbear, 86
Cupric ammonium solution, action of, on cellulose, 5
Curcumin, 87
Dextrin, 4
Dyeing felt hats deep black, 106
" " effect of stiffening and proofing process in, 65, 103
" of wool and felt with coal-tar colours, 105
" of wool and fur, 100
" power of coal-tar dyestuffs, 93
" with mixed coal-tar colours, 106
Dyestuffs, adjectiv, 83, 99
" azo, 98
" classification of, 79
" coal-tar, 90
" " dyeing power of, 93
" " yield of, 91
" mineral, 83
" monogenetic, 81
" pigment, 83
" polygenetic, 82
" substantive, 83
" " artificial, 89
" " natural, 85
Equivalence, law of, 49
Fats, decomposition of, by superheated steam, 35
Felt, dyeing, deep black, 106
" " with coal-tar colours, 105
Felting, dilute acid for promoting, 22
" effect of water in, 21
" fur, 15
" interlocking of scales in, 13
" preparation of fur for, 18
" unsuitability of dead wool for, 18
Fibre, cotton. _See_ Cotton.
" cotton-silk. _See_ Cotton-silk.
" flax. _See_ Flax.
" jute. _See_ Jute.
" silk. _See_ Silk.
" wool. _See_ Wool.
Fibres, action of acids on textile, 5
" " alkaline solution of copper and glycerin on textile, 28
" " alkalis on textile, 5
" " caustic soda on textile , 28
" " copper-oxide-ammonia on textile, 28
" " nitric acid on textile, 28
" " steam on textile, 5
" " sulphuric acid on textile, 27
Fibres, animal, 6
" bast, 3
" vegetable, 1
" " and animal, determining, in mixture, 27
" " and animal, distinguishing, 4, 5
" " and animal, distinguishing and separating, 24
Fibroïn, 7
Flax fibre, action of basic zinc chloride on, 5
" composition of, 3
" structure of, 2
Fraunhofer's lines, 111, 112
Fur, 8
" action of acids on, 23
" " of alkalis on, 24
" " on, in sécretage process, 17
" chrome mordanting of, 77
" composition of, 22
" felting, 15
" finish and strength of felted, effect of boiling water on, 22
" hygroscopicity of, 20
" preparation of, for felting, 18
" sécretage or carrotting of, 17
" stiffening and proofing of felted, 66
" sulphur in, reagents for detection of, 26
Fustic, 99
Gallein, 82, 83
Gallnuts, 99
Garancine, 99
Guy-Lussac tower, 52
Glover tower, 52
Glucose, 4
Greening of black hats, 65
Hæmatein, 76, 78 83, 99
Hair, 8
" cells from, 11
" distinction between, and wool, 12, 14
" dyeing, 26
" growth of, 8
" scales from, 11
" " of, action of reagents on, 12
" scaly structure of, 11
" structure of, 8, 9
" sulphur in, reagents for detection of, 26
Hargreaves & Robinson's process, 53
Hats dyed logwood black, deterioration of, 104
" greening of black, 65
" stiffening and proofing of, 63, 64
" stiffening and proofing of, by Cheetham's process, 66
" stiffening and proofing of, by Continental process, 66
" stiffening and proofing process, effect of, in dyeing, 65, 103
Heat, latent, 32, 33
" " of steam, 34
" " of water, 34
Heddebault's process of separating mixed cotton and woollen goods, 5
Hydrochloric acid, manufacture of, by Hargreaves & Robinson's process, 53
" " manufacture of, by salt-cake process, 53
Ice, heat of liquefaction of, 34
Ice-making machine, Carré, 32
Indican, 85
Indicators, 50, 70
Indigo, 85
" artificial, 86
" blue, 85
" recovery of, from indigo-dyed woollen goods, 24
" vat, 86
" white, 85
Insoluble compounds, precipitation of, from solutions, 38
Iron liquor. _See_ Mordant, iron.
Jute fibre, 3
" composition of, 4
Lac, button, 63
" dye, 62, 99
" seed, 62
" stick, 62
_See also_ Shellac.
Lakes, colour, 75
Latent heat. _See_ Heat.
Leblanc process, 53
Light, analysis of white, 107
" composition of white, 107
" homogeneous or monochromatic, 108, 110
" rays, refraction of, 108
Linen fibre. _See_ Flax.
Litmus, 70, 86
Logwood, 75, 76, 78, 83, 99
Logwood black, 78, 81, 104
" " deterioration of hats dyed with, 104
Madder, 80, 83, 99
Magenta, 76, 80, 83, 91, 97
Marsh gas, 95
Mercuric nitrate, use of, for the sécretage of fur, 17
Merino wool, 15
Methane. _See_ Marsh gas.
Methyl alcohol. _See_ Wood spirit.
" green, 97
" violet, 97
Mirbane, essence of, 96
Molisch's test, 4
Mordant, alumina, 64, 75
" antimony, 76
" iron, 64, 76
" tannin, 76
" tin, 76
Mordanting acid (phenolic) colours, 74
" basic colours, 76
" chrome, 77
" woollen fabrics, 75
Mordants, 69
" fatty acid, 77
Naphthalene, 90, 98
Naphthol yellow, 91
Naphthols, 91, 98
Naphthylamine, 91
Nitric acid, 95
" manufacture of, 52
Nitrobenzene, 96
Nitroprusside of soda, 26
Oils, decomposition of, by superheated steam, 35
Orcèin, 86
Orchil, 85, 86
Orcin, 86
Orellin, 88
Over-chroming, _See_ Chroming.
Paint, Crookes' heat-indicating, 107
Persian berries, 75, 99
Phenol, 90
" constitution of, 98
Phenolic colours. _See_ Acid colours.
Phenolphthalein, 70
Picric acid, 81, 91
" absorption spectrum of, 113
" constitution of, 98
Plumbate of soda, 26
Potassium, decomposition of water by, 25, 30
Proofing mixture, 63
" process, 64
" " Cheetham's, 66
" " Continental, 66
" " effect of, in dyeing, 65, 103
Purpurin, 99
Quercitron, 99
Red liquor. See Mordant, alumina.
Refraction of light rays, 108
Safflower, 85, 87
Salt-cake process, 53
Salts, 49
" acid, 70, 71
" basic, 71
" neutral or normal, 71
" stable, 72
" unstable, 72
Santalin, 99
Santalwood, 99
Sealing-wax, coloured, 103
Sécretage of fur, 17
" process, injury to fur in, 17
Sericin, 7
Shellac, 62
" colouring of, 103
" rosin in, detection of, 63
" solvents for, 63
_See also_ Lac.
Silk fibre, action of acids on, 7
" " " of alkaline solution of, copper and glycerin on, 7
" " " of alkalis on, 7
" " " of basic zinc chloride on, 7
" " bleaching of, 7
" " composition of, 7
" " structure of, 6
" " ungumming of, 7
" glue, 7
" gum, 7
Soap, 60
" alkali in, detection of, 61
" oleic acid, 101
" palm oil, 101
" water in, determination of, 60
Soda. _See_ Alkali.
Solution, 36
" precipitation of insoluble compounds from, 38
Specific gravity, 30
Spectra of coloured substances 112
Spectroscope, 111
Spectrum, 108
" absorption, 113
" continuous, 111
" discontinuous or line, 111
Spirits of salt. _See_ Hydrochloric acid.
Starch, 4
Steam, 31
" latent heat of, 34
Stiffening mixture, 63
" process, 64
" " Cheetham's, 66
" " Continental, 66
" " effect of, in dyeing 65, 103