is of a very complex character, while that from others is comparatively simple. It is this complexity in the composition of the light reflected that makes it so difficult to demonstrate the true laws and facts of colour with pigments or any artificial colouring-matters.

Cause of Colour in Coloured Bodies.-The actual reasons why bodies such as vermilion, magenta, or emerald green are coloured, it is almost impossible to investigate in the present state of knowledge, since the cause, whatever it may be, must be due to the molecular construction of the different compounds about which very little is known; still, we know something of some of the reasons why coloured bodies appear coloured. When light falls upon a substance, the light may be affected in one or two ways; it may be reflected, that is, it may be thrown back from the body; or it may be transmitted, that is, it may pass through, or, in some cases, be absorbed by the body on which it has fallen. As a rule, there is never either complete reflection or complete transmission of light, the most perfectly reflecting body allowing some rays to pass into it. It is by reflected light that we see bodies; when the reflection is complete, or as nearly so as is the case with mercury or a very highly polished plate of silver, the body is nearly invisible; it is only rendered visible because it does not reflect all the light which falls upon it in a regular manner; some is irregularly reflected and it is this light which enables us to see the body. Two kinds of reflection can, therefore, be distinguished-regular and irregular. Regular reflection is that where the light is thrown back in a straight line from the reflecting surface; if this is perfect, only the light that is reflected is seen, the reflector itself is invisible. Irregular reflection is that where the light is thrown back from the reflector in every direction; it is this light which makes the body visible, and it is due to the fact that no matter how apparently even the surface may appear to be, yet it is not even; it is sufficiently rough to cause the light which falls upon it to be irregularly reflected. Then bodies never reflect or absorb the whole of the light which falls upon them, some of it is absorbed; the most perfectly polished plate of silver (which is the most highly reflecting body known). does not reflect the whole of the light which falls upon it, while a piece of black cloth reflects only a little of the light that falls upon it. Upon the character of the reflected light thrown off from a body depends its colour, which is independent of the proportion of the light that falls upon and is reflected by the body. If all the rays of light falling upon it are reflected, then the body appears white; if all the light rays are absorbed, then the body

appears to be black. If, now, some of the spectral rays are absorbed and the rest reflected, then the body appears to be coloured, the colour depending upon the composition of the rays which are reflected; thus the rays from a red body, such as vermilion, are red, as are also those from Derby red and oxide of iron; similarly, the rays from a yellow body, such as chrome yellow or yellow ochre, are yellow, but it does not follow that the rays from all red bodies or from all yellow bodies are identical in composition. If the rays from, say, vermilion, oxide of iron, and crimson lake are passed through a prism, and the spectra of the coloured light which is reflected from each examined, they will be found to be different; that from the crimson lake will contain more blue rays than that from the vermilion, while that from the oxide of iron will contain more of the dark red and indigo rays than either of the others; and it is the same with the other classes of colours. There is no coloured body known which reflects what might be called a pure light, while the spectrum-colours are pure, as has been already stated. It is this compound nature of the light which is reflected from coloured bodies that makes it extremely difficult to demonstrate the true laws of light and colour by the use of pigments.

In the same manner as the coloured light which is reflected from bodies is compound, so that which is transmitted is compound and, usually, the complement of that which is reflected, but this does not always happen. When it is the complement of that which is reflected, then the bodies which give rise to this phenomena are known as dichroic; in other cases both the reflected and transmitted rays are of the same general colour, although there is usually some difference in the actual tint of the two colours.

It is assumed that the coloured bodies have a selective action on the light which falls upon them, reflecting or transmitting, as the case may be, those coloured rays to which they owe their colour, while they absorb all the other rays. White bodies reflect all the rays which fall upon them, black bodies absorb all and are, in consequence, often nearly invisible. As to the character of the rays reflected from red, orange, yellow, green, or other coloured bodies, these will have been inferred from what has been said above.

Colour Theories.-Two theories of colour are in use to explain the coloured effects of light. The old theory, which is mostly due to Brewster, considers that there are three primary colours-viz., red, yellow, and blue; by the proper admixture of which in various proportions all the other colours can be

obtained. The more modern theory, first broached by Young and more fully developed by Helmholtz, considers that there are three primary colours, red, green, and blue, although some authorities add a fourth. However, it must be confessed that while the modern theory accurately explains all the phenomena of colour producible by the use of the spectrum colours, yet the older theory of Brewster more easily explains the phenomena of colour as produced by the admixture of the various colouring-matters, pigments, and dyestuffs in common use; this arises not from any fault in the newer theory, but from the compound nature of the light which is reflected or transmitted from the colouring-matters in question. Of the newer theory it is not intended to deal, although it is advisable for colourists to make themselves acquainted with it; as to the old theory, it will be sufficient to say that when any two of the primary colours are mixed together a so-called secondary colour is produced; thus red and yellow produce orange, red and blue produce violet, while yellow and blue make green. When the secondaries are mixed together they produce what are called tertiary colours, of which there are six, known as buff, citrine, sage, slate, plum, and russet. The nomenclature of these tertiary colours is very indefinite, and different authorities give them different names.

The common theory of red, blue, and yellow is not wholly satisfactory, as it does not account for all the shades which may be produced by the admixture of pigments; thus a mixture of ultramarine, a blue, with yellow ochre, a yellow, does not produce a green, as the theory would expect, but a kind of greenish-grey; this effect can, however, be explained by the blue-red-green theory when we know the kind of rays reflected by the two pigments in question. Reference must be made to text-books on colour for a further development of the subject.

Colours. It has been explained above that the term "colours" is used in two senses-first, to express the sensation which light of various kinds evolved from bodies excites on the retina of the eye, and which sensation is purely functional; second, to denote those bodies which, having the property of selective absorption of coloured rays from the light which falls upon them, appear to be coloured and which have the property of imparting this colour to other bodies; such bodies are known as colouring matters and may be divided into two groups, dyestuffs and pigments; the former are mostly soluble in water and are used solely to dye cotton, wool, or other textile fibres, while the latter are insoluble, and are used in the preparation of paints.

Besides these two classes of coloured bodies there is another group which are distinguished by the fact that while possessing colour yet they cannot impart this colour to other bodies; such are bluestone (sulphate of copper), nitrate of cobalt, chrome alum, &c.

Pigments.-These are a fairly numerous class of colouring matters which are used to give colour to paint. They are mostly derived from the mineral kingdom, although a few are obtained from organic sources. As a class they are distinguished by being insoluble in water, turpentine, and most other solvents with the exception of the strong acids; they are opaque or nearly so; and they should be perfectly inert bodies exercising no action of any kind on any other substance with which they may be mixed. As typical examples of pigments may be taken barytes, oxide of iron, yellow ochre, chrome green, and umber.

In dealing with pigments in detail they will, as a rule, be considered under the divisions of white pigments, red pigments, yellow pigments, and so on; but here and there, deviations from this rule will be made, as in the case of Derby red (which will be dealt with under the head of yellow pigments) and in the case of lakes, where it is thought that the composition and properties of the particular pigments can be more conveniently pointed out, if dealt with in one group.

Paint.-Paint is the name given to a liquid composition which is used very extensively for two purposes-first, to act as a protective substance to preserve the body on which it has been applied from the destructive action of the weather; second, as a decorative agent.

The first object is brought about by making the paint with materials which are not acted upon by the various agents present in the atmosphere, such as water, acid vapours, light, oxygen, that exert a more or less destructive action on bodies which may be exposed to their action. The bodies which have been found to resist this destructive action of the atmospheric influences are the various so-called drying oils, resinous matters, and the pigments. A paint is a liquid composition which will remain liquid until it is applied to the body to be painted, and yet when so applied and afterwards exposed to the atmosphere will dry and leave behind it a firm, hard (yet elastic), and opaque coating, which may be more or less lustrous and be capable of resisting the weather. The opacity of the coating is obtained by using pigments of various kinds, which also tend to increase the resisting power of the paint, and these pigments are mixed with liquid bodies, such as oils and spirits, which are

used partly to obtain a composition that is easy of application, and partly to secure volatility, so that when alone or when mixed with resinous matters, they will evaporate away and leave behind a hard mass firmly binding the pigments to the body over which they have been painted. The liquid bodies which have been found to answer this purpose best are the drying oils, such as linseed oil, which when spread over a surface and exposed to the air absorb oxygen and dry into a hard mass; but as these oils, for various reasons (which will be more fully dealt with later on), cannot be used alone with satisfactory results it becomes necessary to mix them with some solvent, such as turpentine or shale naphtha, which is volatile. In some kinds of paints a little resinous matter is used, which dissolves in the solvent; on exposure the latter evaporates off, leaving the resin behind in the form of a dry coat on the surface to which it has been applied. Paint is always more or less coloured to add to the decorative effect. Its primary purpose, however, is to hide the character of the surface to which it is applied and, as has been pointed out, to protect this surface.

Varnishes. These bodies are very similar to paints in their properties and uses. They differ in giving a transparent lustrous coat of a very resistant character to the destructive action of the weather. They may be coloured; but, if so, transparent colours are used and not, as in the case of paints, opaque pigments. They are composed of a resinous matter dissolved in various oils and solvents, the latter forming the vehicle by means of which the resin is transferred to the surface to be varnished. The special properties of varnishes will be Idealt with later on.