xxxiv INTRODUCTION, 



2. The Colour. — The colour of objects should always he carefully described, aud its 

 cause accurately determined. It most commonly arises from: — 1, partial absorption; 2, 

 the presence of pigment, or other colouring matter ; 3, from iridescence ; 4, from polari- 

 zation, &c. 



(1.) The most common cause is a peculiar property by which a portion of the coloured 

 rays composing the white light which falls upon or is transmitted through an object is 

 absorbed, the remainder being reflected or refracted so as to reach the eye. On examining 

 bodies thus coloured, with whatever powers, their substance is found uniformly coloured, 

 and this colour is unchanged by their immersion in water or oil of turpentine, and is the 

 same in transparent bodies by both transmitted and reflected light. This is commonly 

 regarded as the proper colour of an object. Example: a crystal of blue vitriol. 



(2.) a. In many cases, however, although an object may appear to the naked eye 

 uniformly coloured, on examining it with a high power, the colour, which in fact arises 

 from the above cause, is seen to be confined to certain molecules or granules, whilst the 

 general substance is colourless. These granules may consist of vegetable or animal colour- 

 ing-matters, metallic oxides, &c. The nature of these matters should always be determined, 

 if possible, either by microscopic chemistry — micro-chemical analysis, as it has been called, 

 — or by ordinary chemical analysis. When the colouring-matter is of organic nature, and 

 when its composition cannot be determined, or it has no definite name, it is called pigment. 

 Objects coloured by pigment, metallic oxides, or other colouring matters, are best examined 

 by direct (not oblique) transmitted light, and when immersed in either water or oil of 

 turpentine. These liquids do not change the colour, nor destroy it unless the pigment be 

 soluble in them ; but by rendering the general substance of the object more transparent, 

 they cause the granules to become more distinct. The colour is the same both by 

 transmitted and reflected light. Example : a brown or black hair of an animal, as the 

 mouse. 



b. Sometimes bodies coloured by pigment or other colouring-matters appear under the 

 microscope uniformly dyed, although the colouring-matter consists of an insoluble molecular 

 or granular powder — as a white animal hair first macerated in solution of ferrocyanide of 

 potassium and then in solution of perchloride of iron. Chemical means will alone 

 distinguish this cause of colour from the first, by removing the colouring-matter from the 

 colourless basis. 



(3.) The colours of many objects vary according to the direction of the light transmitted 

 through them, or are only visible by oblique light, and the colours are different by direct 

 and oblique light. These arise from decomposition of white light by either interference or 

 refraction. For the sake of brevity, these may be designated colours from iridescence, 

 because they mostly exhibit the brilliancy and transparency of the colours of the rain- 

 bow. The interference or refraction upon which they depend is ordinarily produced 

 by irregularities of structure, frequently depressions or grooves, and sometimes cavities 

 containing air, &c. Objects exhibiting these colours, Avhich are most brilliant by very 

 'oblique light and under low powers, when examined with a moderately high power by 

 transmitted direct or but slightly oblique light, frequently appear more dull and less 

 brilliant, often dark or black in parts ; and when immersed in oil of turpentine, or some 

 liquid approaching in refractive power the substance of which they are composed, so that 

 their irregularities become filled with it, the colours vanish. Hence colour, when arising 

 fmm iridescence, can readily be distinguished from that arising from general absorption 



