COLOUR, in calico-printing. The term colour in calico-printing isapplied not only to those vegetable, animal, and mineral solutions, which impart their own colour to the cloth on whichthey are applied, but also improperly to those earthy or metallic solutions, which, possessing little or no tingent properties themselves, yet retain or fix the qualities (colours) of other sub stances, when afterwards applied to the cloth. Thus the aeetite of alumina, or prin ter's red liquor, when pure, is almost co lourless,and only becomes red by the pro cess of dyeing, as will be explained here after. The acetite of iron, or iron liquor, in like manner, when used of a determi nate strength, is called black colour, and when weaker, purple colour, though the cloth impregnated with these solutions becomes black or purple, only as being raised like the other in the dye-copper. 1. The colours produced by means of these earthy or metallic solutions (which in the language of science are called mor dants) form the most valuable and impor tant series, whether considered with re gard to the almost infinite variety of shades, or to their solidity and durability. These colours, from the mode in which they are produced, (the mordant being first applied to the cloth, and the colour afterwards raised by dyeing,) are called dyed colours. 2. Sometimes the mordant is previously mixed with a solution of co louring matter, and in that state applied to the cloth, so as to paint or stain it at one operation and without the process of dyeing. Thus another class of colours is produced, many of them possessing great brilliancy indeed, but much inferior to the former in durability. The colours called chemical by calico-printers belong chiefly to this class. 3. In the third and last class we may place all those, where the colouring matter is simply held in solu tion by an acid or alkali, and in this state applied to the cloth without the interven tion of any mordant. To one or other of the foregoing classes may be referred all the colours used in calico-printing, with the exception, however, of those sys tems of colours which have been pro duced by calico-printers in this country, within a short period, by processes and upon principles which have hitherto not been made known. See CALICO-PRINT
ING.
CoLouR of the clouds is thus accounted for by Sir Isaac Newton. Concluding, from a series of experiments, that the transparent parts of bodies, according to their several sizes, reflect rays of one co lour, and transmit those of another, he hence observes, that when vapours are first raised, they are divided into parts too small to cause any reflection at their surfaces, and therefore do not binder the transparency of the air ; but when they begin to coalesce, in order to form drops of rain, and constitute globules of all in termediate sizes, these globules are capa ble of reflecting some colours, and trans mitting others, and thus form clouds of various colours, according to their sizes. Mr. Melville controverts this doctrine, in its application to the red colour of the morning and evening clouds. " Why," he says, " should the particles of the clouds become at that particular time, and never at any other, of such a magni tude as to separate these colours ? And why are they rarely, if ever, seen tinc tured with blue and green, as well as red, orange, or yellow ? Is it not more credi ble, that the separation of rays is made in passing through the horizontal atmo sphere, and that the clouds only reflect and transmit the sun's light, as any half transparent colourless body would do ? For since the atmosphere reflects a greater quantity of blue and violet rays than of the rest,the sun's light transmitted through it ought to incline towards yellow,orange,or red ; especially when it passes through a long tract of air: and thus it is found, that the sun's horizontal light is tinctured with a deep orange, and even red; and the co lour becomes still deeper after sun-set." Hence he concludes that the clouds, ac cording to their different altitudes, may assume all the variety of colours at sun rising and setting, by barely reflecting the sun's incident light as they receive it.