48. Calcareous salts dissolved in water, not only affect the brilliancy of some colours, but possess the inconve nience of weakening the solvent which holds them in solution, and thus extract impel fectly the colouring principle. It is of consequence, therefore, to be able to detect the presence of these salts, that we may avoid the water which contains them, or correct its pre judicial qualities. Chemistry furnishes us with the means not only of discovering the different ingredittnts which exist in water, but of ascertaining with precision the quantity of each. The salts of lime, the most com mon earth which is to he kund in water, are easily de tected by the copious white precipitate which they af ford with the oxalate of ammonia, and those of iron by a solution of galls. In general, water which contains earthy or metallic salts, in such quantity as to be inju rious, readily decomposes soap by a double of the acid combines with the alkali of the soap, while the earthy basis unites with the oil, and forming an earthy soap which is insoluble, produces the cunnin• appear ance observable on such occasions. if then a water be clear, destitute of taste and smell, and capable of dis solving soap readily, it may be regarded proper for the objects of dyeing; and all waters which possess these properties are equally fit for the purpose. But it may be remarked, that waters which hold the earths mecha nically suspended, that is, such as arc muddy, are less than those which hold them in solution : In the first case, they attach themselves but loosely to the stuff; in the second, they are precipitated in a state of minute division, and combining with the mordant, be come intimately united with the cloth.
49. But as it is not always in our power to choose wa ter of the best quality, means have been devised for correcting the injurious properties of this fluid, when it happens to be bad. The most common corrective for this purpose is bran, which is allowed to remain in the water until it acquires a sour taste. Twenty-four bushels of bran arc put into a tub or vat that will contain about 10 hogsheads; a large boiler is filled with water, which, when just ready to boil, is poured into the vat: the acid fermentation soon commences, and in 24 hours the liquor is ready for use. Berthollet conceives that the sour wa ter acts by decomposing the carbonate of lime and mag nesia, from w•ich its acid, being more powerful, disen gages the carbonic acid ; and that in this way the earthy sediment, wiiich is occasioned by boiling, is prevented from taking place.
50. Alkali, the next solvent in order which we men tioned, is employed to dissolve several colours. In ge neral, as Chaptal remarks, the colours which are pro duced by fermentation are less soluble in water than alkali. This is the case with indigo, woad, &c. Thus, boiling water dissolves only a ninth•part of its weight of indigo ; woad communicates very little colour to this fluid ; and annotta can scarcely be dissolved in it at all without the aid of alkali. After the colouring matter has been extracted by an alkali, it is precipitated by the addition of some weak acid.
51. Acids are sometimes employed as solvents of co louring principles. The chemical blue of P.erner, or
the Saxon blue of dyers, is a solution of indigo in con centrated sulphuric acid. Guhliche has proposed to extract the colour of yellow wood, broom, turmeric, &c. by means of the aceto-citric, and simple acetic acid, or any other of the vegetable acids. The nitric and oxy muriatic acids impart a yellow tinge to all animal sub stances ; and the former of these acids is even success fully employed to communicate to silk and wool a very beautiful yellow colour. The nitric acid produces a similar effect on madder ; but the yellow thus developed disappears when the acid is neutralized.
52. The simple colours, or at least those obtained from the decomposition of light by means of the prism, are seven in number, viz. red, orange, yellow, green, blue, indigo, and violet. These colours are supposed to be homogeneous or simple, because when they are transmitted a second time through the prism, they are refracted without undergoing any farther change. This argument, however, is by no means conclusive ; for if any coloured ray be composed of two others, each of these must have the same refrangibility, as it is in con sequence of this circumstance alone that they could oc cupy the same place in the prismatic spectrum ; and, in that case, a second refraction could not separate them. The truth is, that the prism furnishes only one means of the analysis of light, and merely shows that certain rays differ from others in refrangibility.
53. Before the discoveries of Newton, the red, yellow, and blue, were generally supposed to be primary rays, and the rest to be compound colours arising from their intermixture. Thus, red with yellow, produces orange ; yellow with blue, green;' and blue with red, indigo. Another hypothesis has been lately advanced by Priem', and supported by very ingenious reasoning. According to him, the red, green, and violet, are the primary rays ; the red and green giving rise, by their intermixture, to orange or yellow, according as the former or the latter predominates ; the green and violet forming blue; and the violet and red, purple ; and thus, by various modifica tions of the three original colours, all the others are ob tained. We shall give an example of the kind of proof which has been adduced in support of this hypothesis. Let one of the rays, as orange, be selected—a colour which is supposed to be a compound of red and green, the former being in excess. If this colour be really a compound of red and green, and if the arrival of the rays to the part of the spectrum which the orange usually oc cupies be prevented, by the interposition of a substance that allows only the red or the green rays to pass, we ought to find beyond this substance only red or green ; On the other hand, if the orange be simple, these rays will pass through neither of the interposed substances, and beyond this we should have only black. The results corresponded with the hypothesis in a great variety of trials. An additional argument was also derived for its support, by combining the red, green, and blue rays, and thus obtaining white light.