900 COLOURING MATTERS 



few, such as indigo-bluo, are almost insoluble in all menstrua, and can only be made 

 to dissolve by converting them, by means of reducing agents, into other bodies soluble 

 in alkalis. Those which are soluble in water, are, generally speaking, of the greatest 

 importance in the arts, since they admit of more ready application, when they possess 

 this property. 



The behaviour of colouring matters towards acids is often very characteristic. Most 

 colouring matters are completely decomposed by nitric, chloric, manganic, and chromic 

 acids, in consequence of the large proportion of oxygen which these acids contain. With 

 many colouring matters the decomposition takes place even at the ordinary temperature : 

 with others, it only commences when the acid is warmed, especially if the Litter be 

 applied in a state of considerable dilution. Concentrated sulphuric acid also destroys 

 most colouring matters, especially if the acid be heated. It seems to act by depriving 

 them of the elements of water, and thereby converting them into substances containing 

 more carbon than before, as may be inferred from the dark, almost black, colour 

 which they acquire. At the same time the acid generally loses a portion of its 

 oxygen, since sulphurous acid is almost always evolved on heating. Some colouring 

 matters, such as alizarine, are not decomposed by concentrated sulphuric acid even 

 when the latter is raised to the boiling point ; they merely dissolve, forming solutions 

 of various colours, from which they are precipitated unchanged, on the addition of 

 water, when they are insoluble or not easily soluble in the latter. Others, again, like 

 indigo-blue, dissolve in concentrated or fuming sulphuric acid, without being decom- 

 posed, and at the same time enter into combination with the acid, forming true double 

 acids, which are easily soluble in water and combine as such with bases. Many 

 colouring matters undergo a change of colour when exposed to the action of acids, the 

 original colour being restored by the addition of an excess of alkali, and this property 

 is made use of for the detection of acids and alkalis. The colour of an infusion of 

 litmus, for instance, is changed by acids from blue to red, and the blue colour is restored 

 by alkalis. An infusion of the petals of the purple dahlia or of the violet becomes 

 red on the addition of acids, and this red colour changes again to purple or blue with 

 alkalis, an excess of alkali making it green. The yellow colour of rutine becomes 

 deeper with strong acids. In most cases, this alteration of colour depends on a very 

 simple chemical change. Litmus, for example, in the state in which it occurs in 

 commerce, consists of a red colouring matter in combination with ammonia, the com- 

 pound being blue. By the addition of an acid, the ammonia is removed, and the 

 uncombined red colouring matter makes its appearance. Ammonia and most alkalis 

 remove the excess of acid, and, by combining with the red colouring matter, restore 

 the blue colour. "When a colouring matter, like alizarine, is only sparingly soluble in 

 water, its solubility is generally diminished in the presence of a strong acid. Hence, 

 by adding acid to the watery solution, a portion of the colouring matter is usually 

 precipitated. It is very seldom that colouring matters are really found to enter into 

 combination with acids. Indeed, only one, viz. berberine, is capable of acting the 

 part of a true base, and forming definite compounds with acids. Some acids, such as 

 sulphurous and hydrosulphuric acids, do certainly seem to combine with some 

 colouring matters and form with them compounds, in which the colour is completely 

 disguised, and apparently destroyed. If a red rose be suspended in an atmosphere of 

 sulphurous acid it becomes white, but the red colour may be restored by neutralising 

 the acid with some alkali. On this property of sulphurous acid depends the process 

 of bleaching woollen fabrics by means of burning sulphur. In this case the colouring 

 matter is not destroyed, but only disguised by its combination with the acid. 



Most colouring matters are capable of combining with bases. Indeed, their affinity 

 for the latter is generally so marked that they may be considered as belonging to the 

 class of weak acids. Like all other weak acids, they form, with bases, compounds of a 

 very indefinite composition, so much so, that the same compound, prepared on two 

 different occasions, is often found to be differently constituted. Hence the great 

 difficulty experienced by chemists in determining the atomic weight of colouring matters. 

 There are very few of the latter for which several formulae, all equally probable, may 

 not be given, if the compounds with bases be employed for their determination. The 

 compounds of colouring matters with bases hardly ever crystallise. Those with alkalis 

 are mostly soluble in water and amorphous ; those with the alkaline earths, lime, and 

 baryta, are sometimes soluble, sometimes insoluble ; those with the earths and metallic 

 oxides are almost always insoluble in water. The compounds with alkalis are 

 obtained by dissolving the colouring matter in water to which a little alkali is added, 

 and evaporating to dryness : an operation which must be carefully conducted if the 

 colouring matter is one easily affected by oxygen. The insoluble compounds, with 

 earths and metallic oxides, are obtained either by double decomposition of a soluble 

 compound with a soluble salt of the respective base, or by adding to a solution of the 

 colouring matter, in water or any other menstruum, a salt of the base containing some 



