COLOURING MATTERS 899 



acids. A watery solution of hematine, when mixed with an excess of caustic alkali, 

 becomes of a beautiful purple ; but the colour when exposed to the air, almost im- 

 mediately turns brown, the hematine being then completely changed. It is almost 

 needless to observe, that the bodies into which colouring matters are converted by 

 oxidation are incapable, under any circumstances, of returning to their original state. 



The action of reducing agents, that is of bodies having a great affinity for oxygen, 

 on some colouring matters, is very peculiar. If indigo-blue, suspended in water, bo 

 placed in contact with protoxide of iron, protoxide of tin, or an alkaline sulphuret, 

 sulphite, or phosphite, or grape sugar, or, in short, any easily oxidisable body, an 

 excess of some alkali or alkaline earth being present at the same time, it dissolves, 

 forming a pale yellow solution without a trace of blue. This solution contains, in 

 combination with the alkali or alkaline earth, a perfectly white substance, to which 

 the name of reduced indigo has been applied. When an excess of acid is added to the 

 solution, it is precipitated in white flocks. By exposure to the air, either by itself or 

 in a state of solution, reduced indigo rapidly attracts oxygen, and is reconverted into 

 indigo-blue. Hence the surface of the solution, if left to stand in uncovered vessels, 

 becomes covered with a blue film of regenerated indigo-blue. It was for a long time 

 supposed that' reduced indigo was simply deoxidised indigo-blue, and that the process 

 consisted merely in the indigo-blue parting with a portion of its oxygen, which was 

 taken up again on exposure to the air. It has, however, been discovered that in 

 every case water is decomposed during the process of reduction which indigo-blue 

 undergoes, the oxygen of the water combining with the reducing agent, and the 

 hydrogen uniting with the indigo-blue, water being again formed when reduced indigo 

 conies in contact with oxygen. Eeduced indigo is therefore not a body containing less 

 oxygen than indigo-blue, but is a compound of the latter with hydrogen. There are 

 several red colouring matters which possess the same property, that of being converted 

 into colourless compounds by the simultaneous action of reducing agents and 

 alkalis, and of returning to their original state when exposed to the action of oxygen. 

 There can be little doubt that the process consists, in all cases, in the colouring matter 

 combining with hydrogen and parting with it again when the hyduret comes in con- 

 tact with oxygen. 



The action of chlorine on colouring matters is very similar to that of oxygen, though, 

 in general, chlorine acts more energetically. The first effect produced by chlorine, 

 whether it be applied as free chlorine, or in a state of combination with an alkali, or 

 alkaline earth as an hypochlorite, usually consists in a change of colour. Blue and 

 red colouring matters generally become yellow. By the continued action of chlorine 

 all trace of colour disappears, and the final result is the formation of a perfectly white 

 substance, which is usually more easily soluble in water and other menstrua than that 

 from which it was formed. Since it is most commonly by means of chlorine or its 

 compounds that colouring matters are destroyed or got rid of in the arts, as in bleaching 

 fabrics and discharging colours, the process of decomposition which they undergo by 

 means of chlorine has attracted a good deal of attention, and the nature of the chemical 

 changes, which take place in the course of it, has often been made a subject of dispute, 

 though the matter is one possessing more of a theoretical than a practical interest. It 

 is a well-known fact that many organic bodies are decomposed when they are brought 

 into contact, in a dry state, with dry chlorine gas. The decomposition consists in the 

 elimination of a portion of the hydrogen of the substance and its substitution by 

 chlorine. When water is present at the same time, the decomposition is, however, not 

 so simple. It is well known that chlorine decomposes water, combining with the 

 hydrogen of the latter and setting its oxygen at liberty, and it has been asserted, that 

 in the bleaching of colouring matters by means of chlorine when moisture is usually 

 present, this always takes place in the first instance, and that it is in fact the oxygen 

 which effects their destruction, not the chlorine. This appears, indeed, to be the case 

 occasionally. Kubian, for instance, the body from which alizarine is derived, gives, 

 when decomposed with chloride of lime, phthalic acid, a beautifully-crystallised sub- 

 stance, containing no chlorine, which is also produced by the action of nitric acid on 

 rubian, and is, therefore, truly a product of oxidation. In many cases, however, it is 

 certain that the chlorine itself also enters into the composition of the new bodies 

 produced by its action on colouring matters. When, for instance, the chlorine acts 

 on indigo-blue, chlorisatine is formed, which is indigo-blue, in which one atom of 

 hydrogen is replaced by one of chlorine, plus two atoms of oxygen, the latter being 

 derived from the decomposition of water. 



The behaviour of colouring matters towards water and other solvents is very various. 

 Some colouring matters, such as those of logwood and brazilwood, are very easily 

 soluble in water. Others, such as the colouring matters of madder and quercitron bark, 

 are only sparingly soluble in water. Many, especially the so-called resinous ones, are 

 insoluble in water, but more or less soluble in alcohol and ether, or alkaline liquids. A 



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