THE ACTION OF MORDANTS 115 



tively charged, it will tend to combine with basiphil (negatively 

 charged) tissue-constituents, such as the nucleic acids. It will not 

 tend to combine with acidophil (positively charged) constituents, 

 such as the haemoglobin of red blood-corpuscles. 



It might be thought that the hydrated aluminium ion, carrying 

 a triple positive charge, would have a greater tendency to be 

 attracted to sites of negative charge in the tissues than would the 

 doubly charged aluminium-purpurine ion. If so, little dyeing 

 would be likely to occur; yet the solution dyes strongly. The 

 explanation appears to be as follows. When some of the water 

 molecules of a coordination complex, such as the hydrated alu- 

 minium ion, are replaced by other molecules, the solubility of the 

 complex in water is reduced. This reduction in solubility will 

 favour the deposition of the aluminium-purpurine ion on the sites 

 of negative charge, while the hydrated aluminium ion has a strong 

 tendency to remain dissolved. ^^^ 



A few of the hydrated aluminium ions are likely to take up two 

 purpurine ions ; some may take up three. The result will be a reduc- 

 tion of the positive charge respectively to one or nought; in the 

 latter case there would be precipitation of an insoluble pigment. 

 On the analogy of the compound of purpurine with cobalt,^^^ it 

 must be regarded as probable that one aluminium atom can 

 accept chelate bonds from three molecules of purpurine. This, 

 however, is not certain. A great deal of study has been devoted to 

 the attempt to discover the exact composition of the flaming red 

 dye, Turkey red, which has been known in the East for many 

 hundreds of years. It appears certain that although this substance 

 contains calcium, yet the chelate bonds are all with aluminium, 

 and that only two alizarine molecules are thus joined to each 

 aluminium atom: of the two remaining covalencies of this metal, 

 one still retains a water molecule (and the other is linked through 

 oxygen to calcium)."^ 



Compounds between mordants and dyes are known as lakes. 

 The artists' water colour, madder lake, is of this nature. This 

 insoluble substance is similar in composition to Turkey red. It is 

 distributed by the artist's brush in the form of a fine suspension. 

 When a mordant and a dye are dissolved together, a lake may 



