240 DYEING 



crystals of phosphomolybdic acid. The composition of these 

 crystals is not quite constant, but approximates to H3P04(Mo03)] 2, 

 with water of crystallization. 



The function of phosphomolybdic acid in Mallory's and similar 

 techniques was explained by the researches of Mollendorff ^^^ and 

 Seki.461 



If an Irish bull be permissible, one may say shortly that phos- 

 phomolybdic acid acts as a colourless acid dye (for it scarcely 

 colours the tissues). Luckily one can convert it into a coloured 

 substance by exposure to bright light. A blue lower oxide of moly- 

 bdenum, of indeterminate composition, is produced. If a micro- 

 scopical section be soaked in a solution of phosphomolybdic acid 

 and then exposed to light, the blue colour reveals that it was 

 present chiefly in the collagen; much less in the cytoplasm; less 

 again in muscle; and least of all in red blood corpuscles. This is 

 exactly the same distribution as is shown by methyl blue, and 

 phosphomolybdic acid thus acts as though it were a very slowly 

 diffusing acid dye. The anion is large, and its size is increased by 

 hydration. 



If a section be treated with phosphomolybdic acid and then with 

 one of the background or levelling dyes at low concentration in 

 the presence of the same acid, the background dye will colour 

 nothing except the red blood-corpuscles. Thus the phospho- 

 molybdic acid acts as a dye-excluder towards the background dye. 

 One is reminded of the use in the textile industry of 'resists', or 

 substances that prevent the subsequent action of dyes.''^ A mixture 

 can be made of normal and 'resisted' wool, and this gives a varie- 

 gated effect when dyed. Some of the substances used are colourless 

 sodium sulphonates, which act very much as though they were 

 colourless dyes. 



When a section is first treated with a typical background dye, 

 or with some other dye (such as acid fuchsine or azocarmine) that 

 diffuses more easily than aniline blue, and then with phospho- 

 molybdic (or phosphotungstic) acid, the latter competes with the 

 dye wherever it can enter. It enters the collagen most easily. If 

 the treatment be stopped at the right moment, the dye is turned 

 out of the collagen, but left in the cytoplasm, muscle, and red 

 blood-corpuscles. If now the section be rinsed and treated with 

 aniline blue or a similar dye, the collagen will be coloured ex- 

 clusively by this. 



The treatment with phosphomolybdic acid also helps differ- 



