178 CHAPTER XIII. 



agent in them is a compound of haematein with alumina 

 (much as in carmine the active agent is a compound of car- 

 minic acid with alumina, cf. 206).* These solutions of 

 alumina-hgematein lakes are in general the ones employed 

 for substantive staining (see 202). Besides these, some 

 other compounds, viz. those of chrome, iron, copper, vana- 

 dium, and molybdenum, are also employed in histology, but 

 mostly in the adjective way of staining ( 202). The com- 

 pounds of haematoxylin with the other heavy or alkaline 

 metals have been tried, but do not afford histological stains. 

 Neglecting all these for the present, let us return to the 

 consideration of the stains composed of the alumina lakes. 



The tirst difficulty with which the worker with these 

 solutions has to contend is that of getting his haematoxylin 

 duly oxidised into haematein, in order to the formation of the 

 desired hasmatein-alumina compound, or lake. If this be 

 done by the hitherto customary process of leaving the solu- 

 tions to " ripen " by the action of the air, it is necessary to 

 wait for a long time before the reaction is obtained. During 

 all this time, it may be weeks or months, there is no means, 

 except repeated trial, of ascertaining whether the solution at 

 any moment contains sufficient hsematein to afford a good 

 stain. And here a second difficulty arises ; the oxidising 

 process continuing, the solutions become (t over-ripe ; " the 

 haematein, through further oxidation, passes over into colour- 

 less compounds, and the solutions begin to precipitate. They 

 are therefore, in reality, a mixture in constantly varying 

 proportions of " unripe," "ripe/' and " over-ripe " con- 

 stituents (the first and last being useless for staining pur- 

 poses), and, in consequence, their staining power is very 

 inconstant. 



The great point in Mayer's work is that not haematoxylin, 

 but hsematein, should be taken in the first instance for making 

 the staining solutions. This at once relieves us from the 

 tedious and uncertain process of "ripening" in the old way. 

 We have a ripe solution to begin with, and we know that it 

 must be ripe. A discovery of Unna's, to be mentioned below, 



* I restrict this assertion to the case of the alurmna-haematein stains, as 

 it seems possible that in the iron or chrome processes the colouring agent 

 may be haBinatoxylin, or even some higher oxidation product of it than 

 hsematein. Cf. MAYEE in Anat. Anz., xiii, 1897, p. 318. 



