METHODS. 3 



can of course only be done when the sections are to be 

 mounted in glycerine. 



When picric or chromic acid is used, the object 

 should be immersed in it for several hours ; the length 

 of time varies with different material, and, in the case 

 of chromic acid, with the strength of the solution used, 

 from a few minutes to twenty-four hours. The objects 

 must then be washed with dilute alcohol (50 per cent.), 

 and then placed in stronger alcohol (70 per cent.), and 

 finally in absolute alcohol (or 90 per cent.), which 

 must be changed so long as any colour is still extracted 

 from the objects. They may be preserved in this for 

 future use. 



When osmic acid is used, the fixing effect is pro- 

 duced much more rapidly; in the case of simple 

 structures, such as unicellular or filamentous Algae 

 a few minutes (5 15) generally suffices; in the case of 

 more complex structures, such as ovules, sporangia, 

 growing-points, &c., the object may be left in the acid 

 till it looks black on the exterior : it must be then well 

 washed with dilute alcohol (50 per cent.), and left in it 

 for \some time, and be then removed to 70 per cent. 

 The sections are best mounted in dilute glycerine. In 

 some cases osmic acid produces an excessive blackening 

 of the cells, which can be removed by treatment with 

 chlorine-water. 



It is advisable in cases in which the cell-walls tend 

 to swell up excessively (as in many Algas) to use solu- 

 tions of picric, chromic, and osmic acids, to which an 

 equal volume of absolute alcohol has been added. 



Of these hardening reagents the most serviceable are 

 absolute alcohol, or 90 per cent, alcohol, and picric acid. 



B 2 



