COLLECTING, FIXING, &c. ii 



continued, the washing may be considered complete when the alcohol 

 no longer clears, but retains the brown colour. The specimens may 

 now be transferred to 92 per cent, alcohol, in which they may remain 

 indefinitely. 



5. Pianc Acid SohUion. 



A saturated solution of picric acid in 92 per cent, alcohol. 



This solution is used chiefly for fixing large pieces of tissue, its 

 action being very rapid. The specimens must remain in the solution 

 for several hours, or until the solution has completely penetrated 

 through them ; they must then be transferred to alcohol, which must be 

 changed at intervals for a day or two to wash out the acid as far as 

 possible. Vegetable tissue takes up picric acid very quickly and 

 retains it so persistently that even after prolonged and repeated 

 washings, it is scarcely possible to eradicate the acid until the tissue is 

 sectionised, and the sections passed through two or three changes of 

 alcohol . 



6. For7italdehyde. 



Formaldehyde, 40 per cent., 3 cc. 

 Water ... ... 97 cc. 



This is also made up in one to five per cent, solutions, according to 

 requirements. A three per cent, solution being, perhaps, the most 

 useful for all-round work. It is suited to most delicate objects, 

 but is used more frequently in animal than in vegetable work. A 

 two per cent, solution of formaldehyde may also be used as a mounting 

 medium, but a stronger solution has a tendency to turn the objects 

 opaque after they have been mounted for some weeks, and on this 

 account I prefer carbolised water as a mountant. 



