io6 
R. C. McLean. 
b. Size of Material. The thickest sections soak out best and 
material in bulk better than any sections. For this reason it is 
best to complete the recovery before proceeding to section, rather 
than trust to swelling out the latter after cutting. 
c. Concentration of Medium. An aqueous solution of Potassium 
Hydrate of 4% to 8% strength. The former gives good results with 
Thallophytes, but a concentration not less than the latter is advis¬ 
able for vascular material. 
d. Time. Transfer the pieces from distilled water to Potash 
and leave them to soak for 6 to 9 days according to size and the 
degree of alteration in form which the material has undergone. 
A decided improvement has been noticed if the soaking in 
Potash is allowed to proceed normally for about half the total time 
given, and thereafter the solution gradually concentrated down to 
about one-third of its original volume, during the remaining days. 
This may be advantageously performed under reduced pressure 
(see below). 
e. Temperature. A moderate rise in temperature (up to 
70°C.) during the preliminary soaking in water is all to the good and 
shortens the time in Potash to some extent. Prolongation of the 
water soaking also has this effect. 
Heating in the Potash means ruination to cellulose tissues. 
The action of Potash upon the tissues is not perfectly even. 
Thin walled parenchyma are perfectly restored. Epidermis, 
collenchyma, sclerenchyma, xylem and xylem parenchyma are 
equally good. Phloem and cambium are the least satisfactory, 
although if no great amount of sclerenchyma be present, so that 
drying does not produce destructive internal strains, these too may 
be completely restored. 
The cognate problem of the removal of air is best resolved by 
the application of reduced pressure during the soaking in Potash, 
which may be associated with the gradual concentration of the 
medium advocated above. It is greatly aided, however, by perfect¬ 
ing the initial penetration with Absolute Alcohol in the manner 
above indicated. 
The last process is neutralization. For this purpose mineral 
acids should be avoided. It is best accomplished by weak acetic 
acid (15-20% of Glacial) changed several times, which causes no 
noticeable solution of the membranes. Wash until the water, 
after standing with the object in it for several hours, appears 
neutral to litmus. If the material is very deeply coloured the acetic 
