MOUNTING IN CANADA BALSAM. 37 



this nine or ten times, the mixture (which will then be about a 

 90 per cent, alcohol) can be removed with the pipette as com- 

 pletely as possible, and absolute alcohol added. For the sake 

 of economy at each stage up to the last the strongest methylated 

 spirit may be used without any very serious reduction in the per- 

 centage strength of the mixture, 1 but it is well then after the 

 complete removal of the last stage of mixture as above to first 

 add pure methylated spirit; then pipette it off and add abso- 

 lute alcohol. If the dehydration be naturally very complete, 

 as for example with alcohol material dyed with an alcohol stain, 

 the preparation may usually be transferred direct to Canada 

 balsam ; usually, however, the replacement of the dehydrant is 

 desirable. This is best effected with xylol (or oil of cloves may be 

 used). Xylol is very sensitive to the smallest traces of water, 

 which would make the preparation milky, and hence more or 

 less opaque ; previous dehydration must therefore be complete. 

 Milkiness is best observed against a black background, and, if 

 present, the section must again be placed in absolute alcohol, 

 and once more in xylol. Finally, mounting consists in trans- 

 ferring the preparation to a solution of Canada balsam in 

 turpentine or in xylol as thick as treacle, and covering with a 

 cover-glass. This will in time set hard ; but it is preferable 

 when quite set to run a line of gold size round the edge of 

 the cover-glass, overlapping it as little as possible. This last 

 precaution is quite necessary if at any time an oil-immersion 

 lens is to be used, since Canada balsam is soluble in the immersion 

 oil. Further notes on mounting will be found in Chapter VIII. 



1 An accurate percentage table will be found in one of the appendices* to 

 this book. 



