266 Modern Microscopy 



Test. — Ordinary methylated spirit of the shops turns 

 turbid upon the addition of water, and is useless for 

 scientific work. The other variety can be mixed with 

 water to any extent and remain clear. We speak of this 

 special methylated spirit as 90 per cent, alcohol. 



Dehydrating and Clearing.— After fixation, the object 

 is passed through a few changes of the 90 per cent, 

 alcohol — say, three changes of twenty-four hours each 

 — to rid it of the acetic and to complete the harden- 

 ing. It is then passed into absolute alcohol from two 

 to several hours, according to the size and density of 

 the tissue, in order that it may be thoroughly dehydrated. 

 From the absolute alcohol it is transferred to a mixture of 

 pure xylol and absolute alcohol, the exact proportion of 

 which is not of importance, half and half being a useful 

 combination. In this xylol-alcohol the object must remain 

 for several hours until partially cleared. Subsequently the 

 clearing is completed in pure xylol. When quite clear, 

 which may readily be seen by the transparent and hyaline 

 character of the object, it is ready for the embedding bath. 

 Great care must be taken that the object is thoroughly 

 cleared by the xylol, otherwise failure will result from the in- 

 ability of the paraffin- wax to mix with alcohol (see p. 269). 



Final Paraffin Infiltrating Bath. — Previous to the 

 final infiltrating - bath of paraffin, it is well to place 

 the object when taken from xylol into a vessel contain- 

 ing at the bottom sufficient cold and solid paraffin to 

 cover the object when melted. Place then the object 

 from the last xylol-bath upon the cold paraffin, and cover 

 the object with pure xylol ; the whole should then be 

 placed upon the top of the oven, protected from dust by 

 the sheet of plate- glass already mentioned (p. 259). Here 

 the paraffin will slowly melt and mix with the evaporating 

 xylol ; after a couple of hours or so a more rapid evapora- 

 tion of the xylol may be brought about by placing the 

 vessel inside the oven for a few hours. Subsequently the 

 object is transferred to pure melted paraffin of the melting- 



