124 A MANUAL OF BACTERIOLOGY 



or an alcoholic one. After staining they are well rinsed 

 in water or alcohol to remove the excess of stain, and are 

 then dehydrated and cleared before being mounted. For 

 dehydrating, if they have been washed in water, they 

 should be well rinsed in methylated spirit 1 to remove the 

 excess of water, and then transferred to absolute alcohol 

 for a few seconds to two minutes, the time varying with 

 the size and thickness of the section and depth of staining 

 required. In many cases for instance, when the anilin 

 dyes have been used for staining the sections must be 

 passed as rapidly as possible, consistent with thorough 

 dehydration, through the absolute alcohol to avoid remov- 

 ing too much of the colour. If it is important to avoid 

 any decolor isation, anilin oil may be used for dehydration, 

 as in Weigert's method (pp. 120 and 127). For clearing, 

 xylol or cedar oil is the best agent, for neither dissolves 

 the anilin dyes ; they will only clear, however, out of 

 absolute alcohol, hence the preliminary rinsing of water- 

 washed sections with methylated spirit to prevent dilution 

 of the subsequent bath of absolute alcohol. Oil of cloves 

 can also be employed, but has the disadvantage that it 

 dissolves the anilin dyes, and the colour of stained sections 

 treated with it is apt to be less permanent ; it has the 

 advantage, however, of clearing out of methylated spirit, 

 absolute alcohol being unnecessary. The alcohol and 

 clearing agents are conveniently placed in watch-glasses 

 or small shallow glass capsules. The section is known to 

 be cleared when it appears quite transparent and almost 

 invisible when the watch-glass or capsule containing it is 

 held over a dark surface. If after two minutes in the 

 clearing agent the section still appears cloudy and opaque, 

 it has not been sufficiently dehydrated, and should be 



1 Absolute alcohol may of course be employed instead of the first bath 

 of methylated (or rectified) spirit, but methylated answers just as well and 

 is less expensive (but see note, p. 102). 



