CLEARING AND MOUNTING SECTIONS. 413 



position on the slide, and the jelly, just sufficient of which has 

 been melted, should be dropped on the section, and a cover, 

 previously breathed ui)on it as before, placed over it. The slide is 

 afterwards to be set aside until the jelly becomes firm, when the 

 cover may be ringed with Bell's cement. Other convenient prepar- 

 ations of glycerine, which set at the edges of the cover and thus 

 fix it to the slide, contain gum arabic as an ingredient. Hoyer's 

 medium contains in addition, chloral hydrate or acetate of potash, 

 according as it is to be used with sections stained with carmine or 

 h?ematoxylin, or with aniline colours. 



Canada Balsam. 



Of resinous media, Canada balsam is at once the type and the 



best in use. The raw material is not very suitable, however, since 



it contains a certain amount of oily matter, which prevents it 



setting satisfactorily. It is therefore desirable to heat it gently in 



an oven, until it is of such consistence that it becomes brittle 



when cold. By then dissolving in benzol, or xylol, in the proportion 



of about loo grammes to 50 c.c, it is rendered fit for use. If the 



menstruum be required to evaporate very slowly, xylol should be 



employed ; but for general purposes the benzol solution will be 



found preferable. Before these solutions can be applied to the 



sections, the latter must be dehydrated by means of methylated or 



absolute alcohol. When the former is employed, the sections must 



afterwards be "cleared" by immersion in oil of bergamot or oil of 



cloves, before mounting. After absolute alcohol, however, oil of 



cedar or xylol will act more satisfactorily. Oil of cloves is very 



generally used, but it is apt to dissolve out aniline colours and 



render objects very brittle, if they are left in it very long. As a 



rule, it is best to leave them in the clearing liquid just long enough 



to effect the desired purpose (entire removal of alcohol, indicated 



by the sections appearing perfectly translucent), then remove and 



mount straightway, by placing upon clean cover-glasses, covering 



with a drop of the benzol-balsam and immediately inverting upon 



a clean side which has been slightly warmed to remove the film of 



surface moisture always present upon glass exposed to ordinary 



temperatures. If any air-bubbles appear, gentle warming and 



careful manipulation of the cover-glass with a mounted needle will 



