74 BASIC METHODS FOR EXPERIMENTS 



to be used. On one slide should be placed as many sections as 

 possible. 



Mounting the Sections 



The best microscope slides available should be used. Avoid 

 if possible the use of those of a greenish tint. Procure therefore 

 clear white slides with ground edges, free from defects. It is a 

 good plan to select from the boxes of slides when first opened 

 those of uniform thickness discarding the thicker ones and those 

 that are too thin. Those selected are washed singly with clean 

 hands in warm water and Castile soap, thoroughly rinsed and 

 placed upright in covered jars of 95 per cent alcohol. If how- 

 ever the slides remain murky, they are placed one by one in 

 jars of fuming nitric acid for about 24 hours. Next they are 

 washed in running tap-water, care being exercised not to scratch 

 them. They are then stored in 95 per cent alcohol until needed. 



The slide to be used is removed from the alcohol which should 

 run off without beading. It is then dried with clean bird's-eye 

 cloth free from lint. Next a drop of Mayer's albumen fixative 

 is conveyed to it by means of a glass rod. The drop of fixative 

 is rubbed evenly over the slide by the index finger previously 

 washed in soap and water and cleansed in ether. For sections 

 that have been fixed in solutions containing chromic acid one 

 uses more, and for those containing picric acid or formol, less, 

 of the albumen fixative. Experience alone teaches the amount 

 to use; too little means loss of sections, too much clouds the 

 sections with stained masses of albumen. 



The strips of paraffin are now placed consecutively on the 

 slide in as nearly parallel rows as possible, the first cut end of 

 each strip being about 5 millimeters away from the left end 

 of the slide. Three or four drops of distilled water are run 

 under the sections and the strips brought more closely parallel. 

 The slide is now gently warmed by holding it above the elec- 

 tric hot-plate or over the feeble flame of an alcohol lamp. But 

 the paraffin must not melt! In this wise the ribbons stretch and 

 become easy to arrange in perfectly straight parallel rows. The 

 water is drained from the slide; the slide is gently warmed 

 again to insure equal adhesion of the strips throughout their 

 extent; it is labelled on the right by scratching, with a splinter 



