36 II. FOOD RESERVES IN SITU. 



them in dilute watery solution of tannin for about ten minutes, 

 and then, after careful washing in water, treating with a 2 per 

 cent, solution of osmic acid. After washing out the osmic acid 

 the preparations may be put up in glycerine or in glycerine- 

 jelly. The crystalloids are permanently stained brown (tannate 

 of albumen). By the same reagent (osmic acid) the oil, if 

 not removed, is slowly blackened a peculiarity which fat oils 

 have in common with ethereal oils ; this reaction is, however, 

 not characteristic, as many other organic substances become 

 blackened by osmic acid. Or, after hardening, place for about 

 an hour in a 25 per cent, watery solution of tannin, wash in 

 water, and then the sections may be placed for an hour in 10-20 

 per cent, solution of ferrous sulphate, again washed, and mounted 

 in Canada balsam. The crystalloids are then a deep-blue colour. 

 Mounting in Canada Balsam. To mount a preparation suc- 

 cessfully in Canada balsam requires special preliminary treat- 

 ment. It must be remembered that no trace of water must be 

 introduced into Canada balsam ; hence perfect dehydration of 

 the section becomes necessary. There are usually, indeed, three 

 stages in the method of mounting a preparation in this mountant ; 

 dehydration, replacing the dehydrant by some fluid which is 

 perfectly miscible with Canada balsam, and the mounting itself. 

 Dehydration is usually effected by alcohol. In the present 

 instance, though the material had been placed for a time in 

 alcohol, stains more or less watery may have been used ; it will, 

 however, suffice to place the section for a few minutes in absolute 

 alcohol to secure dehydration ; but it will be readily understood 

 that no section coloured with a stain which, like magenta or most 

 of the aniline dyes, is soluble in alcohol, can be treated in this 

 way, for the colour will, of course, be more or less completely 

 dissolved out. Sections stained with these cannot therefore be 

 dehydrated in this (the ordinary) manner. Hereafter one may 

 wish to mount in Canada balsam very watery preparations the 

 direct transfer of which to strong or absolute alcohol may lead to 

 shrinkage or collapse, so that gradual dehydration becomes neces- 

 sary. Various processes are in vogue ; a very simple one, and 

 one I [ED.] have found quite effective, and relatively economical, 

 is to place the sections in a 10 per cent, solution of alcohol, made 

 by adding 4f c.c. water to ^ c.c. absolute alcohol ; then at intervals 

 of a few minutes each removing with a pipette about 1 c.c. of the 

 mixture then adding 1 c.c. absolute alcohol ; when you have done 



