280 JIANIPULATION. 



Glycerine.— TWis fluid (the sweet principle of fats and oils), 

 lately recommended by Mr. Warington for preserving delicate 

 animal and vegetable tissues, has many advantages over other 

 fluids, not only in keeping the green colours of infusoria, but, 

 as it evaporates very slowly, it may be employed in cells 

 without so much care being necessary in the cementing of 

 their covers. It is miscible with water in all proportions; 

 the most convenient strength for use will be one part of 

 glycerine to two of distilled water ; if made weaker than this, 

 confervas will readily grow in it when exposed to the air, but 

 not when sealed up in cells. If pure glycerine be employed 

 it wUl act on the marine-glue, and from its highly refracting 

 properties, many delicate structures wiU be entirely lost in it, 

 as in Canada balsam. The best glycerine is procured in the 

 manufacture of lead plaster (the emplastrum plumbi of the 

 Pharmacopceia) ; it also remains in great abundance after the 

 formation of soap ; but in this latter case it always is mixed 

 with some free alkali, which renders it unfit for use. 



Castor Oil was first recommended by Mr. Warington, in 

 1844, as a medium for mounting certain classes of objects, 

 especially crystals; he has also found that it answers equally 

 well for minute fungi and parasitic insects ; before being em- 

 ployed, it should be carefully examined, to see that it be per- 

 fectly free from all crystalline deposit. 



Chromic Acid is useful for mounting some very delicate 

 preparations; it was first used by Mr. Warington, in 1842, 

 as a preservative agent, being part of a patent for a new mode 

 of tanning ; it has since been employed by Mr. Bowman and 

 M. Briicke to harden the vitreous humour of the human and 

 other eyes, in order to detect its real structure. Chromic 

 acid may be procured in the crystalline state, and should be 

 dissolved in so much water as will render the tint of the 

 solution a pale straw colour. A much weaker solution than 

 this wiU keep most animal tissues. 



Salt and Hater. — A solution made in the proportion of five 

 grains of salt to an ounce of distilled water will answer for 

 keeping very many animal and vegetable structures ; it was 

 first recommended by Dr. Cook, more than twenty years ago. 



