18 



CoRKOsiYE Sublimate (HgClg). A saturated solution of this 

 exceedingly poisonous substance in fresh water (or sea-water for 

 marine animals) is a very useful killing and preserving fluid. Its 

 action is extremely rapid, and is made even more so by heating. 

 The solution is best made by putting more of the crystals in a 

 vessel of water than will dissolve ; but as solution takes place 

 slowly the mixture should be made some hours I^efore it is likely 

 to be wanted. As the solution is used more water can be added 

 at intervals. So long as some of the crystals remain undissolved 

 the solution will remain " saturated." For delicate specimens 

 which are not calcareous it is useful to add a few drops of glacial 

 acetic acid to the solution. Steel instruments should not he 

 brought into contact with solutions of corrosive sul^limate. 



Objects hardened with corrosive sublimate should be soaked 

 for some hours in water or, better, in iodised alcohol (alcohol 

 70 per cent. 100 parts ; tincture of iodine 2 • 5 parts) before lieing 

 placed in alcohol for keeping. Otherwise the sublimate may 

 crystallise out in the tissues, and spoil the specimens, when 

 subsequently mounted for the microscope. 



N.B. — The saturated solution of sublimate looks exactly like 

 plain water, and the bottle containing it should be labelled 

 " Poison." 



Formalin. The commercial formalin as bought is a solution 

 of the gaseous compound formaldehyde in water. Its strength is 

 about 40 per cent. For use it may be diluted with several times 

 its bulk of water ; hence it is very convenient for certain purposes, 

 owing to its portability. It is also much cheaper than spirit. The 

 strength most convenient for preserving animals is a 10 per cent, 

 solution of formalin (or, which is the same thing, a 4 per cent, 

 solution of formaldehyde). This is made by diluting one volume 

 of commercial formalin with 9 volumes of water. After having 

 been preserved in this solution specimens may be kept in a 

 weaker one — say 5 per cent. (5 parts commercial formalin to 

 95 of water). 



Note. — As formalin does not give good results with many kinds 

 of animals, some caution must be used in employing it. It 

 is advisable not to preserve all the specimens of a kind in 

 it, but to keep some in alcohol. Further, formalin contains a 

 certain amount of acid, which is injurious to animals with 

 calcareous structures, such as worms with calcareous tubes, or 

 jaws in which the hardening matter is calcareous. Hence 

 Polychffites should generally be preserved in alcohol rather than 

 in formalin. For marine animals the formalin may be diluted 

 with sea-water instead of with fresh water — an advantage which 

 this reagent has over alcohol. N.B. — Formalin should not be 

 used for preserving Nematodes, as it is liable to cause the 

 specimens to burst. 



