THE USE OF FORMALIN AS A PRESERVATIVE 



FLUID. 



BY JAMES HOKSELL. 



For considerably more than a year I have had the action of 

 Formalin as a preservative fluid under constant observatiou, and, as 

 the results may prove useful to many readers, I will now summarise 

 them as much as possible. 



The fluid as commercially sold is a 40 % aqueous solution of 

 formaldehyde, but in use, for ease in reckoning percentages, this 

 must be considered as representing 100 %. Thus a 5 % solution of 

 formalin is made up of 5 parts of the fluid as commercially sold, 

 diluted with 95 parts of water. 



The chief biological value of formalin is essentially in the pre- 

 paration and preservation of specimens intended for dissection and 

 Museum purposes ; in microscopical technique — for reasons to be 

 mentioned later — its use is entirely secondary and limited. 



Reviewing its application among the various phyla of animals we 

 find that : — 



SPONGES are most beautifully preserved by simple immersion 

 without previous fixation. No contraction of even the most delicate 

 membrane is observable, and where thsre is natural transparency or 

 semi-opalescence, this is perfectly retained — Halisarca is a good 

 instance in point : spirit specimens become opaque and leathery- 

 looking ; formalin ones retain the characteristic jelly-like appearance 

 with, however, a sensible stiffening that is just sufficient to become 

 valuable as diminishing the danger of injury to the specimen in 

 handling, &c. 



Hydrozoa. Hydroid stocks of the smaller sizes, being usually 

 required for microscopical examination, mounted in balsam, are best 

 preserved in the ordinary way by fixing and subsequent grading into 

 spirit ; this, because I find that the staining of formalin-preserved 

 specimens is inferior to that of well fixed spirit ones, as differention 

 is comparatively poor in the former case. For Medusae, great and 

 small, and for many Siphonophores, formalin finds one of its most 

 valuable applications. For instance, the large Medusas, Aurelia, 

 Chrysaora, and Rhizostoma are preserved perfectly by simple 

 immersion in a 5% solution. No special care whatever is needed, 

 except that the animals should be perfectly healthy and active when 

 put in. They may remain in the original fluid indefinitely, though 

 in the case of animals containing a maximum of water in the tissues, 

 as do the large medusas, it is advantageous to change to a fresh 

 solution in the course of several days. The advisability for this 

 depends very greatly upon the relative volume of the fluid contained 

 in the receptacle as compared with the bulk of the animals immersed 



