MICROSCOPIC PREPARATIONS IN MARINE ZOOLOGY 319 



way changing their form. This is termed " fixing." There 

 are many " fixing " reagents viz. chromic acid, bichromate 

 of potash, corrosive sublimate, osmic acid, etc. and each 

 has its votaries. 



A good all round fixing solution, cheaper, and some 

 say better, than osmic can be made thus, Water 4 

 ounces, corrosive sublimate 40 grains, picric acid 60 

 grains, formalin 8 ounces. Dissolve the corrosive in hot 

 water. When cold, add the picric, then the formalin. 

 (This is known as " Bouin's Fluid.") 



For working on the first subject I shall select viz. 

 sponges the osmic is, I think, the best. It does not 

 allow of much subsequent " staining " of the preparation, 

 but it leaves it of a brown tint, which demonstrates as well 

 as any stain. 



The little sponge, sycon, which has either just been 

 gathered or kept as directed in the last chapter, is freed 

 from any adherent particles of sand, etc., and is dropped 

 quickly into a small, stoppered bottle containing distilled 

 water and some of the osmic acid solution, about equal 

 parts just enough to submerge the sponge need be em- 

 ployed, as it cannot be used again. 



In a little while the sponge will turn brown, and then 

 black. Let it remain there a few hours, and then wash it 

 in distilled water, and place it in dilute spirits of wine (say 

 half water). In a few hours place it in stronger spirit, and 

 after a longer period, say next day, place it in absolute 

 alcohol. 



In a water bath that is, any small jam pot, that can 

 be stood in a small saucepan, glue-pot fashion dissolve, say, 

 an ounce of paraffin-wax (not letting the saucepan contain 

 water to boil over into the wax). Now remove the sponge 

 from the absolute alcohol, just drain it, but not to dryness, 

 and place it in a little pot in oil of cloves, pressing it gently 

 down beneath the surface. When it has been there a while, 



