162 



The Naturalist. 



pipette to inject it full of the acid. Time should uot be giTen for 

 the blood to coagulate and fasten the organs together, nor should the 

 object be allowed to remain for any length of time in a turbid liquid. 

 Too much stress cannot be laid on this simple rule, for in its non- 

 observance lies the occasion of so many unsatisfactory results. How 

 long an object ought to remain in the acid depends greatly on its 

 nature. Usually a few hours is sufficient, but for large objects, and 

 thdse containing a large per-centage of water, a longer time is neces- 

 sary, and also an occasional stirring up or change of liquid. Often, 

 if left in the acid a whole day, the objects appear to sustain no 

 injury. While in chromic acid the tissues are apt to become too 

 brittle unless taken out in time, in picrosulphuric acid there is a 

 certain death to the cells, but no true hardening. In cases where 

 carbonate of lime, in the form of gypsum, forms a part of the animal 

 structure, this would be acted on by the sulphuric acid, and there 

 would be a softening rather than a hardening of the whole structure. 

 When the objects have remained a sufficient time in the acid, they 

 should be transferred to 70 % alchohol. As long as any colour is 

 given to the alcohol it must be continually changed, for until every 

 trace of picric acid has been removed the object will not stain 

 properly. This is often a tedious process, and requires long washing 

 and continuous stirring up of the liquid. Small objects may be 

 conveniently placed under the microscope to see if any tinge of the 

 yellow picric acid remains. When every trace of colour has been 

 dissolved out, the preservative process is complete, and the objects 

 may be kept in 90 % alcohol for luither study. 



Picro-sulphutic acid must not be used for animals containing 

 carbonate of lime, where that is desired to be preserved. For 

 example, in Echinoderms the carbonate of lime is dissolved by the 

 picrosulphuric acid, and then thrown down as crystals of gypsum in 

 the tissues. In its use with vertebrates it must always be borne in 

 mind that the acid swells up the connective tissues. Prof. Emery 

 finds this process very useful for embryos of vertebrates and for 

 fishes, but they should not be allowed to remain in the acid more than 

 three or four hours. Kleinenberg has recently used creosote made 

 from, beechwood tar along with the acid, to avoid the swelling-up of 

 the tissues, with very lair results. Although this method is con- 

 siderably the best for preserving Crustacea as a rule, it will not do at 

 all for the parasitic species. It causes the object to swell up to such 

 an extent as often to rupture the walls. In order to avoid the 

 deposition of gypsum crystals in those objects containing carbonate 



