ZOOLOGICAL STATION IN NAPLES. 101 



Picro-nitric acid may be used. It is prepared as follows : — 

 Water ... ... ... ... 95 parts. 



Nitric acid (25 per cent. N2O5) ... 5 „ 



Picric acid (as much as will dissolve).* 



Picro-nitric acid also dissolves carbonate of lime, but it holds 

 it in solution, and thus the formation of crystals of gypsum is 

 avoided. In the presence of much carbonate of lime, the rapid 

 production of carbonic acid is liable to result in mechanical 

 injury of the tissues ; hence, in many cases, chromic acid is 

 preferable to picro-nitric acid. 



Picro-nitric acid is, in most respects, an excellent preservative 

 medium, and as a rule will be found to be a good alternative in 

 those cases where picro-sulphuric acid fails to give satisfactory 

 results. Dr. Mayer commends it very strongly, and states that 

 with eggs containing a large amount of yelk material, like those of 

 Palinurus^ it gives better results than nitric, picric, or picro-sulphu- 

 ric acid. It is not so readily removed from objects as picro- 

 sulphuric acid, and for this reason the latter acid should be used 

 wherever it gives equally good preparations. 



2. Alcohol. — In the preparation of animals or parts of 

 animals for museums or histological study, it is well known that 

 the chief difficulties are met in the process of killing. Alcohol, 

 as co7mno7ily used for this purpose by collectors, has little more 

 than its convenience to recommend it. Dr. Mayer has called 

 attention to the following disadvantages attending its use in the 

 case of marine animals : — 



(i) In thick-walled animals, particularly those provided with 

 chitinous envelopes, alcohol causes a more or less strong macera- 

 tion of the internal parts, which often ends in putrefaction. 



(2) In the case of smaller Crustacea, — e.g,^ Amphipods and 

 Isopods, — it gives rise to precipitates in the body-fluids, and thus 

 solders the organs together in such a manner as often to defy 

 separation even by experienced hands. 



(3) It fixes most of the salts of the water adhering to the 



* This mixture is used undiluted. 



