FORMALIN 349 



water, certain salts in the latter are precipitated, and 

 the solution becomes turbid. (2) It is sold as a con- 

 centrated solution, and for our purposes it has to be 

 considerably diluted with water ; it is therefore a con- 

 venient fluid to take on board a ship, where storage 

 space is frequently limited, and on expeditions, when 

 weights have to be carefully considered. 



As a rule most marine animals can be satisfactorily 

 preserved in formalin, but there are exceptions, where 

 formalin is inferior to alcohol, or even quite useless. 

 It destroys completely certain kinds of calcareous 

 structures, such as the spicules in calcareous sponges 

 and the calcareous skeleton of some larvae. The dis- 

 solving away of calcareous substances is due to formic 

 acid, which is nearly always present in formalin. On 

 the other hand, shells of molluscs may be left for 

 years in formalin without showing any apparent 

 destruction. The action of formalin upon animal 

 tissues is variable, depending upon the chemical com- 

 position of the tissue ; it may harden, soften, or destroy. 



There is one group of animals for which formalin is 

 not suitable — namely, the Ctenophora. Its effect upon 

 the Ctenophores is interesting and worth mentioning 

 here. Pleurobrachia looks splendid after it has been 

 killed in formalin 5 per cent, in sea water, and remains 

 all right for a few months. Later on its jelly shows 

 visible signs of shrinkage. The shrinkage continues 

 until the specimen becomes quite shrivelled up, and 

 eventually disintegrates. Beroe frequently shrivels 

 up in the course of a few days, or the process of 

 shrinkage may be extended over a few months. Bolina, 

 one of the most delicate Ctenophores, dissolves at once 

 in formalin. 



Strength of Solution. — Before dealing with the 

 strengths of the solutions suitable for the preservation 



