16 FIXING. 



The process of washing out is often greatly facilitated by 

 heat. Picric acid, for instance, is nearly twice as soluble in 

 alcohol warmed to 40 C. as in alcohol at the normal tempe- 

 rature (Fol). 



In the case of marine organisms it may be stated as a general 

 rule that their tissues are more refractory to the action of 

 reagents than are the tissues of corresponding fresh- water or 

 terrestrial forms, and fixing solutions should in consequence 

 be stronger (about two to three times stronger, according to 

 Langerhans) . 



Marine animals ought to be freed from the sea-water adhe- 

 rent to their surfaces before treating them either with alcohol 

 or any fixing reagent that precipitates the salts of sea water. 

 If this be not done, the precipitated salts will form on the 

 surfaces of the organisms a crust that prevents the penetra- 

 tion of reagents to the interior, thus allowing maceration to 

 be set up, and hindering the penetration of staining fluids. 

 Fixing solutions for marine organisms should therefore be 

 such as serve to keep in a state of solution, and finally remove, 

 the salts in question. They should never be made with sea 

 water as a menstruum, as some workers have inconsiderately 

 proposed. If alcohol be employed it should be acidified with 

 hydrochloric acid. Picro-sulphuric acid is a reagent that 

 fulfils the conditions here spoken of. (On this subject see 

 Paul Mayer, in Mitth. ZooL Stat. Neapel, ii (1881), p. 1, 

 et seq. See also the abstract in Journ. Roy. Mic. Soc. (N.S.), 

 ii (1882), pp.866 881, and that in Amer. Natural, xvi (1882), 

 pp. 697706.) 



