[7] PRESERVATION OF MICROSCOPIC MATERIALS. 613 



A couvenieut way for the traveling- collector to carry the " stock" for 

 this mixture is to have the solid ingredieuts put up in bulk by an apothe- 

 cary. The bichromate of potash should be pulverized in a mortar, so 

 as to dissolve tlie more readily, and mixed with sodic sulphate in the 

 proportion of 5 parts to 2 of the latter. This mass can then be weighed 

 out into parcels of 1 ounce, 1 dram, 1 scruple, which is exactly the 

 quantity of solid material needed to mix with 1 quart of water (wine 

 measure). These parcels containing the dry powder ready mixed can 

 then be carried conveniently and the fluid mixed, to the amount of a 

 quart at a time, whenever required. 



After the objects have been kept in this solution for a month or so, 

 the}" should be transferred to about forty times their own volume of 

 water and the water changed every day for two or three days, in order 

 to get rid of the bichromate of potash, when the specimens may be 

 transferred to 70 per cent, alcohol. It has been objected that Mullers 

 fluid ijroduces precipitates in the cavities of objects, but there are few 

 preservatives which do not, and on account of the convenience with 

 which it may be used by collectors, it is, next to alcohol, probably, one 

 of the most useful of all hardening and preservative compounds. 



Ghromio acidJ.^-Solutions of this substance have been extensively used 

 for the hardening of embryological and histological materials, and either 

 alone, or, better still, in combination with other substances, is still rec- 

 ognized as one of the most useful reagents for this purpose. Whitman, 

 speaking of it, says : " Chromic solutions have, in common with osmio 

 acid, the peculiarity of hardening by virtue of the chemical combina- 

 tions which they form with cell-substances, and all the consequent dis- 

 advantages with respect to staining. The use of chromic acid in the 

 zoological station of iS^aples maj- be said to have been largely superseded 

 by picro-sulphuric acid, corrosive sublimate, and MerJceUs fluid, for it is 

 now seldom used except in combination with other fluids. It is some- 

 times mixed with Kleinenberg's fluid, for example, when a higher de- 

 gree of hardening is required than can be obtained by the use of the 

 latter fluid alone. It is a common error to use too strong solutions of 

 chromic acid, and to allow them to act too long. Good results are in 

 some cases obtained when the objects are treated witb a weak solution 

 (one-third to one-half of 1 per cent.) and removed soon after they are com- 

 pletely dead." Weak solutions of one-half to one-fourth of 1 per cent., or 

 even less, are also recommended by Semper, who allows it to act only for 

 a short time, or until the cells are killed. But it is important in any 

 case that as much as possible of the acid should be extracted by sub- 

 sequent immersion of specimens treated with it in water or weak alco- 

 hol, since its presence often renders subsequent staining with carmine 

 difficult. It interferes with the staining by means of the aniline dyes 

 much less perceptibly, and in the case of some of those most diffusible 

 and soluble in alcohol or water scarcely at all. Flemming uses ex- 

 ceedingly dilute solutions of chromic acid in order to fix the cleavage 



