HYDROFLUORIC ACID. 295 



the more so as it more effectually hinders any collapsing of the structures 

 that might result from the withdrawal of their supporting calcareous 

 elements. 



Picric acid fluids are good media, as though their action is slow they pre- 

 serve tissues well, and leave them in a good state for subsequent staining. 



568. Phosphoric Acid. 10 to 15 per cent. (HAUG, 1. c., in 559). 

 Somewhat slow, staining not good. 



569. Lactic Acid. 10 per cent, or more. Fairly rapid, preserves well, 

 and may be recommended (HAUG, 1. c.). 



570. Chromic Acid is employed in strengths of from O'l per cent, to 2 

 per cent., the maceration lasting two or three weeks (in the case of bone). 

 It is better to take the acid weak at first, and increase the strength gradually. 

 In any way the action is extremely slow, and it is therefore better to take 

 one of the mixtures of chromic acid with a more energetic agent. 



571. Chromic and Nitric Acid. Dissolve 15 grms. pure chromic acid 

 in 7 oz. of distilled water, to which 30 minims of nitric acid are afterwards to 

 be added. Macerate for three or four weeks, changing the fluid frequently 

 (Marsh). 



FOL takes 70 volumes of 1 per cent, chromic acid, 3 of nitric acid, and 200 

 of water (Lehrb., p. 112). 



See also 564. It remains to be added that even with the addition of 

 nitric or hydrochloric acid the action is excessively slow, frequently requiring 

 months to be complete. 



572. Arsenic Acid. 4 per cent, aqueous solution, used at a temperature 

 of 30 to 40 C. (SQUIRE'S Methods and Formulse, &c., p. 11). 



573. Glycerin. Alum-Carmine. It should be remembered that these 

 commonly used reagents dissolve carbonate of lime ; they must therefore be 

 avoided in the preparation of structures containing delicate calcareous ele- 

 ments that it is wished to preserve (calcareous sponges, larvae of Echino- 

 dermata, &c.). 



Desilicification. 



574. Hydrofluoric Acid (MAYEB'S method, Zool. Anz., 1881, No. 97, 

 p. 593). The objects from which it is desired to remove siliceous parts are 

 brought in alcohol into a glass vessel coated internally with paraffin (other- 

 wise the glass would be corroded by the acid). Hydrofluoric acid is then 

 added drop by drop (the operator taking great care to avoid the fumes, which 

 attack mucous membranes with great energy). A Wagnerella borealis may 

 thus be completely desilicified in a few minutes. Small pieces of siliceous 

 sponges will require a few hours, or at most a day. The tissues do not 

 suffer ; and if they have been previously stained with acetic acid carmine the 

 stain does not suffer ; at least, this was so in the case of Wagnerella. 



This dangerous method is best avoided. As regards sponges, I would 

 point out that if well imbedded, good sections may be made from them with- 



