CORROSION, DECALCIFICATION, ETC. 327 



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) . Small pieces 

 of siliceous sponges will be completely desilicified in a few 

 hours, or at most a day. The tissues do not suffer. 



This dangerous method is best avoided as far as possible. As regards 

 sponges, I would point out that if well imbedded good sections may be 

 made from them without previous removal of the spicula. The spicula 

 appear to be cut; probably they break very sharply when touched by the 

 knife. Knives are of course not improved by cutting such sections. 



ROUSSEAU imbeds the objects in celloidin, as described 558, then brings 

 the block, in a covered caoutchouc dish, for a day or two into a mixture of 

 50 c.c. alcohol and 20 to 30 drops of hydrofluoric acid, and washes out the 

 acid with alcohol containing carbonate of lithia in powder. 



Bleaching. 



575. MAYER'S Chlorine Method (Hitth. Zool. 8 tat. Neapel, 

 ii, 1881, p. 8). Put into a glass tube a few crystals of 

 chlorate of potash, add two or three drops of hydrochloric 

 acid, and as soon as the green colour of the evolving chlorine 

 has begun to show itself, add a few cubic centimetres of 

 alcohol of 50 to 70 per cent. Now put the objects (which 

 must have previously been soaked in alcohol of 70 to 90 per 

 cent.) into the tube. They float at first, but eventually 

 sink. They will be found bleached in from a quarter of an 

 hour to one or two days, without the tissues having suffered. 

 Only in obstinate cases should the liquid be warmed or more 

 acid taken. Sections on slides may be bleached in this way. 

 Instead of hydrochloric acid nitric acid may be taken, in 

 which case the active agent evolved is oxygen instead of 

 chlorine. 



This method serves both for removing natural pigments, 

 such as those of the skin or of the eyes of Arthropods, and 

 also for bleaching material that has been blackened by 

 osmic acid, and, according to renewed experiments of 

 MAYER'S, is to be preferred to the peroxide of hydrogen 

 method. 



For bleaching chitin of insects, not alcohol but water 

 should be added to the chlorate and acid (MAYER, Arch. Anat. 

 Phys., 1874, p. 321). 



