14 HABDENING AGENTS. 



called dehydrating tubes. The plaster of paris diaphragm can be 

 made by first constructing a mould of the desired size, with a paper 

 bottom and a card-board hoop for the outside. This must be 

 placed on a level surface. The plaster of paris is then mixed with 

 water and poured into the mould to about the depth of 1 c. m. 

 While it is yet soft the three legs can be inserted near the edge and 

 holes for the dehydrating tubes cut in the disk with a knife or 

 pressed out with glass tubing of convenient.size. When the plaster 

 is dry the hoop can be removed and the disk placed in position in 

 the jar, which is then filled with alcohol to within about 2 c. m. of 

 the under side of the plaster. The dehydrating tubes should be 

 about 12 c. m. long and can be made by cutting off the bottom of 

 large test tubes. In one end is placed a diaphragm of chamois 

 skin, which can be fastened in position by means of a spring made 

 of steel wire or ribbon and forced with the chamois skin in the 

 tube. A rubber band around the tubes prevents them from falling 

 through the holes in the disk and enables them to be lowered to 

 any desired depth in the alcohol. 



The tissue to be dehydrated is packed closely in the dehyrat- 

 ing tube and just enough 50 per cent, alcohol added to cover it. 

 This is then quickly lowered through the hole in the disk, until the 

 two liquids are at a level. After from 12 to 24 hours, by osmosis, the 

 two liquids will be of the same strength. The tissue can then be 

 taken out and placed in the infiltrating bath at once. This method of 

 hardening has been tried on nearly all kinds of plant tissue, and in 

 almost every case it was found to be successful. For the most del- 

 icate tissues where slow hardening is desired, 5 per cent, alcohol 

 can be placed in the dehydrating tube, and thick chamois skin used 

 for a diaphragm, while for some of the more delicate algae it has been 

 found advisable to use as low as 1 per cent, alcohol in the tube. 



The strength of the alcohol in the jar can be kept up by add- 

 ing to it from time to time some calcic chloride, which will in no 

 way injure the alcohol. The jar should be tall enough to allow 

 the cover to be kept on while the tubes are in position and thus 

 prevent evaporation. 



An apparatus of such a form with a dozen dehydrating tubes 

 can be used for months without changing the alcohol. Other har- 

 dening agents such as picric, chromic, acetic, or osmic acids can be 



