DEHYDRATING AND STAINING 



247 



to dissolve the corrosive sublimate that still remains, and 

 thus prevent the later formation of crystals of that substance. 

 Such crystals would not appear immediately, but ever in- 

 creasingly, as the preparation is kept. Put a few drops of 

 iodine into the 70 per cent alcohol containing the object, 

 leave a few minutes, and, if the yellow color caused by the 

 iodine has disappeared, pour off the alcohol and use more 

 70 per cent alcohol with iodine, as before. The bleaching in- 

 dicates that some corrosive sublimate remains. Repeat until 

 the yellow color does not fade. Then transfer to clear 70 

 per cent alcohol. At this point either staining, or prepara- 

 tion for so doing, begins. 



In case the stain you wish to use is a 70 per cent alcoholic 

 solution, it may be used immediately. Otherwise, the object 

 must be run through the grades of alcohol, up or down as the 

 case may be, to that medium in which the stain to be used is 

 dissolved. If an aqueous stain such as alum carmine is to 

 be used, pass through 50 per cent and 35 per cent alcohol to 

 water. If a 95 per cent alcoholic stain is to be used, pass 

 through 80 per cent and 95 per cent alcohol. 



The time an object should be treated with stain varies 

 with the stain and the size of the object. Alum carmine 

 should be used from six to twenty hours, according to cir- 

 cumstances. Borax carmine should be used for from five 

 minutes to half an hour. Aceto-carmine, used for killing and 

 staining, acts very rapidly. Delafield's hematoxylin (a dark 

 wine-colored solution in water) requires ten minutes to half 

 an hour. In all these cases, examination of the objects them- 

 selves is the only means of deciding when staining is suffi- 

 cient. It is usually best to overstain slightly and then to 

 bleach out, as certain parts of the protoplasmic structure 

 will retain the stain better than others, and thus better dif- 

 ferentiation will be secured. After staining, bring the tissues 

 gradually into 70 per cent alcohol, and then treat with acidu- 

 lated alcohol to remove excess of stain. After this, every 

 trace of the acid must be removed by washing in clean al- 



