176 GUIDANCE IN MAKING PERMANENT PREPARATIONS. 



their size. Three minutes will suffice for killing hydroids in 

 sublimate-acetic. 



Washing. All objects must be thoroughly washed, after using 

 most killing agents. This may be done with repeated changes 

 of fresh water or with alcohol, beginning with a low grade and 

 gradually working up to 70 percent. With most small objects 

 alcohol is preferable, but if the object is large this is too expen- 

 sive. 



In case a fixing agent is used which is an alcoholic solution, 

 wash out in the same grade of alcohol used in making the fixing 

 agent. 



Dehydrating and Staining. From water, all objects should 

 be placed successively in 35 percent, 50 percent, and 70 percent 

 alcohol, five to fifteen minutes in each for small objects. In 

 subsequent changes from one grade to another allow about the 

 same time. All tissues killed in a corrosive sublimate mix- 

 ture should now be treated with a weak solution of iodin, 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 increas- 

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

 into the 70 percent alcohol containing the object, leave a few 

 minutes, and, if the yellow color caused by the iodin has disap- 

 peared, turn off the alcohol and use more 70 percent alcohol 

 with iodin, as before. The bleaching indicates that some cor- 

 rosive sublimate remains. Repeat until the yellow color does 

 not fade. Then wash in clear 70 percent alcohol. At this point 

 either staining, or preparation for so doing, begins. 



In case the stain you wish to use is a 70 percent 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 percent and 35 percent alcohol to water. 

 If a 95 percent alcoholic stain is to be used, pass through 80 per- 

 cent and 95 percent alcohol. 



