PRESERVATION OF SPECIMENS IN PLASTICS 4T) 



ency to harden rapidly if exposed to room temperature for any length 

 of time . 



6. When the partial polymerization has been completed, allow the 

 residual monomer in the vented stopper to evaporate, and if necessary 

 wash away any partial polymer that may have accumulated in it, using 

 acetone or a mixture consisting of 80 parts of toluene and 20 parts of 

 'denatured alcohol. The residual sirup in the flask may be left to com- 

 bine with the new lot of monomer if the operation is to be repeated 

 soon; otherwise it should be thinned with a small amount of monomer 

 and poured into the low viscosity partial polymer storage jar, and the 

 flask then rinsed with acetone or the toluene-alcohol solvent mixture. 



Other methods. — Two other procedures may be employed, if desired, 

 for preparing partial polymer sirup. One that requires considerable 

 amount of lapsed time but very little attention consists in preparing, by 

 ^application of heat or ultraviolet radiation, a small amount of polymer 

 1 to 2 inches thick in the bottom of a flask or other convenient glass 

 container and then pouring over it catalyzed monomer to a depth of 

 several inches. This is allowed to stand at room temperature until it 

 has become partially polymerized to the desired degree through the 

 "seeding" action of the polymer. During the period of standing, it is 

 well to shake the flask several times a day in order to maintain a fairly 

 uniform consistency throughout the sirup, and it is furthermore advis- 

 able to keep the flask stored in the refrigerator except during working 

 hours when it can be inspected with suitable regularity to observe the 

 progress of the polymerization. 



The third procedure produces comparatively quick results but requires 

 close attention and considerable manipulation to keep the reaction 

 under control. It is similar to the water-bath method, except that the 

 heat is applied directly to the vessel containing the monomer, and con- 

 sists in placing the flask directly on an electric hot plate for 3 to 5 min- 

 utes and then holding it about an inch above the heated surface until 

 the sirup attains the desired viscosity. The flask should be provided 

 with a vented stopper as explained under water-bath method, and should 

 preferably be not more than one-third filled with monomer. The hot 

 plate should be operated with its switch at the low heat position, and 

 the flask should be kept more or less continuously in motion after the 

 first few minutes, as a precaution" against localized overheating. 



Whenever any tendency toward excessive boiling is noted, the flask 

 should be removed at once from the heat source and shaken vigorously, 

 and, if necessary, chilled in cold water. When all spontaneous bubbling 

 ceases, heating may be resumed and continued cautiously until partial 

 polymerization is completed. Thorough final chilling is important in 

 order to prevent uncontrolled continuance of the polymerization reac- 

 tion during storage. 



SELECTION OF SPECIMENS 



Preservation of biological specimens by embedding them in blocks of 

 plastic formed by polymerization of methacrylate esters is applicable to 

 both plant and animal material, but it is limited to durably pigmented 

 specimens that can be dehydrated satisfactorily and are strong and rigid 

 enough to withstand deformation while the plastic is hardening. All 

 such specimens must, furthermore, be free from extractable substances 



