CHAP TEE IX. 99 



As soon as the added thick collodion (of which only just enough to 

 cover the object should have been taken) has so far sunk down that 

 the object begins to lie dry, fresh thick solution is added, and the 

 whole is left as before. (If the first layer of collodion has become 

 too dry, it should be moistened with a drop of ether before adding 

 the fresh collodion.) Provision should be again made for slow 

 evaporation, either in one of the ways above indicated, or which is 

 perhaps better by setting the objects under a hermetically fitting 

 bell-jar, which is lifted for a few seconds only once or twice a day. 

 I have frequently found it advantageous to set the objects under a 

 bell- jar, together with a dish containing alcohol, so that the evapora- 

 tion is gone through in an atmosphere of alcohol. This is especially 

 indicated for very large objects. The whole process of adding fresh 

 collodion and placing the objects under the required conditions of 

 evaporation is repeated every few hours for, if need be, two or three 

 days. 



When the mass has attained a consistency such that the ball of a 

 finger (not the nail) no longer leaves an impress on it, it should be 

 scooped out of the dish or mould, or have the paper removed if it 

 has been imbedded in paper, and be submitted to the next stage of 

 the hardening process. (If the mass is found to be not quite hard 

 enough to come away safely, it should be put for a day or two into 

 weak alcohol, 30 to 70 per cent.) 



163. Hardening, Definitive. Several methods are available for 

 the definitive hardening process. One of these is the chloroform 

 method, due to VIALLANES (Rech. sur I'Hist. et le Dev. des Insectes, 

 1883, p. 129). 



It consists in bringing the objects into chloroform. In some cases 

 a few hours' immersion is sufficient to give the requisite consistence. 

 In no case have my specimens required more than three days. The 

 collodion frequently becomes opaque on being put into the chloro- 

 form, but regains its transparency after a time. 



Small objects may be hardened by chloroform without preliminary 

 hardening by evaporation. All that is necessary is to expose the mass 

 to the air for a few seconds until a membrane has formed on it, and 

 then bring it into chloroform. If the mass is in a test-tube this may 

 be filled up with chloroform and left for two or three days if need be. 

 By this time the collodion mass will be considerably hardened, and 

 also somewhat shrunk, so that it can be shaken out of the tube. It 

 is then brought into fresh chloroform in a larger vessel, where it 

 remains for a few more days until it is ready for cutting. But 

 sufficient hardening is sometimes obtained in a few hours. 



72 



