HAEDENING. 197 



The objects being imbedded, and in the stage at which we 

 left them at the end of 299, the treatment should be as 

 follows : 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. Provision 

 should be made for slow evaporation, either in one of the ways 

 above indicated, or, which is perhaps better, by setting the 

 objects under an hermetically fitting bell- jar, which is lifted for 

 a few seconds only once or twice a day. I have sometimes 

 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. 



When the mass has attained a consistency such that the 

 ball of a finger 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.) 



Several methods are available for the definitive hardening 

 process. One of these is the chloroform method, due to VIAL- 

 LANES (Rech. sur I* Hist, et le Dev. des Insectes, 1883, p. 129). 

 I recommend this method for small objects, because I find it 

 more certain and more rapid than the alcohol method, and 

 preferable on account of a superior consistency it gives to the 

 mass. (ScHiEFFEKDECKER does not find this, v. Zeit.f. wiss. Mik., 

 v, 4, 1888, p. 506.) For large objects the method is inferior 

 to the alcohol method, because the rapid hardening of the 

 external layers is an obstacle to the diffusion necessary to the 

 hardening of the inner layers. 



The method consists in bringing the objects into chloroform. 

 " Under the influence of this reagent the collodion coagulates 

 into a mass having the consistence of wax, but having also 

 an elasticity that renders it unbreakable, and having besides 

 the precious quality of being admirably transparent, and 

 possessing exactly the index of refraction of glass/' 



In some cases a few hours' immersion is sufficient to give 

 the requisite consistence. In no case have my specimens re- 

 quired more than three days. But the length of time required 



