24 PRACTICAL MICROSCOPY. 



Celloidin is the proprietary name of a sort of pyroxylin, very solu- 

 ble in a mixture of ether and alcohol, producing a collodion. If 

 thick collodion be exposed for a few moments to the air it becomes 

 semi-solid not unlike boiled egg-albumen; and to this property is due 

 the value of a solution of celloidin in histology. It may be used as 

 follows: 



To a mixture of equal parts of ether and alcohol add celloidin,* 

 until the thickest possible solution has been obtained. 



A piece of alcohol-hardened tissue having been selected and kept 

 for the preceeding twenty-four hours in a mixture of equal parts of 

 alcohol and ether, is placed in about an ounce of the solution, and 

 allowed to remain twenty-four hours. The bottle containing the 

 whole should be well corked to prevent evaporation. 



The tissue after infiltration is to be placed on a cork support and 

 allowed to remain in the open air for a few minutes, after which it 

 should be plunged into a mixture of alcohol two parts, water one 

 part. Here it may remain for twenty-four hours, or until wanted. 



Cut in the usual way using a mixture of alcohol two parts, water one 

 part, for flooding the knife; the sections should be finally preserved in 

 the. same instead of pure alcohol which would dissolve the celloidin. 



In infiltrating the tissue with the collodion it is best, especially if 

 it be very dense in parts, to use, first, a thin and subsequently 

 the thick solution. A more perfect infiltration is often obtained in 

 this way. In some cases I have been obliged to continue the m acera 

 tion for several days. The solution should be kept in well-stoppered 

 bottles, as the ether is exceedingly volatile. Should the collodion at 

 any time become solid from evaporation, it may be easily dissolved 

 by adding the ether and alcohol mixture. 



The process is of inestimable value where delicate parts are weakly 

 supported, and where it is important to preserve the normal relations. 

 The gelatin-like collodion permeates every space and, as it is not to be 

 removed in the future handling of the sections, it affords a support to 

 portions that would otherwise be lost or distorted. It offers no ob- 

 struction to the light, being perfectly translucent and nearly colorless. 



HARDENING BY FREEZING, ETC. 



I do not recommend the freezing process. 



Other fixing and hardening methods, which are of special applica- 

 tion only, will be introduced in our future work as occasion may de- 

 mand. 



* I find, after repeated trial, that the ordinary soluble gun-cotton, such as is 

 employed by photographers, is in no way inferior to the celloidin. 



