REAGENTS 37 



Some very minute objects, like bacteria and the smaller Cyano- 

 phyceae, may be dehydrated by heating them until all water is drawn 

 off, but, of course, this shows merely the form, with little or nothing 

 of the internal structure. 



CLEARING AGENTS 



Clearing agents are so named because they render objects transpar- 

 ent. When clearing agents are used to precede infiltration with paraf- 

 fin, the clearing is merely incidental, the real purpose being to replace 

 the dehydrating agent with a solvent of paraffin. The clearing is use- 

 ful, even in this case, because it indicates when the replacing has be- 

 come complete. 



When the clearing agent is used to precede infiltration with paraffin, 

 the material should always be most thoroughly dehydrated with abso- 

 lute alcohol before beginning with the clearing agent. When the clear- 

 ing agent is used to clear sections or small objects just before mounting 

 in balsam, absolutely perfect dehydration is not necessary with all 

 clearing agents. Bergamot oil, carbolic acid, and Eycleshymer's clear- 

 ing fluid (ecjual parts of bergamot oil, carboHc acid, and cedar oil) will 

 clear readily from 95 per cent alcohol. Sections to be cleared in xylol 

 or clove oil should be dehydrated in "absolute" alcohol. If the abso- 

 lute alcohol is below 99 per cent, xylol will not clear perfectly; but 

 clove oil clears readily from 99 per cent. If the absolute alcohol is not 

 up to 99 per cent it is a good practice to go from the alcohol to clove 

 oil; and then, from clove oil to xylol. 



Water may be removed by distilling or by putting a "drier" into the 

 alcohol. Put some calcined copper sulphate into the bottle of "abso- 

 lute" alcohol, shake, and allow to stand for 24 hours. Then pour off and 

 add fresh copper sulphate, shake, and repeat the operation until the 

 fresh copper sulphate no longer gets conspicuously blue when put into 

 the alcohol. When the alcohol can be mixed with xylol without becom- 

 ing milky, it may be called absolute alcohol. 



Some put a httle calcium sulphate into the "absolute" alcohol and 

 keep it there, pouring off the alcohol very gently as it is needed. 



By any of these methods, 99 per cent alcohol can be brought up to 

 usable absolute alcohol. Distilling, followed by a drier, will bring 95 

 per cent alcohol up to a usable absolute alcohol. 



