1893.] MICKOSCOHCAL JOtJRNAL. 30 



If the washing is not carefully done the specimen will continue to 

 blacken in the alcohol. This should be avoided. 



BicJiromatcs, — The bichromates of potassium and of ammonia 

 are advantageously used, instead of chromic acid ; sometimes with 

 it. A solution of pure bichromate of ammonia in water (3 per 

 cent.) is made, or the mixtures called Midler's or Erlicki's fluids 

 are used. 



Midler's fluid docs not differ in its properties from the pure 

 bichromate. Its formula is : Water, lOO ; bichromate of potas- 

 sium, 2; sulphate of soda, i. 



Erlicki's fluid fixes better and hardens quicker than the pure 

 bichromates, especially if it is placed in a stove heated to about 

 37° C. It is useful for the central nervous system ; in 8 days a 

 remarkable consistency is obtained. The formula is : Bichromate 

 of potassium, 2 grms. ; sulphate of copper, \ grm. ; water, loo. 



In a general way bichromate solutions should be used to fix 

 vascular specimens when the blood-corpuscles are to be preserved ; 

 also for injections with Prussian blue, and whenever the nucleus 

 is to be stained with hoematoxylin. Tissues should remain in the 

 chromic salt for at least lo days la longer time will do no harm), 

 and carefully washed in many changes of water for 24 hours. 

 They are then ready to be imbedded for sectioning. 



Picric Acid. — This is used in a saturated aqueous solution. 

 .... Picric acid is a fixative of the first order ; it is advantage- 

 ously used, instead of the bichromates, for the study of blood- 

 vessels. Small pieces (few mm.) should be left in the solution for 

 not longer than 24 hours. Transfer to strong alcohol. 



Bichloride of JSIercury. — To prepare the solution, 



dissolve an excess of the bichloride in distilled water. To fix the 

 tissues of vertebrates, use it cold. Very small pieces should remain 

 in it only till fixation is complete. When entirely whitened (in 

 from 5 to 6 hours), transfer to strong alcohol and renew it many 

 times. Do not use metallic instruments in the manipulation of 

 the specimens 



Hardening. 



The foregoing fixing agents often harden the tissues sufficiently 

 to allow of sectioning. It is possible to make sections free hand 

 after the action of alcohol, of osmic acid, of picric acid, or of the 

 bichromates, yet it is necessary, especially with the last mentioned, 

 that the action be prolonged for several months. But great expe- 

 rience is here needed, and it is better to complete the hardening, 

 as may be done by placing the fixed tissues in strong alcohol, but 

 as the beginner will often find the piece not hard enough to sec- 

 tion in this way he should have recourse to methods of imbedding. 



The imbedding or the imbibition by the organs of the proper 

 substance is intended to increase the consistency of the tissues 

 and to prevent the disarrangement of the elements or of the parts 

 of small specimens (embryos, for example). There are many 



