WITH THE MICROSCOPE. 57 



particularly useful for hardening portions of the nervous system 

 previous to cutting thin sections. The solution is prepared by dis- 

 solving sufficient of the crystallized acid in distilled water to rendtr 

 the liquid of a pale straw colour. 



The crystallized acid may be prepared by decomposing ico 

 measures of a saturated solution of bichromate of potassa, by the 

 addition of 120 to 150 measures of pure concentrated sulphuric acid. 

 As the mixture becomes cool, crystals of chromic acid are deposited, 

 which should be dried and well pressed on a porous tile, by which 

 means the greater part of the sulphuric acid is removed, and the 

 crystals obtained nearly pure. 



105. Preservative Gelatine. This substance was first employed 

 for preserving microscopical textures by Mr. H. Deane, who gives 

 the following directions for its preparation : 



Gelatine ... ... ... ... i ounce. 



Honey ... ... ... ... 4 ounces. 



Spirits of wine ... ... ... -i- ounce. 



Creosote 6 drops. 



Soak the gelatine in water until soft, and to it add the honey 

 which has been previously raised to the boiling-point in another 

 vessel. Next, let the mixture be boiled, and after it has cooled some- 

 what, the creosote dissolved in the spirits of wine is to be added. 

 Lastly, filter through thick flannel to clarify it. 



When required for use, the bottle containing the mixture must be 

 slightly warmed, and a drop placed on the preparation upon the 

 glass slide, which should also be warmed a little. Next, the glass 

 cover, after having been breathed upon, is to be laid on with the 

 usual precautions. The edges may be covered with a coating of the 

 Brunswick black varnish. Care must be taken that the surface of 

 the drop does not become dry before the application of the glass 

 cover ; and the inclusion of air-bubbles must be carefully avoided. 



106. Gelatine ami ciyeerine. A mixture of gelatine and glycerine 

 makes a very valuable medium for preserving different animal and 

 vegetable structures. 



The mixture may be made as follows : A certain quantity of 

 gelatine or isinglass is allowed to soak for some time in cold water, 

 until it swells up and becomes soft. It is then placed in a glass 

 vessel and melted by the heat of warm water. It may be clarified if 

 necessary, by first adding to the cool gelatine a little white of egg, 

 then boiling the mixture, and filtering through fine flannel. To this 

 fluid, an equal quantity of strong glycerine is added and well mixed 

 with it. This mixture may be kept for any length of time, and a 



