178 THE MICROSCOPE AND HOW TO USE IT. 



be employed, while for transparent animals dark colours are pre- 

 ferable. Chrome yellow serves the purpose. It is easily obtained 

 by mixing solutions of bichromate of potassium and acetate of 

 lead. A copious yellow precipitate is formed, which must be 

 washed on the filter, then exposed in the air until nearly dry. 

 The pigment, after it is reduced to a pulpy state, is added to an 

 ordinary aqueous solution of gelatine, and the mass then filtered 

 warm through linen. If a blue mass, the gelatine may be dis- 

 solved directly in liquid Prussian blue and filtered through paper. 

 Chloroform and alcohol are the best means of killing annelids for 

 this purpose ; fresh water may be also used for some marine 

 species. A leech is placed in water in which is a small quantity 

 of chloroform ; after a few moments, it sinks to the bottom and 

 remains motionless. It is better if it remains in the solution for 

 one or two days before beginning to inject it. A well-stoppered 

 bottle must be used, as chloroform evaporates so quickly. The 

 best form of syringe consists of a glass tube drawn to a fine point 

 at one extremity and furnished at the other with a rubber tube. 

 Preparatory to injecting, the glass should be plunged in warm 

 water for a few minutes. Then expel the water and fill with the 

 injection fluid by sucking the air from the rubber tube. If the 

 mass is turned into the large end of the glass, granules are intro- 

 duced which are large enough to obstruct the narrow opening at 

 the small end. Insert the cannular end in the vessel, clasp both 

 with forceps, then force the fluid, by aspiration, through the 

 rubber tube, which is held in the mouth. When the injection is 

 finished, place the animal in cold water, to stiffen the mass. — 

 M. T., Zool. Stat. Neapd. 



Hardening Injected Tissues— Injected tissues must be hard- 

 ened in spirit. After the first day's immersion, they should be 

 transferred to fresh spirit for two more days, and then again into 

 fresh spirit, and kept in this until ready for cutting into sections. 

 It is never necessary to use absolute alcohol, and it is seldom 

 needful to place the tissues first in weak spirit and gradually to 

 increase the strength up to perfectly anhydrous alcohol. The 

 length of time required for hardening depends upon the kind of 

 tissue, its size, and, to a certain extent, upon the quantity of spirit 

 used. The smaller the size of the tissue, the more rapidly will 



