ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 891 



Dissolve in a little water some very pure gum arabic, so as to 

 obtain a liquid having tbe consistency of a thick syrup.* Pour a 

 little into a watch-glass, so as not to quite fill it. Then add from six 

 to ten drops of pure glycerine, and with a small stirrer carefully mix 

 the gum with the glycerine until it forms a homogeneous mass. Then 

 lay the preparations on the surface of the liquid, and with needles 

 press them into it. 



This doue leave the whole to dry, which takes from one to four 

 days, according to the condition of the air. The gum will assume 

 the consistency of cartilage ; without being soft it is supple and 

 yields to the finger. The cake of gum is then cut into squares or 

 strips, corresponding with the preparations, and removed. A plate of 

 gum enclosing the preparations is thus detached without difficulty 

 from the bottom of the watch-glass. These plates are turned over 

 and again allowed to dry until they are wanted for use ; they may 

 be preserved in good condition almost indefinitely, the gum, when 

 mixed with a sufficient quantity of glycerine, never becoming hard or 

 brittle. 



The following are points to be noted : — Between the limits of 

 6 to 10 drops of glycerine above mentioned, the proportions most 

 suitable to the nature of the object under examination and to the 

 season of the year may be found by experimental trials. Too much 

 glycerine prevents the gum from acquiring sufficient toughness, too 

 little allows it to become brittle. In the winter or in rainy weather 

 less glycerine should be added than in the summer or in dry weather. 

 It is often well to soak the object in glycerine before putting it into 

 the gum ; the quantity of glycerine thus absorbed by the object 

 being taken into consideration, and less added directly to the gum. 



With a stove or by the help of the sun the gum can be very 

 quickly dried, but in most cases it is a question of patience. It is 

 one of the great advantages of the gum and glycerine tliat they dry 

 so gradually ; they are generally liquid the first day, pasty the 

 second, and cartilaginous the third. The object having remained in 

 this liquid for twenty-four hours is perfectly soaked, the gum having 

 penetrated into all the interstices of the cells and the sections pre- 

 serve the relations of organs which are not directly connected. With 

 soap or gelatine the imbibition is in many cases less perfect, because, 

 unless a high temperature is maintained for a long time, the solidifi- 

 cation of the mass takes place too quickly and does not allow the 

 liquid to penetrate so deeply into the tissues. 



When the strips are removed from the watch-glass, it is better to 

 wait until they have assumed such a consistency that they cannot be 

 easily bent. It is after having waited almost a week that the author 

 has always obtained the best sections. 



Gum alone rapidly becomes hard and brittle ; the effect of the 

 glycerine is to preserve it almost indefinitely in a cartilaginous 

 consistency. Another advantage of the method is the perfect trans- 

 parency of the substance surrounding the object to be cut, so that it 



* Solutions of gum, sold under the name of strong white liquid glue, may 

 also he used. They have the advantage of having a uniform consistency. 



