126 T>, E. GODDARD ON MANIPULATION WITH CANADA BALSAM 



nearly colourless, and more or less viscid. Its boiling point, as far 

 as I can ascertain by experiment, is about 160 centegrade (320 

 Fah.) If exposed to the action of the atmosphere, it becomes thick ; 

 when heated repeatedly, it is rendered brittle, and often passes to 

 a brown, or straw colour, lighter or darker, according to the degree 

 of heat to which it has been subjected. If brought into contact 

 with humid matter, a white cloudiness ensues that will render the 

 medium useless. It is soluble, to a greater or less extent, in cam- 

 phine, alcohol, naphtha, benzole, chloroform, eether, and crystal oil. 

 It is insoluble in water. When I first attempted to prepare my 

 own slides, I was misled, and met with many failures by following 

 the adyice contained in some books and papers recommending the 

 use of "old balsam." My objections to it are, it is too thick to 

 enter the minute vessels in animal and vegetable structures, and 

 retains air-bubbles instead of occupying their space. To render it 

 available, it requires heating more or less ; this, often repeated, 

 renders it too brittle to trust. It is often discoloured, and some- 

 times presents a very yellow appearance. It has, however, the 

 advantage of hardening rapidly ; but, even this, under some cir- 

 cumstances, is much against its use. The Canada balsam obtained 

 at opticians will generally answer very well. It should be of such 

 consistence as will drop readily from a glass rod ; if the drops are 

 long in falling, it is a proof that the medium is too thick. When 

 such is the case, it can be made fit for use by the addition of cam- 

 phine or turpentine. The balsam must be warmed in a water bath 

 (about 50 cent.) and placed under the exhausted receiver of an air 

 pmnp. The question may here be asked — What kind of vessel is 

 the best to keep the medium ? 



I object to corked bottles, r because the cork is liable to adhere to 

 the glass, and small pieces get broken away and fall into the balsam. 

 A stoppered bottle is also open to criticism. When the balsam soils 

 the neck, the stopper adheres to it, requiring the aid of heat to 

 loosen it, and the hardened balsam prevents the total exclusion of 

 the air, and the medium gets thick. 



I prefer using a conical capped gum -pot, the outside neck of 

 which is ground, the cap thereby fitting like a stopper. If any 

 balsam falls on it, it may be cleared off without any fear of soiling 

 the medium. 



So much for the general properties of Canada balsam. We will 

 now notice, in the 



