D. E. GODDARD ON MANIPULATION WITH CANADA BALSAM. 127 



Second place, some of the sources of failure, and how they may- 

 be overcome. 



Foremost among these must be considered those plagues of the 

 amateur — air-bubbles ; these generally arise from three sources. 



1. — Bad, or thick balsam, in which case it must be thinned, as 

 before mentioned. 



2. — Expansion of air contained in cellular tissue, or minute or 

 intricate vessels. The only method by which such annoyances can 

 be avoided is by lengthened soaking in camphine or turpentine, and 

 submitting the preparation to the action of an air pump. 



The 3rd source of air-bubbles is the application of too great 

 heat to the slide, by which the balsam is boiled. The method by 

 which this can be corrected I shall notice presently. 



I may here perhaps caution the student against discarding pre- 

 parations simply because air-bubbles are present. When the balsam 

 is in good condition, and no heat, or, at most, very little has been 

 applied, he will generally find they will disappear in a longer or 

 shorter period, according to the nature of the specimen mounted. 

 Many slides that I have thrown aside and forgotten, have, in the 

 course of a few months, been discovered in my " spoiled box " in a 

 beautiful state of preservation, every bubble having vanished. The 

 object to be attained by mounting in balsam is to render the speci- 

 men transparent, which would otherwise be too opaque for observa- 

 tion by transmitted light. Most objects for the polariscope require 

 to be mounted in some medium. Balsam and glycerine are the 

 favourites for some objects, such as crystals, that would be decom- 

 posed by balsam, or dissolved by glycerine. Castor oil may be used 

 with advantage. 



I do not wish it to be understood that I advise the use of 

 Canada balsam for every kind of preparation ; it will be for the 

 student to find by practise how far it may be made available. 



It should be established as an axiom in microscopical manipula- 

 tion — That the specimen should invariably be soaked for a longer or 

 shorter period in the medium in which it is to be preserved. Thus, 

 when balsam is the medium, soak in turpentine, or what is infinitely 

 better, camphine ; when glycerine is used, let the specimen be 

 placed in it, and in all cases submitted to the air pump. A cheap 

 and very effective form of air pump can be obtained from Mr. 

 Baker, 244, Holborn ; the price is, I think, 18s. 



When preparing insects for the microscope, I know it is usual 



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