TEST- FLUIDS. 125 



and the subsequent sediments will consist almost ex- 

 clusively of diatoms, the sizes of which will be so 

 graduated that the earliest sediments may be ex- 

 amined with the low powers, the next with the medium 

 powers, while the latest will require the higher powers 

 a separation which is attended with great conve- 

 nience/'* small portions of the sediment should then 

 be mounted in Canada balsam, or set up dry between 

 two pieces of thin glass. For mounting microscopic 

 objects you will require a pair of fine-pointed forceps 

 for holding the objects to be mounted, a pair of stout 

 needles fixed in handles, a spring dipt for holding 

 down the covers whilst the balsam is cooling, and a 

 small spirit-lamp. 



Canada balsam a natural combination of resin 

 with the essential oil of turpentine may be procured 

 from any druggist It is thick and viscid, but becomes 

 softer on the application of heat ; you must be careful 

 to keep it very clean and to exclude the air, which 

 would render it too thick for immediate use. To 

 mount in Canada balsam, place a drop on the glass 

 slide by means of a glass rod, then apply gentle heat, 

 immerse the object in it, and if there are no air- 

 bubbles, place the glass cover on, apply the spring 

 clip, and set aside for the balsam to harden. You will, 

 however, have need to exercise much patience ; for 

 no sooner is the object placed in the balsam than all 

 at once many air-bubbles make their unwelcome ap- 

 pearance ; you must, therefore, boil the balsam over 

 the spirit-lamp, if the texture of your object will allow 

 you to do so, and the heat will probably drive out the 

 intruding bubbles. It is advisable to prepare some 

 objects before mounting in Canada balsam by soak- 

 ing in oil of turpentine for some minutes. Insect 



* Carpenter, page 315. 



Sold by Messrs. Baker, Mr. Collins, and others. 



