H. M0RLAND ON MOUNTING MEDIA FOR DIATOMS. 109 



covering-glasses the same will yield to a slight extent, and the 

 diatoms either get crnshed between the slip and cover, or be 

 transferred from the latter to the former. 



Fluids. — Although certain of these media, such as biniodide 

 of mercury with iodide of potassium, as well as oil of cassia, 

 can be obtained with fairly high refractive indices, yet I cannot 

 too emphatically condemn them for use with the higher powers 

 of the microscope, simply from the fact that the diatoms will 

 not remain on the cover-glass, but must necessarily fall to the 

 bottom of the cell, which consequently must be very shallow, 

 otherwise the diatoms will be beyond the focus of the objective. 

 With shallow cells in fluid mounts the diatoms can easily get 

 crushed on cleaning the cover-glasses. If it were not for these 

 fatal objections, I should be disposed to regard oil of cassia 

 very favourably as a mounting medium, as these essential 

 oils give great brilliancy, but whether they can be effectually 

 sealed for a permanency I cannot say. I once mounted a slide 

 in oil of cloves, and it remained perfect for some considerable 

 time, but eventually a bubble made its appearance. I have 

 never seen a slide of diatoms mounted in biniodide of mercury 

 and iodide of potassium, and am inclined to think that this 

 medium is very little used.-" 



Resins. — This class of media is about the most generally 

 used for mounting diatoms, and for ordinary mounts they will 

 be found hard to beat ; they differ somewhat from each other 

 in their way of working, and in their refractive indices. In 

 speaking of them I will take them seriatim. 



Canada Balsam. — This is the oldest and most generally 

 known of the resin media ; it requires hardening by means of 

 heat, and if not overdone there is still left a certain amount of 

 toughness which enables the slide to withstand ordinary fair 

 handling. The only objection to my mind against this medium 

 is that its refractive index is not sufficiently high for the new 

 immersion lenses, that is, for the finer kinds of diatoms ; still, 

 for the coarser kinds, say four out of five species at the very 

 least, Canada balsam, taken all round, still remains as good as 



* Since writing the above I have learnt, with respect to the solution of 

 biniodide of mercury and iodide of potassium, that the medium is of such 

 high specific gravity, viz. 3'02, that any diatoms which fnay chance to be- 

 come detached will float in the fluid and press upwards against the 

 covering-glass, instead of falling to the bottom of the cell. — [H.M.] 



