136 PREPARATION AND MOUNTING 
part of alcohol should be added to the water. If it be 
desired to exhibit the stipitate forms in their natural para- 
sitism upon other aquatic plants, the entire mass may be 
mounted in Deane’s gelatine in a deeper cell; and such a 
preparation is a very beautiful object for black-ground 
illumination. If, on the other hand, the minute structure of 
the silicious envelopes is the feature to be brought into 
view, the fresh diatoms must be boiled in nitric or hydro- 
chloric acid” (which process is fully described in Chapter 
III.). It is very convenient to have many of these objects 
mounted by two or more of the above methods; and if they 
are to be studied, this is indispensable. Mr. Hepworth 
once showed me about one hundred slides which he had 
mounted in various ways, for no other purpose than the 
study of the fly’s foot. 
My friend, Mr. Rylands, successfully mounts the diatoms 
in the state in which he finds them, and gave me the follow- 
ing method as that which he always employs. He says that 
he has had no failures, and hitherto has found his specimens 
unchanged. Take a shallow ring cell of asphalt or black 
varnish (which must be at least three weeks old), and on the 
cell, whilst revolving, add a ring of benzole and gold-size 
mixed in equal proportions. In a minute or two pure 
distilled water is put in the cell until the surface is slightly 
convex. The object having been already floated on to the 
cover (the vessel used for this purpose being an ordinary 
indian-ink pallet), is now inverted and laid carefully upon 
the water in the cell. By these means the object may be 
laid down without being removed. The superfluous moisture 
must not be ejected by pressure, but a wetted camel-hair 
pencil, the size made in an ordinary quill, being partially 
dried by drawing through the lips, must be used repeatedly 
to absorb it, which the pencil will draw by capillary 
attraction as it is very slowly turned round. When the 
cover comes in contact with the benzole and gold-size ring, 
there is no longer any fear of the object being removed, and 
a slight pressure with the end of the cedar stick of the 
