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. Hep worth 

 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 



