42 PREPARATION AND MOUNTING 



3. Mounted dry or in balsam (see Chapter III.), after the 

 cleansing process already described. 



I will here give Mr. Rylands' method of mounting them dry, 

 the fluid and balsam preparations being noticed in their respec- 

 tive chapters. The slide with the ring of asphalt, or black var- 

 nish, should have been prepared some weeks previously, in order to 

 allow it to dry thoroughly. When required, it must be held over 

 the spirit-lamp or Bunsen's burner until the ring of varnish is 

 softened. The burnt cover, having been heated at the same time, 

 must then be taken in the forceps and pressed upon the softened 

 varnish until it adheres all round. When cold, an outer ring of 

 asphalt completes the slide. 



Such is the method which my friend Mr. T. G. Eylands em- 

 ploys in the preparation of diatoms for the microscope. I have 

 said enough concerning his results. It is to be feared, however, 

 that to some these several modes of operation may appear lengthy 

 and complicated ; but if read carefully, and the experiments tried, 

 they will be found simple enough in practice, and to occupy much 

 less time than an intelligible description would lead the novice to 

 believe necessary. 



One of the most fertile as well as the most curious magazines of 

 Diatomaceae is guano. The siliceous forms contained therein have 

 been devoured by sea-birds and passed through the stomach unin- 

 jured, and after lying for ages may be cleaned and classified. 

 Many of these are not elsewhere met with, so that the student 

 who is desirous to enter into the study of Diatomacea3 must be in- 

 structed as to the best mode of obtaining them from this source. 

 The particulars to be observed so closely resemble those before 

 mentioned in the treatment of the ordinary diatoms, that it will 

 be sufficiently explicit to give the outlines of the process. The 

 guano must be first washed in pure water, allowed to subside per- 

 fectly, and the liquid then poured off. This must be repeated 

 until the top fluid is clear, and care taken not to decant the liquid 

 until perfect subsidence has taken place. The deposit must then 

 be treated with hydrochloric acid with a gentle heat for an hour 

 or two, adding a little fresh acid at intervals as long as it excites 

 any effervescence. After this nitric acid must be substituted for 



