70 PREPARATION AND MOUNTING 



now describe it. A circle of bitumen about one-third! 

 smaller than the covering glass ia drawn beforehand on my 

 slides. When I wish to make a preparation, instead of 

 coating, as formerly, the first circle with a second layer of 

 bitumen, I form a second circle of it outside the first, and 

 as near as possible to it, and each of the two circles has its 

 own advantage ; the first, in fact, while forming the cell,, 

 serves as a support for the covering glass, and thus pre- 

 serves the Diatom acese from any breakage ; it offers, besides, 

 a serious obstacle to the spreading of the more liquid bitu- 

 men of which the outer circle is composed ; and the lattei 

 closes the cell by fixing the cover, which, when the prepara- 

 tion is dry, may be covered with a final circle of bitumen 

 It is of course understood that I am speaking of prepara- 

 tions made in the dry way only, and not with balsam." 



One of the most fertile as well as the most curious maga- 

 zines of Diatomaceae is guano. The siliceous forms con- 

 tained therein have been devoured by sea-birds and passed 

 through the stomach uninjured, and after lying for ages- 

 may be cleaned and classified. Many of these are not else- 

 where met with, so that the student who is desirous to enter 

 into the study of Diatomacea3 must be instructed as to the 

 best mode of obtaining them from this source. The par- 

 ticulars to be observed so closely resemble those before men- 

 tioned 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 perfectly, and the liquid then poured off. This 

 must be repeated until the top fl'uid 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 the hydro- 

 chloric, and the heat kept up to almost boiling-point for 

 another hour at least, adding a little fresh acid as before. 

 When this ceases to act, the deposit must be allowed to 



