One of the most fertile as well as the most curious magazines of Diatomaceæ is _guano_. The siliceous forms contained 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 elsewhere met with, so that the student who is desirous to enter into the study of Diatomaceæ must be instructed 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 perfectly, 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 the hydrochloric, 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 settle perfectly and the acid poured off. All traces of the acid must now be washed away with pure water, when the remains will be Diatomaceæ, the sand contained in the guano, and a few other forms. Some of these may be mounted dry, as before mentioned, but the greater portion should be put up in Canada balsam as described in Chapter III. Such is the ordinary method for the treatment of guano; but Mr. Rylands’ mode of proceeding with ordinary Diatomaceæ (before given) will be found equally successful with these deposits. The fossil Infusoria (as they were formerly called) are now termed Diatomaceæ, and are found in various parts of the world--“Bermuda earth,” “Berg-mehl” from Norway, deposit from Mourne Mountain in Ireland, &c. They are found in immense quantities, and afford the microscopist innumerable objects. The same treatment as that usually employed for the Diatomaceæ must be followed with these deposits, but as they are sometimes obtained in hard masses, disintegration is first necessary. To effect this, they are usually boiled for a short time in diluted _liquor potassæ_, which will soon cause the mass to fall into a mud-like deposit. Water must then be immediately added, in order that all further action of the _liquor potassæ_ may be stopped, otherwise the objects searched for will be dissolved. For this reason it is necessary to understand what substance is being dealt with, because some deposits are much finer and acted upon more readily than others.