494 



PHYSICAL GEOGRAPHY. 



with plenty of clean water. When we have recent gatherings of fila- 

 mentous or stipitate forms of diatomacese, which we desire to preserve in 

 the natural condition, they should be immersed for about twenty-four 

 hours in alcohol to dissolve out the endochrome. If this does not 

 answer, it will be well to soak the mass of diatoms or plants upon which 

 they are adherent in a solution of hypochlorite of soda, an impure variety 

 of which is sold in the shops under the name of Labarraque's disin- 

 fectant, for about the same length of time. This will generally destroy 

 all color, and leave the specimens transparent. It is best, however, in 

 many cases not to remove the endochrome, but leave it, and mount the 

 specimens in such a way as to show them in as natural a condition as 

 possible. How this may be done will be described hereafter. 



Guanos. The preparation of these substances so as to obtain the 

 microscopic organisms they may contain is rather difficult, tedious, and 

 dirty, and should only be undertaken by a person somewhat versed in 

 chemical manipulations, and in a proper room as a laboratory, where 

 there is no danger of harm resulting from the fumes evolved. As the 

 ammoniacal guanos are those which contain the most diatoms, and con- 

 sequently which answer best to clean, we will begin with them, and take 

 as a type that which comes from the islands on the coast of Peru. As it 

 comes into commerce this guano is a moist powder of a light iron-rust 

 color, smelling strongly of ammonia, and having scattered throughout its 

 mass lumps of ammoniacal salts of a more or less solid consistency. 

 The guano should be thinly spread out upon a stiff piece of paper and 

 exposed to the air, and, preferably, to a moderate heat for several days or 

 even weeks. In this way most of the moisture and much of the ammo- 

 nia will evaporate, and less acid will be required to clean the guano. It 

 will now have become much lighter in color, and crumble to a dry powder. 

 A tin pan is now about half filled with a solution of common washing 

 soda in clean filtered water, and placed over some source of heat, as on 

 a stove. The strength of this solution is not a matter of any great 

 moment, and must vary with the guano manipulated. As soon as it 

 begins to boil, the guano is dropped gradually in, a little at a time, while 

 the liquid is stirred with a glass rod or stick of wood. Considerable 

 effervescence takes place, ammonia being given off, and therefore it must 

 be kept continually stirred, and care exercised to prevent its boiling over. 



