496 PHYSICAL GEOGRAPHY. 



down, it is again boiled in the alkaline solution and in water alternately, 

 until it has all been disintegrated. It is then all permitted to settle for 

 at least three hours, when it is thoroughly washed and boiled in hydro- 

 chloric acid for about half an hour. There is then added an equal 

 amount of nitric acid, and the boiling continued for a short time. It is 

 then washed and heated in sulphuric acid, with the addition of bichro- 

 mate of potash and hydrochloric acid. 



All mixed gatherings of diatomacese, and particularly all muds and 

 deposits, should be separated into densities, so that for the most part the 

 larger forms are collected together, free from sand, and separate from 

 the smaller species and broken specimens. This is done by using a 

 number of beaker glasses, of various sizes, in the following manner: 

 Into a one-ounce beaker the cleaned diatoms are placed, and the vessel 

 filled with water. It is then well stirred up by means of a glass rod, and, 

 after resting about five seconds, poured off carefully into a six-ounce vessel 

 so as not to disturb the sand which has settled. Again the vessel is filled 

 up with water, stirred, allowed to settle for the same length of time, and 

 poured into the same vessel. This is repeated until it has been done at 

 least six times, when we shall find all of the sand, free from diatoms, in the 

 small beaker. This can be thrown away, and as soon as the material in 

 the large beaker has settled it is returned to the small one, and the same 

 process gone through with, only extending the time of settling now to 

 about ten seconds. The next density is that which settles in twenty 

 seconds ; and so on, five or six densities may be obtained, and if carefully 

 prepared they will be found to contain forms varying very much one from 

 the other. The large species of Triceratium, Aulacodiscus, and the like, 

 will be found in the coarsest density, and the broken diatoms in the 

 lightest. 



Preserving and mounting specimens so as to have them in a condition 

 for study at any future time. Of course, when possible, diatomacese 

 should be studied in the living condition. But there are many forms 

 which have not been as yet found living, and these can only be studied 

 as dead skeletons ; and, in fact, it is in the dead skeletons of the diato- 

 maceae that many of the most marked characteristics are to be found; 

 and on such characteristics species have been founded. Besides, the 

 most beautiful sculpturing of the valves is only to be seen after every- 



