OF THE FREE DIOTAMACE^E. 
2£ 
very often they are not to be seen at all ; but the contents 
of the valves can be made out clearly enough. For this 
last purpose, then, the plants should be put away in small 
bottles, filled with very dilute alcohol. When required for 
use, let the bottle be well shaken, insert a glass rod, and 
some of the frustules are sure to cling to it, and be drawn 
out. The drop may then be placed on a slide, and the 
mixture allowed to evaporate. 
If, as is often the case, especially with the fossil species, 
the valves will not adhere to the slide, recourse must be 
had to a very thin solution of gum arabic. By placing a 
minute drop on the glass, and then immersing the specimens, 
their adhesiveness is ensured. However, this plan should 
only be resorted to in extreme cases, for the residuum left 
by even the purest gum is very apt to interfere with the 
beauty of the preparation, and to spoil the delicate markings 
of the valves. 
