OF THE FREE DIOTAMACE.E. 29 



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. 



