On the Investigation of Microscopic Forms. 225 



Experiments were made upon a sample of the Rappahannock 

 infusorial earth, which had been given to me by Prof. Wm. B. 

 Eogers, in its natural state. By immersing the valves in various 

 liquids, I finally ascertained that in strong sulphuric acid they 

 became either invisible or very nearly so, while the grains of sand 

 on the slide retained their distinctness perfectly. It was curious 

 to observe how by diluting the acid with water, the valves again 

 became visible and distinct in outline markings. By igniting this 

 earth I produced a slight change in the index of refraction of the 

 silica composing the valves, so that afterwards they were visible 

 with tolerable distinctness in the same sample of sulphuric acid. 



Index of refraction of water 1 • 336 



„ „ sulpli. acid 1-435 



„ „ diatoms 1*435 



„ „ quartz 1 ■ 548 



„ ,, Canada balsam 1 • 548 



This Table shows that the index of refraction of the diatoms is 

 about half-way between that of water and Canada balsam, thus ex- 

 plaining the fact that they appear about equally distinct in both of 

 these media. — Sillimans American Journal, vol. xxxiii., Jan., 1862. 



Troy, Nov. 26th, 1861. 



