8 DR BREWSTER on the Existence of Two New Fluids 



finger on the microscope caused the air-bubble to tremble and 

 oscillate in this miscroscopic level. Hence the new fluid is dis- 

 tinguished by a second physical property, no less remarkable than 

 the first. 



Although I now entertained no doubt of the accuracy of the 

 conclusion, that the fluid was a new one, yet I conceived it 

 might be possible to obtain at least an approximate measure 

 of its refractive power, and thus to put its novelty beyond the 

 reach of a doubt. In order to do this, it became necessary to 

 observe the manner in which the total reflexion of the upper 

 surface of the cavity was modified by the contact of the fluid, 

 and, if possible, to measure the angle at which total reflexion was 

 effected, by the separating surface of the fluid and the solid. For 

 this purpose I took a plate of topaz AB, Fig. 2., with a stratum 

 of cavities m ri, perfectly parallel to the natural surface of the 

 plate. I then placed upon each surface the rectangular prisms 

 ABC, ABD, and introduced between them a thin film of oil of 

 cassia. Rays of light RS, RS were then allowed to fall upon the 

 stratum of cavities m n, so that the rays reflected from the upper 

 surface of the cavity could be examined by a microscope whose 

 object lens is LL. Upon making this arrangement, the stratum 

 of cavities was seen in the most beautiful manner. The vacuity 

 V, Fig. 3. of a cavity seen in this way, shone with all the bril- 

 liancy of total reflexion, the separating surface of the new fluid 

 ABCD, and the cavity, exhibited a faint grey tint, while the sur- 

 rounding portions of the solid topaz were comparatively black. 

 The variations which the vacuity V undergoes by heat are now 

 finely seen, and at a temperature of 80 it vanishes in a brilliant 

 speck, leaving the whole of the cavity ABCD of the same uni- 

 form tint as in Fig. 4. 



The phenomena now described are not so distinctly seen 

 when the stratum m n is deeply seated beneath the surface of 



