26 DR BREWSTER on the Existence of Two New Fluids 



vertically between two rectangular prisms of glass ; and having 

 examined in succession the light reflected from the surfaces mp 

 and no, I found that it had suffered total reflexion, both from 

 the side cd and the side gh of the vacuity, and consequently 

 that the vacuity occupied the whole thickness of the cavity. 

 After the heat was applied, the sides c d and gh continued equal- 

 ly luminous, and when eg and dh had retreated to mn and po, 

 as shewn in Fig. 28., it became quite manifest, that the space 

 mnop was not filled with the expanded fluid, but with the fluid 

 in the state of vapour. The coloured rings at first appeared both 

 on the faces cd and gh, and when the whole was converted into 

 vapour they disappeared, and the light reflected from both the 

 surfaces mp, no, which was now uniform, was not that of total 

 reflexion, nor yet that of the expanded fluid, but of an interme- 

 diate intensity, corresponding to that of a dense vapour, with a 

 refractive power much lower than 1.21066. 



There is another set of phenomena of exquisite beauty to an 

 optical observer, which seem to arise either from the decompo- 

 sition of the fluid, or the condensation of gaseous matter in the 

 vacuity. 



When heat is applied to the cavity, the new fluid has its sur- 

 face in a state of constant agitation, resembling in the closest 

 manner a surface into which a fluid is discharging itself by drops. 

 When the vacuity is just filled up, one or more drops quit the 

 point where the vacuity disappeared, and pass along the surface 

 of the cavity, like a drop of oil adhering to it in close con- 

 tact, and never mixing with the fluid. Each of these drops be- 

 gins in a short time to spread circularly, and to exhibit within 

 its disc an immense number of close coloured rings. By slow 

 cooling the drops become thinner, and the rings less numerous, 

 and more completely displayed, till they entirely disappear at a 

 particular temperature. When the cooling is effected quickly, the 



