24 DR BREWSTER on the Existence of Two New Fluids 



concentric coloured rings of thin plates, produced by the extreme 

 thinness of each of the fluids at the edges. 



If we now raise the temperature of the room gradually to 

 58, we shall observe a brown spot appear in the centre of the 

 vacuity V. This spot marks the visible commencement of eva- 

 poration from the new fluid below, and arises from the attenuated 

 vapour which attaches itself to the roof of the cavity. As the 

 heat increases, the brown spot enlarges, and becomes very dark. 

 It is then succeeded by white, and one or more rings rise in the 

 centre of the vacuity. The vapour then seems to form a drop, 

 and all the rings disappear, by retiring to the centre, but only to 

 reappear with new lustre. During the application of heat, the 

 circle efgh is in a state of constant contraction and dilatation, 

 like the pupil of the eye when exposed to light, being always 

 greatest when the rings disappear, and contracting its dimensions 

 when they are again formed. 



When the vaporisation is so feeble as to shew itself only by 

 a single ring of one or two tints of the second order, they may 

 be made to disappear instantly by the slight degree of heat 

 produced by a single breath upon the crystal ; aud the same ef- 

 fect is produced by the approximation of a heated body. When 

 the heat reaches the fluid, however, it makes it throw off fresh 

 vapour, and the rings again appear. 



If we put a drop of ether upon the crystal when the rings 

 are in a state of rapid play, the cold occasioned by its evapora- 

 tion immediately causes them to disappear, till the temperature 

 again rises. 



When the temperature is perfectly uniform, the rings remain 

 stationary, and it is interesting to observe the first ring produced 

 by the vapour swelling out to meet the first ring at the margin 

 of the fluid, and sometimes coming so near it, that the darkest 

 parts of both form a broad black band. 



