HEAT 391 



terming beautiful curves, and the surface breaking into sym- 

 metrical ripples, which the lens will focus on the screen. On 

 turning out the burner the movement will gradually decrease 

 in energy, until suddenly contact is established, and the 

 water bursts into a cloud of steam. 



Arrange in the field a smaller silver dish about two inches 

 diameter, which has had the bottom carefully flattened for 

 about one inch in the centre, convex side uppermost as in the 

 first experiment. By carefully adjusting a small ring on the 

 end of a wire about 1 mm. above its surface, and heating the 

 basin and in a much less degree the wire, hot water darkened 

 with ink or other dye can be gently delivered from the pipette 

 so as to be held in place by the small ring thus immersed 

 in it. Projection on the screen will then demonstrate that 

 there is a small clear space between the silver and the water, 

 through which the light from the lantern passes. 



Heating of the water is desirable in these experiments, in 

 order to avoid cold water lowering the temperature of the 

 metal underneath to a point which will not maintain the 

 spheroidal state. 



231. Radiant Heat in the Spectrum. The demonstra- 

 tion of dark heat rays has already been mentioned under 

 Calorcsccnce, in 180. The quantitative examination of the 

 spectrum demands expensive apparatus in the shape of a 

 complete train of prisms and lenses in rock-salt (which alone 

 will show the real heat maximum in the ultra-red region), 

 and a sensitive thermo-pile of the linear form, guarded by a 

 narrow perpendicular slit. Only the general scheme can 

 be given here in fig. 216, where s is the slit of the lantern 

 through which come parallel rays produced by a rock-salt 

 lens in the lantern itself, A is the salt focussing lens, B the 

 rock-salt prism. Then v B G B representing violet, blue, green, 

 and red rays, the thermo-pile will show the greatest heat at 

 about P, and will cease to give evidence of it at o, nearly as 

 far from B as the violet rays are. For these experiments a 



