HEAT 385 



tion two images of the jar, formed by rays at right angles 

 with those direct from the jar to the spectators. The great 

 advantage of this method is, that the spectators only need 

 to be about the same height as the jar, to receive perfectly 

 polarised light all round; and the complementary effect 

 can always be seen at the same moment in the mirrors, so 

 that when the jar extinguishes direct light, the mirror 

 images are bright. This having been demonstrated, we 

 finally cover the top of the jar with a large quartz plate Q. 

 The jar itself will now glow with all the rotational colours 

 as the polariser is rotated ; differently -coloured images 

 being seen at the same time in the mirrors M, and the 

 whole forming a demonstration of indescribably delicate 

 beauty. It is convenient to have the mirrors hinged 

 together like a book, and to stand both jar and mirrors on 

 a circular piece of board, if no higher support be needed, in 

 order that the whole may be turned to face in succession all 

 sides of the room. 



CHAPTER XXin 



HEAT 



A. NUMBER of experiments showing the effects, nature, and 

 qualitative relations of heat are easily capable of projection. 

 Some specimen examples may suffice. 



224. Expansion. As regards the expansion of solids, 

 Gravesande's ring and the various socket forms of apparatus 

 need no remark. Any ordinary form of pyrometer is readily 

 projected in action by the shadow method ( 109) ; or a small 







