448 



Prof. Tyndall on Calorescence. 



immediate, I was at first surprised at the time necessary to pro- 

 duce it. We are here reminded of Franklin's experiments on 

 cloths of different colours, and his conclusion that dark colours 

 are the best absorbers. This conclusion, however, might readily 

 be pushed too far. Franklin's colours were of a special kind, 

 and their deportment by no means warrants a general conclu- 

 sion. The invisible rays of the sun possess, according to Miiller, 

 twice the energy of the visible ones. A white substance may 

 absorb the former, while a dark substance — dark because of its 

 absorption of the feeblest portion of the radiation — may not do 

 so. The white powder of alum and the dark powder of iodine, 

 exposed to the action of a source in which the invisible rays 

 greatly surpass the visible in calorific power, exhibit a deport- 

 ment at direct variance with the popular notion that dark colours 

 are the best absorbers. 



§ 10. In conclusion, I would briefly refer to a few experiments 

 made to determine the calorescence obtainable through glasses 

 of various colours. In the first column of the subjoined Table 

 the colour of the glass is given ; in the second column the effect 

 observed when a brilliant spectrum was regarded through the 

 glass is stated; and in the third column the appearance of a leaf 

 of platinized platinum when placed at the focus, after the con- 

 verged beam had passed through the glass, is mentioned. 



Colour of glass. Prismatic examination. Calorescence. 



Dark red . . . Red only transmitted . . . Dull white heat. 

 Mean red . . . Red only transmitted . . . White heat. 

 Yellow intercepted with greatest 

 power. 



All the blue end absorbed . . 



Light red 

 Yellow 



Bright white. 



Green . . . . 



Dark purple . . 



Mean purple . . 



Light purple . . 

 Dark blue . 



Mean blue . . . 



Light blue . . . 



Another blue glass. 

 Black glass No. 1 . 

 Black glass No. 2 . 

 Black glass No. 3 . 



{Besides the green, a dull red 

 fringe and a blue band were 

 transmitted. 

 I Extreme blue and red trans- 

 j mitted. 

 Central portion of spectrum cut 

 out. 

 i Dims the whole spectrum, but 

 ) chiefly absorbs the green. 

 I Transmits the blue,a green band, 

 I and a band in the extreme red. 

 I Blue; ayellowish-green band and 

 I the extreme red transmitted. 

 ( Transmits a series of bands — 

 ) blue and green, a red band next 

 ) orange, then a dark-red band, 

 ( and finally extreme red. 



I Vivid red with bright 

 j yellow in centre. 



No incandescence. 



Vivid orange. 

 Vivid orange. 

 Vivid orange. 

 Red heat. 

 Reddish-pink heat. 



Pink heat, 

 into red. 



Pink heat. 



passing 



Dims all the spectrum: white I Barely visible red . 



light transmitted. \ J 



Whitish-green light transmitted Dull red. 



Deep-red light transmitted . | "hemiddle™ 186 m 



