Prof. Tyndall on Calorescence. 449 



The extremely remarkable fact here reveals itself, that when 

 the beam of the electric lamp is sifted by certain blue glasses, the 

 platinum at the focus glows with a distinct pink colour. Every 

 care was taken to avoid subjective illusion here. The pink 

 colour was also obtained at the focus of invisible rays. With- 

 drawing all the glasses, and filtering the beam by a solution of 

 iodine alone, platinum was raised nearly to whiteness at the 

 focus. On introducing the pale-blue glass between the iodine- 

 cell and the focus, the calorescence of the platinum was greatly 

 enfeebled — so much so, that a darkened room was necessary to 

 bring it out in full distinctness ; when seen, however, the ther- 

 mograph was pink. A disk of carbonized paper being exposed 

 at the obscure focus, rose at once to vivid whiteness when the 

 blue glass was absent ; but when present, the colour of the light 

 emitted by the carbon was first a distinct pink ; the attack of 

 the atmospheric oxygen soon changes this colour, the combus- 

 tion of the carbon extending on all sides as a white-hot circle. 

 If subsequent experiments should confirm this result, it would 

 follow that there is a gap in the calorescence, the atoms of the 

 platinum vibrating in red and blue periods, and not in interme- 

 diate ones. But I wish here to say that further experiments, 

 which I hope shortly to make, are necessary to satisfy my own 

 mind as to the cause of this phenomenon. 



* The incandescent thermograph of the coal-points being ob- 

 tained, a very light-red glass introduced between the opake solu- 

 tion and the platinum reduced the thermograph both in size and 

 brilliancy. A second red glass, of deeper colour, rendered the 

 thermograph still smaller and feebler. A dark-red glass reduced 

 it still more — the visible surface being in this case extremely 

 minute, and the heat a dull red merely. When, instead of the 

 coloured glass, a sheet of pure-white glass was introduced, the 

 image of the coal-points stamped upon the platinum-foil was 

 scarcely diminished in brilliancy. A thick piece of glass of deep 

 ruby-red proved equally transparent; its introduction scarcely 

 changed the vividness of the thermograph. The colouring- 

 matter in this instance was the element gold, not the compound 

 suboxide of copper employed in the other red glasses. Many 

 specimens of gold-jelly, prepared by Mr. Faraday for his investi- 

 gation of the colours of gold, though of a depth approaching to 

 absolute blackness, showed themselves eminently transparent to 

 the obscure heat-rays ; their introduction scarcely dimmed the 

 brilliancy of the thermograph. Hence it would appear that even 

 the metals themselves, in certain states of aggregation, share 

 that high diathermic power which the elementary metalloids 

 have been found to display. 



I have just said that a sheet of pure- white glass, when inter- 



