PEOFESSOR TTNDALL ON CALORESCENCE. 13 



quarter of an inch deep, through which cold water was caused to circulate. The cell 

 containing the solution was moreover surrounded by a jacket, and the current, having 

 completed its course round the aperture, passed round the solution. Thus the appara- 

 tus was kept cold. The neck of the cell was stopped by a closely-fitting cork ,• through 

 this passed a piece of glass tubing, which, when the cell was placed upon its stage, 

 ended at a considerable distance from the focus of the mirror. Experiments on com- 

 bustion might therefore be carried on at the focus without fear of igniting the small 

 amount of vapour which even under the improved conditions might escape from the 

 bisulphide of carbon. The arrangement will be at once understood by reference to 

 Plate I. figs, i a & b, which show the camera, lamp, and filter both from the side and 

 from the front, ipy is the mirror from which the reflected cone of rays passes, first 

 through the rock-salt window (unshaded), and afterwards through the iodine filter m n. 

 The rays converge to the focus k, where they would form an invisible image of the lower 

 carbon point ; the image of the upper would be thrown below k ; and both images spring 

 vividly forth when a leaf of platinized platinum is exposed at the focus. At ss (Plate I. 

 fig. 4 a) is shown, in section, the annular space in which the cold water circulates. 

 Fig. 4 b (Plate I.) shows the manner in which the water enters this space and passes 

 from it to the jacket surrounding the iodine-cell m. 



With this arrangement, and a battery of fifty cells, the following results were ob- 

 tained : — 



A piece of silver-leaf, fastened to a wire ring and tarnished by exposure to the fumes 

 of sulphide of ammonium, being held in the dark focus, the film flashed out occasionally 

 into vivid redness. 



Copper-leaf tarnished in a similar manner, when placed at the focus, was raised to 

 redness. 



A piece of platinized platinum foil was supported in an exhausted receiver, the vessel 

 being so placed that the focus fell upon the platinum. The heat of the focus was 

 instantly converted into light, a clearly defined and inverted image of the points being 

 stamped upon the metal. Fig. 5 (Plate I.) represents the thermograph of the carbons. 



Blackened paper was now substituted for the platinum in the exhausted receiver. 

 Placed at the focus of invisible rays, the paper was instantly pierced, a cloud of smoke 

 was poured through the opening, and fell like a cascade to the bottom of the receiver. 

 The paper seemed to burn without incandescence. Here also a thermograph of the coal 

 points was stamped out. When black paper is placed at the focus, where the thermal 

 image is well defined, it is always pierced in two points, answering to the images of the 

 two carbons. The superior heat of the positive carbon is shown by the fact that its 

 image first pierces the paper ; it bums out a large space, and shows its peculiarly crater- 

 like top, while the negative carbon usually pierces a small hole. 



Paper reddened by the iodide of mercury had its colour discharged at the places on 

 which the invisible image of the coal points fell upon it. 



