Non-luminous Radiation emitted by a Gas Flame. 



57 



of the distance from the source, and as the flame and chimney were of 

 considerable size, the position of the source was not well defined ; it 

 was also necessary to assume that the amount of absorption due to the 

 air, or to the aqueous vapour it contained, was negligible, or at least 

 that it could be allowed for. 



An ordinary thermopile with fifty-four couples and a low resistance 

 Nobili's galvanometer were used. The galvanometer needles, which 

 were rendered as nearly astatic as possible, were suspended in the 

 ordinary way by a silk fibre, a plane mirror was attached to them, and 

 the image of a slit placed in the slide-holder of a magic lantern 

 thrown by reflection from the mirror on to a scale placed about 

 0"8 metre from the galvanometer. The scale was divided into half 

 centimetres, and the position of the line of light was read to 1 mm. by 

 estimation. A deflection of 4° on the circle of the galvanometer cor- 

 responded to about five divisions on the scale. 



In order that the galvanometer should have a fairly fixed zero, it 

 was found necessary to place a weak magnet, a slightly magnetised 

 knitting needle, at a short distance from it ; this reduced the sensibility 

 of the instrument, but it was still much more sensitive to the feeble 

 thermoelectric currents than a low resistance (0*5 ohm) Thomson gal- 

 vanometer of the ordinary pattern. 



The galvanometer coil was nearly perpendicular to the magnetic 

 meridian, and the adjusting magnet so placed that the needles were 

 parallel to the axis of the coil. 



The thermopile was fixed at the end of a horizontal stand, to which 

 a divided scale was attached, and was connected by covered copper 

 wires with the galvanometer, a three-way plug being inserted in 

 the circuit. 



Both faces of the thermopile, which had been carefully blackened 

 with camphor smoke, were exposed ; the radiation from the lamp 

 whose light-giving power was to be measured fell on the one, and the 

 radiation from a compensating source of heat, an Argand with a metal 

 chimney, on the other. Tin-plate screens were placed on either side of 

 the pile, and the space between them stuffed with cotton wool, to 

 screen it from air currents and from the radiation of surrounding 

 objects, it being impossible to use the reflecting cones with the method 

 of measurement employed. 



Four cells of wood well soaked in paraffin were used to hold the 

 water and alum solution; their ends were closed with pieces of crown 

 glass, cut from the same plate of Messrs. Chance's manufacture, 

 which were pressed against the ends of the cells by wooden pressure- 

 plates and screws, a washer of vulcanised sheet-india-rubber being 

 interposed between the cell and the glass. This arrangement enabled 

 the same pair of plates to be used with all four cells. The plates 

 were 1*5 mm. thick, the cells were 15 cm. deep, 10 cm. wide, and 



