NICHOLS. — THE VISIBLE RADIATION FROM CARBON. 93 



The iucrease of temperature as the flame approaches the junction is 

 gradual at first: but at a distance of about 0.4 cm. from the median 

 plane, the curve suddenly becomes steep. It is probable that this 

 distance measures the thickness of the layer of non-luminous gas which 

 surrounds the visible flame. Outside of this region, the junction is 

 heated almost altogether by radiation. As soon as it penetrates the 

 column of moving gas, however, heat is brought to it principally by 

 convection. Before the surface of the luminous mantle is reached the 

 curve shows indications of approaching a maximum. 



Upon pushing the flame still nearer to the junction so that the latter 

 penetrated the luminous region, an accumulation of lampblack began to 

 form upon the wire, with fall of temperature ; a process so rapid that at 

 the end of two minutes a button of carbon several millimeters in diameter 

 is formed. This is finally torn loose from the wire by its own weight; 

 whereupon the deposition of a new mass begins. I attempted by watch- 

 ing the breaking away of the carbon from the wire, which occurred at 

 regular intervals, to determine the temperature of the wire before the 

 coating of carbon had begun to show itself again. The highest temper- 

 ature which it was possible to observe in this way was nearly one hundred 

 decrees below that in the luminous layer, and it was obvious from the 

 movement of the galvanometer needle that the junction was being rapidly 

 cooled by the deposition. 



Junction II. (diameter 0.01598 cm.) was now substituted for Junction 

 I., and a similar set of readings were made. This junction, as had been 

 anticipated, showed higher temperatures. It was found possible, owing 

 to the small diameter and consequently high temperature of the wire, to 

 penetrate further into the flame before the deposition of carbon began, 

 so that measurements with the junction actually within the luminous 

 layer could be made. The general form of the curve, as will be seen by 

 inspection of the figure (curve b) is the same as that obtained with Junc- 

 tion I. After penetrating the luminous mantle to a small fraction of a 

 millimeter, carbon began to gather upon this junction likewise, wilh 

 lowering of temperature, as in the case of Junction I. The attempt to 

 read temperatures immediately after the dropping of the accumulated 

 carbon showed that the highest temperature which could thus be ob- 

 served was again about one hundred degrees below the temperature of 

 the luminous mantle. It was clear in this case, as before, from the rapid 

 fall of temperature already going on, that this reading has no significance. 



Similar readings with Junction III. (diameter 0.0108 cm.) gave a third 

 curve of the same type as those plotted from the reading made with I. 



