as the arc was very unsteady, and when the pressure was raised it
was almost impossible to keep an arc going, partly because the arc
broke when it was elongated the least bit, and partly because a com
plete lantern of soot treess grew all round the crater, and seemed to
short-circuit the arc from time to time
The arc being very unsteady, no satisfactory reading of the voltage and current was possible. varied from 6080, and the amperes kept continually varying from 1520. At 40 1bs. with 20 amperes the volts varied from 50 60. The crater was not well developed, so that the radiation observation, even at low pressures, was not very satisfactory, while at high pressures the arc was too short to see into the crater at all, and the lantern of soot trees hida considerable length, 3 or 4 mm. of the negative carbon besides. The radiomicrometer gave 440 divisions with a good arc in air, and 380 with the moderately good crater in hydrogen. But this difference is no greater than would often occur with a good and moderately good crater, so that there is not any proof of a difference of temperature dne to cooling power of hydrogen. These experi- ments showed us that it was quite hopeless to get any measures of radiation under pressure with hydrogen.
We finally tried an atmosphere of carbon dioxide cylinder of liquid CO2, which was connected to our are box by a copper tube and stop valve. The arc burned fairly well in this and, except for the difficulty of getting a sufficiently long are at pressures above 150 lbs., some pretty satisfactory measures of radiation were obtained. We found that whenever the pressure was suddenly reduced, there was a fog formed in the box, which cut off the light enormously. Also by looking down the steel tube, which is closed at its end by a lens, we could see powerful convection currents in the gas which scattered a lot of light. At high pressure the refraction due to these currents prevented any sort of an image of the crater being formed while the pressure was varying. While the pressure was steady a good image could be formed. This tube is nearly 3 ft. in length, and onlyin in bore, and it would natnrally take time for the gas to settle down throughout its length. We propose to have this tube removed, and the apertnre in the box closed by a strong piece of plain glass, and to form an image of the carbons by a lens placed at a snit At from 60 to 80 lbs. pressure the voltage We used a gas, This we expect will remove the difficulty able distance outside. arising from these convection crrrents.
The result of all these experiments so far is that it would require more evidence than we have been able to get, to affirm that either the temperature of the crater of the arc is raised or lowered by pressure. We got some very concordant observations, which showed the temperature to be lowered with prossure, and in which at the time we could see no evidence of absorption by fog, but then, at other
