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336
N. T. GORDON
]J. S. M. P. E.
walled cylindrical glass jacket. The space of 1.7 cm. between the double walls was filled in turn with water, 0.5 per cent CuCl2 solution, and 1 per cent CuCl2 solution. The visible light was measured with a photometer, and the radiant energy by means of a thermopile and sensitive galvanometer.
Table II contains the results of these experiments, the data for the 400 watt lamp being kindly furnished by Dr. G. R. Fonda, and by Mr. F. A. Benford and Dr. S. Dushman for the 150 watt lamp. There appears to be some discrepancy for the values obtained with water,
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but I think these may be explained as being due to different experimental conditions. The fact which is most striking, however, is the advantage of a copper chloride solution over water for the relatively high ratio of absorption of heat to light. A layer of CuCl2 1.7 cm. thick transmits only 8 per cent of the radiant energy for constant light intensity, and this 8 per cent must lie almost entirely in the visible spectrum. However, as is so often the case, certain difficulties must be overcome before copper chloride may be used satisfactorily. These difficulties include hydrolysis of the salt, corrosion of metal parts, and the formation of deposits when slight evaporation occurs. It seemed expedient, therefore, to proceed experimentally with a unit using water as the absorbing medium.