jets and clouds of hydrogen cooler than the luminous prominences, and so looking black when projected on a background of the hotter gas. Tacchini and Respighi have kept up a careful and systematic record of chromospheric phenomena in a statistical way.
During the past four years the most important investigations upon the solar radiation have been those of Langley. The results of the Mount Whitney Expedition of 1881, with those of certain supplementary investigations, were published in 1884. They fully confirm his earlier conclusion, that the previously received value of the solar constant should be largely increased, and from his data he fixes it at about thirty calories per square metre per minute instead of twenty-five. Of course, this involves a corresponding increase of twenty per cent in all the figures which are given to illustrate the immensity of the solar radiation in various ways.
It is worth noting also that Langley, following many French authorities, prefers to employ a smaller heat-unit than the calory used by the writer—the gramme-degree instead of the kilogramme-degree. His solar constant is the number of these small calories received per minute upon a square centimetre, and therefore on his scale of notation stands as three instead of thirty. We mention it, because this discordance in the definition of the calory has led to some confusion among those not entirely familiar with the subject. If we were to follow strictly the so-called c.g.s. system, the solar constant would be represented by a number still sixty times smaller—viz., 0⋅050 (gramme-degrees per centimetre per second).
Professor Langley has also, with the bolometer, carried the investigation of the invisible portion of the sun's heat-spectrum far beyond any of his predecessors, and has discovered and measured the wavelength of a large number of absorption-bands in this region; his results are confirmed by those of Becquerel and Abney, the latter operating by means of photography, and the former by means of the effect of the invisible infra-red radiations in quenching the phosphorescence of a suitably prepared screen.
Langley finds that the solar spectrum seems to terminate abruptly at a wave-length of about thirty thousand on Ångström's scale: he does not find in the sun's heat any of the long-waved, slowly-pulsating radiations, such as are emitted by bodies at or below the temperature of boiling water. We might think that they had been absorbed in the earth's atmosphere, were it not that he finds just these rays relatively abundant in the spectrum of lunar heat. He also finds them in the heat-spectrum of the electric arc, so that it is difficult to suppose that they do not originally exist in the solar spectrum. Unless there is some hidden fallacy or error in some of the observations, we are almost driven to admit that they have been absorbed in interplanetary space. But probably it will be best to await further confirmation of the experimental results before accepting so remarkable a conclusion.
