3 o THE POPULAR SCIENCE MONTHLY. 



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 

 Blount Whitney Expedition of 1881, with those of certain supple- 

 mentary 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 au- 

 thorities, prefers to employ a smaller heat-unit than the calory used 

 by the writer — the gramme-degree instead of the kilogranwne-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 discord- 

 ance 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 rep- 

 resented 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 inves- 

 tigation of the invisible portion of the sun's heat-spectrum far beyond 

 any of his predecessors, and has discovered and measured the wave- 

 length 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 Angstrom'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 

 lu:it-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 ex- 

 perimental results before accepting so remarkable a conclusion. 



