518 Prof. S. P. Langley on the Invisible 



It may be asked if we can, after all, feel sure of the character 

 of such minute amounts of heat in the presence of the already 

 described reflected and diffused heat from the upper spectrum, 

 considering the possibility that something may go wrong in 

 the elaborate arrangement of the sifting-train, and leave us 

 (as everything we study now is invisible) without ocular 

 warning of the fact. I may reply that we have lately found 

 an admirable check upon the efficiency of our optical devices 

 in the behaviour of that familiar substance lampblack, which 

 all physicists use either on thermometers, thermopiles, or 

 bolometers. All of us know probably that it is not absolutely 

 non-selective, as it used to be thought, and that it has a ten- 

 dency to transmit the infra-red with greater freedom than the 

 visible spectrum ; but the statement I am about to make may 

 excite surprise. It is that when a very perfectly polished 

 rock-salt plate is covered by a sheet of lampblack of such 

 thickness as to transmit less than one per cent, of ordinary 

 white light, it transmits about ninety per cent, of the radiations 

 belonging to these extreme wave-lengths. In other words, it 

 has become a transparent body to rays of this wave-length, 

 while it exercises an intermediate degree of absorption on 

 intermediate rays, so that by the amount of their absorption 

 by lampblack we have a test by which these latter may be 

 independently identified. 



We have given this study not only to map new lines and 

 bands in the region between 3^ and 5^, but also to verify 

 the existence of an all but infinitesimal amount of heat at 

 much greater wave-lengths. We have applied great pains to 

 this latter, not so much on account of its own importance as 

 on account of the important conclusions to be drawn from it 

 later ; for if it is true that in this extreme spectral region, 

 corresponding to temperatures much below the boiling-point 

 and even below that of melting ice, the amount of the solar 

 heat is trivial, it is also true that the fact of its existing at all 

 is of very significant interest to the meteorologist ; for it is 

 obviously here, at temperatures below 100° C, that the rays 

 which make up the nocturnal as well as diurnal radiations 

 from the soil of our own planet are to be found. We observe 

 that if such rays can enter the air from the sun, they can go 

 out even from an icy soil (and still more from an ordinary 

 one), to whose radiations it hence appears the atmosphere is 

 more or less permeable. Heat, then, apparently escapes in 

 some very minute degree even from the surface of the Arctic 

 regions, not only by convection but by direct radiation 

 through the atmosphere towards space. Meteorological ques- 

 tions of great interest, to which we shall return in a later 



