78 Intelligence and Miscellaneous Articles. 



ON THE LAW OF ABSORPTION OF RADIATIONS THROUGH BODIES, 



AND ITS EMPLOYMENT IN QUANTITATIVE SPECTRAL ANALYSIS 



(PART I.). BY G. GOVI. 



"When we interpose an absorbent medium in the path of the 

 white light which passes through the slit of a spectroscope, we 

 usually see dark bands appear in different parts of the spectrum, 

 which there diminish the brightness of the colours or even com- 

 pletely extinguishes them. It is seldom that these bands do not 

 invade a great number of contiguous wave-lengths, which they ob- 

 scure in spreading more feebly on both sides of a more intense line 

 of absorption. If the thickness of the absorbent medium be aug- 

 mented, fresh shaded bands often appear between the former ones ; 

 but what never fails to be produced is the strengthening of the 

 first bands and their progressive dilatation ; so that, for a certain 

 thickness of the medium, the entire spectrum is invaded by the 

 shade, and so much enfeebled that it may be regarded as quite ex- 

 tinguished. 



This progressive widening of the absorption-bands singularly 

 reminds one of the increase in number and the dilatation of the 

 bright lines which several observers have verified in the spectra of 

 incandescent gases in proportion as their rarefaction is diminished 

 and their temperature augmented ; so that it is quite possible the 

 two phenomena may correspond and be complementary the one to 

 the other for one and the same substance. Moreover all the radia- 

 tions, visible and invisible, of the spectrum present analogous phe- 

 nomena ; and if we here speak of the luminous radiations only, it is 

 solely because their study is much more convenient and more usual 

 than that of the ultra-red or uLra-violet radiations. 



It is obvious, from what has just been said, that the absorptive 

 power of a substance is not sufficiently characterized by such or 

 such a dark band appearing in the spectrum of the white light 

 which has passed through a certain thickness of it, and that, in 

 order to define it perfectly, we must know all the modifications it 

 can determine in the spectrum, from the slightest and most limited 

 up to that which produces sensible extinction of all the radiations. 

 In other words, we do not truly know the absorptive power of a 

 substance unless we have determined its coefficients of absorption 

 for all the wave-lengths that can be studied, from those correspond- 

 ing to obscure heat to those met with at the limit of photogenic 

 action. 



This is why Sir J. Herschel, and many others after him, have 

 attempted to construct, through points, the curves which were to 

 express the values of the absorption- coefficients, as functions of 

 the wave-lengths, for different bodies ; but the discontinuity of the 

 artifices employed, and the absence of every photometric measure, 

 have hitherto permitted only very incomplete results to be obtained. 

 It is nevertheless not impossible to obtain a more rigorous defini- 

 tion of the absorptive power of bodies, either by making directly 

 apparent to sight the curves themselves of equal chromatic absorp- 



