Page:On the Influence of Carbonic Acid in the Air upon the Temperature of the Ground.pdf/12

strong absorption-bands in this interval, among which those marked $\rho$, $\sigma$, $\tau$, and $\phi$ are the most prominent^[1], and these absorption-bands belong most probably to the aqueous vapour, That Paschen has not observed any emission by water-vapour, in this interval may very well be accounted for by the fact that his heat-spectrum had a very small intensity for these short-waved rays. But it may be conceded that the absorption-coefficient for aqueous vapour at this angle in Table II. is not very accurate (probably too great), in consequence of the little importance that Langley attached to the corresponding observations. After this occurs in Langley's spectrum the great absorption-band $\psi$ at the angle 39.45 ($\lambda =1.4~\mu$), where in Paschen's curve the emission first becomes sensible ($\log y=-0.1105$ in Table II.). At wave-lengths of greater value we find according to Paschen strong absorption-bands at $\lambda =1.83~\mu$ ($\Omega$ in Langley's spectrum), i. e. in the neighbourhood of 39°.30 and at $\lambda =2.64~\mu$; (Langley's ${\text{X}}$) a little above the angle 39°.15. In accordance with this I have found rather large absorption-coefficients for aqueous vapour at these angles ($\log y=-0.0952$ and $-0.0862$ resp.). From $\lambda =3.0~\mu$ to $\lambda =4.7~\mu$ thereafter, according to Paschen the absorption is very small, in agreement with my calculation ($\log y=-0.0068$ at 39°, corresponding to $\lambda =4.3~\mu$). From this point the absorption increases again and presents new maxima at $\lambda =5.5~\mu$, $\lambda =6.6~\mu$, and $\lambda =7.7~\mu$, i. e. in the vicinity of the angles 38°.45 ($\lambda =5.6~\mu$) and 38°.30 ($\lambda =7.1~\mu$). In this region the absorption of the water-vapour is continuous over the whole interval, in consequence of which the great absorption-coefficient in this part ($\log y=-0.3114$ and $-0.2362$) becomes intelligible. In consequence of the decreasing intensity of the emission-spectrum of aqueous vapour in Paschen's curve we cannot pursue the details of it closely, but it seems as if the emission of the water-vapour would also be considerable at $\lambda =8.7~\mu$ (39°.15), which corresponds with the great absorption-coefficient ($\log y=-0.1983$) at this place. The observations of Paschen are not extended further, ending at $\lambda =9.5~\mu$, which corresponds to an angle of 39°.08.

For carbonic acid we find at first the value zero at 40°, in agreement with the figures of Paschen and Ångström^[2]. The absorption of carbonic acid first assumes a sensible value at

1. Langley, Ann. Ch. et Phys. sér. 6, t. xvii, pp. 323 and 326, 1889, Prof. Papers, No. 15, plate 12. Lamansky attributed his absorption-bands, which probably had this place, to the absorbing power of aqueous vapour (Pogg. Ann. cxlvi. p. 200, 1872).
2. It must be remembered that at this point the spectrum of Paschen was very weak, so that the coincidence with his figure may be accidental.