Page:On Radiation.djvu/51

 absorptions effected by a series of vapours at a pressure of $1⁄60$th of an atmosphere:—

Bisulphide of carbon is the most transparent vapour in this list; and acetic ether the most opaque; $1⁄60$th of an atmosphere of the former, however, produces 47 times the effect of a whole atmosphere of air, while $1⁄60$th of an atmosphere of the latter produces 612 times the effect of a whole atmosphere of air. Reducing dry air to the pressure of the acetic ether here employed, and comparing them then together, the quantity of wave-motion intercepted by the latter would be many thousand times that intercepted by the air.

Any one of these vapours discharged in the free atmosphere, in front of a body emitting obscure rays, intercepts more or less of the radiation. A similar effect is produced by perfumes diffused in the air, though their attenuation is known to be almost