absorptions effected by a series of vapours at a pressure of 160th of an atmosphere:—
| Name of vapour. | Absorption. | ||
| Bisulphide of carbon | 47 | ||
| Iodide of methyl | 115 | ||
| Benzol | 136 | ||
| Amylene | 321 | ||
| Sulphuric ether | 440 | ||
| Formic ether | 548 | ||
| Acetic ether | 612 | ||
Bisulphide of carbon is the most transparent vapour in this list; and acetic ether the most opaque; 160th of an atmosphere of the former, however, produces 47 times the effect of a whole atmosphere of air, while 160th 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
