The supervisory lamp associated with the peg in the wanted subscriber's jack glows from the time that the peg is inserted until the subscriber responds, when it darkens, in which condition it remains until the subscriber restores the receiver to the hook and causes the lamp to light up again.
The saturated aqueous solution is colourless and fumes strongly on exposure to air; after a time it darkens in colour owing to liberation of iodine.
Silk is readily distinguished from wool and other animal fibres by the action of an alkaline solution of oxide of lead, which darkens wool, &c., owing to the sulphur they contain, but does not affect silk, which is free from that body.
On exposure to light it rapidly darkens, a behaviour utilized in photography.
Creating smoky eye makeup essentially involves a makeup application that darkens the lids and highlights the brow bone.