_To introduce Clouds_ (Vol. viii., p. 451.) as desired by your
correspondent [Greek: S]., the negative must be treated in the sky by
solution of cyanide of potassium laid on in the form desired with a camel's
hair pencil. This discharges a portion of the reduced silver, and allows
the light to penetrate; but great care is required to stop the action by
well washing in water before the process has gone too far. White clouds are
produced by painting them in with a black pigment mixed in size.