Pour into a glass tube, about ten inches long, and one inch in
diameter, a little water coloured with pink or other dye; then fill
it up gradually and carefully with colourless water, so as not to mix
them: apply heat at the bottom of the tube, and the coloured water will
ascend and be diffused throughout the whole.
The circulation of warm water may be very pleasingly shown, by heating
water in a tube similar to the foregoing; the water having diffused in
it some particles of amber, or other light substance not soluble in
water.