be considered as a semi-transparent medium, which appears to us yellow or reddish before a light ground, but blue before a dark one.
161.
If we now turn our attention to fluid mediums, we find that water, deprived in a very slight degree of its transparency, produces the same effects.
162.
The infusion of the lignum nephriticum (guilandina Linnæi), which formerly excited so much attention, is only a semi-transparent liquor, which in dark wooden cups must appear blue, but held towards the sun in a transparent glass must exhibit a yellow appearance.
163.
A drop of scented water, of spirit varnish, of several metallic solutions, may be employed to give various degrees of opacity to water for such experiments. Spirit of soap perhaps answers best.
164.
The bottom