Tyndall's investigations of the transparency and opacity of gases and vapours for radiant heat, which occupied him during many years (1859-1871), are frequently considered his chief scientific work.
Sometimes this principle has weight, and sometimes it has not; sometimes it is free fire and sometimes it is fire combined with the earthy element; sometimes it passes through the pores of vessels, sometimes these are impervious to it; it explains both causticity and non-causticity, transparency and opacity, colours and their absence; it is a veritable Proteus changing in form at each instant."
Semi-opacity and opacity are usually produced by the addition to the glass-mixtures of materials which will remain in suspension in the glass, such as oxide of tin, oxide of arsenic, phosphate of lime, cryolite or a mixture of felspar and fluorspar.
Substances having high specific gravity, malleability, opacity, and especially a peculiar lustre, the term "metal" became generic for all substances with these properties.
Closely associated with the effect of continental immobility are the effects dependent on the low specific heat and the opacity of the lands, in contrast with the high specific heat and partial transparence of the ocean waters.