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.
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.
He examined the opacity of the air for sound in connexion with lighthouse and siren work, and he finally clinched the proof of what had been already substantially demonstrated by several others, viz.
The enteron arises as a space in the endoderm, and an opacity - the primitive streak - appears at the hind end of the blastopore (fig.
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.