Of flake charcoal is generally sufficient for the insulation of the holds, though for deck-houses and other parts exposed to the sun the thickness must be greater.
The blue-bird with his azure plumes, the thrush clad all in brown, the robin jerking his spasmodic throat, the oriole drifting like a flake of fire, the jolly bobolink and his happy mate, the mocking-bird imitating the notes of all, the red-bird with his one sweet trill, and the busy little wren, are all making the trees in our front yard ring with their glad song.
The bluebird, breathing from his azure plumes The fragrance borrowed from the myrtle blooms; The thrush, poor wanderer, dropping meekly down, Clad in his remnant of autumnal brown; The oriole, drifting like a flake of fire Rent by a whirlwind from a blazing spire; The robin, jerking his spasmodic throat, Repeats imperious, his staccato note; The crack-brained bobolink courts his crazy mate, Poised on a bullrush tipsy with his weight: Nay, in his cage the lone canary sings, Feels the soft air, and spreads his idle wings.
These cracks coincide with planes of easy separation or of gliding in the crystal; they are especially useful in helping to determine the crystallographic orientation of a cleavage flake of mica when crystal faces are absent.
Of flake charcoal and vegetable silica, or II of small pumice, are required to give the same protection as 7 in.
The acute bisectrix of the optic axes never deviates from the normal to the basal plane by more than a degree or two, hence a cleavage flake of mica will always show an optic figure in convergent light when placed on the stage of a polarizing microscope.