in which case, when chopped, put them in the corner of a napkin or cloth, wash them in water, squeeze them dry, then put them back on the board, and chop finer, or sometimes only rubbing the pan or the meat with the onion is quite sufficient.” (Soyer.)
O′NYX. A sub-species of quartz often wrought into small ornamental articles. Among jewellers, any stone exhibiting layers of two or more colours, strongly contrasted, is called an ‘onyx.’ A regularly and richly banded agate of this class is much prized for cameos. The Sardonyx of the ancients is a variety of onyx. It is so called from Sard or Sarda, a rich dark-red variety of carnelian, stripes of which in the stone alternate with white stripes. See Gems.
O′PAL. A mineral allied to agate and chalcedony, but distinguished by its peculiar resinous lustre. The variety most admired as a gem is the precious or noble opal, which is remarkable for its beautiful play of colours. The finest opals come from Hungary, and although usually very small, will, if of good quality, realise as much as £5 each. Their value increases in a much greater ratio than their increase in size. They are also found in Saxony and in North America. The largest known specimen of an opal is that in the Imperial Cabinet at Vienna, the dimensions of which are five inches by two and a half. Girasol, Cacholong, Hyalite, and Menilite are varieties of opal. See Gems and Pastes.
O′OLITE. A variety of limestone composed of a number of collections of small round particles, bound together by a calcareous cement. The resemblance the mass is supposed to bear to the roe of a fish has caused oolite to be called roestone. When the grains are of larger dimensions it is called peastone. The little spherical bodies of which the stone is composed are mostly formed of concentric layers of carbonate of lime arranged round a grain of sand, a fragment of shell or some other nucleus.
The building stones of Caen, Portland, and Bath are all oolitic limestones. When first quarried they are mostly soft, a circumstance which admits of their being easily sawn and carved. They harden by exposure to the air.
In geology the term “oolite” has a wider signification, and is applied to an important division of the middle secondary rocks of England, underlying the chalk formation and rich in the interesting fossil remains.
OPHTHAL′MIA. Syn. Ophthalmitis, L. Inflammation of the eye. In ordinary cases this disease is confined to the external membrane of the globe of the eye or to the eyelids; but it occasionally attacks the sclerotica, cornea, choroid coat, and retina.
The common causes of ordinary or conjunctival ophthalmia are the sudden exposure of the organ to a cold easterly wind, to dust, gritty particles, or to any external irritation or injury.
The symptoms are, in part, those common
to local inflammation. The eye or eyelids become more or less bloodshot, swollen, and tender, and a sensation resembling that induced by the presence of particles of sand or some gritty substance, accompanied by much heat and a pricking pain, is almost constantly experienced. The secretion becomes yellowish and glutinous, and during the night frequently glues, as it were, the lids together. Sometimes only one eye is attacked, but after two or three days the disease extends to the other.