OXYGEN 



OYSTER 



685 



Owing to the intensity with which many of these 

 combinations take place, this gas has the power of 

 supporting Combustion (q.v. ) in an eminent degree. 

 It is only slightly soluble in water ; 100 cubic inches 

 of that liquid dissolving 4'11 cubic inches of gas 

 at 32, and only 2 "99 inches at 59. It is slightly 

 heavier than air, its specific gravity being 1 '1056. 



Oxygen gas is not only respirable, but is essential 

 to the snp]K)rt of animal life ; and hence it was 

 termed vital air by some of the older chemists. A 

 small animal placed in a bell-glass containing pure 

 oxygen will not be suffocated as soon as if it were 

 placed in the same glass filled with atmospheric 

 air. For further details on this property of oxygen, 

 the reader is referred to the article RESPIRATION. 



Oxygen is the most abundant and the most 

 widely distributed of all the elements. In its free 

 state (mixed but not combined with nitrogen) it 

 constitutes about a fifth of the bulk, and consider- 

 ably more than a fifth of the weight, of the atmo- 

 sphere. In combination with hydrogen, it forms 

 eight-ninths of all the water on the globe ; and in 

 combination with silicon, calcium, aluminium, &c., 

 it enters largely into all the solid constituents of 

 the earth's crust ; silica in its various forms of 

 sand, common quartz, flint, &c. chalk, limestone, 

 marble, and all the varieties of clay, containing 

 about half their weight of oxygen. It is, more- 

 over, found in the tissues and fluids of all forms of 

 animal and vegetable life, none of which can sup- 

 port existence independently of this element. 



There are various laboratory methods of obtain- 

 ing oxygen on the small scale, the simplest of 

 which consists in the exposure of certain metallic 

 oxides to a high temperature. It was originally 

 obtained by its discoverer, Dr Priestley, from the 

 red oxide of mercury, which, when heated to about 

 750, resolves itself into metallic mercury and 

 oxygen gas. It may be obtained similarly from 

 red oxide and peroxide of lend, the resulting pro- 

 ducts being protoxide of lead and oxygen. 



The ordinary laboratory method commonly em- 

 ployed to obtain an abundant supply of oxygen 

 consists in heatinj* chlorate of potash, KCIO 3> 

 which yields up all its oxygen (amounting to 39'16 

 per cent.), and leaves a residue of chloride of 

 potassium. One ounce of this salt yields nearly 

 two gallons of oxygen gas. It is found by experi- 

 ment that if the chlorate of potash is mixed with 

 about a fourth of its weight of Mack oxide of copper, 

 or of binoxide of manganese, the evolution of the 

 gas is greatly facilitated, although the oxides do not 

 seem to undergo any change during the process. 



Various processes have been proposed for obtain- 

 ing oxygen on the large scale, but only in recent 

 years has the commercial production of the gas 

 been carried out sufficiently cheaply to enable 

 oxygen to be employed extensively for industrial 

 pn rposes. The_ method employed by Erin's Oxygen 

 Company consists in passing air under pressure 

 over barium oxide, BaO, heated to a temperature 

 of dull redness. In this way a quantity of barium 

 peroxide, BaO,, is formed, and this can be made 

 to aj;ain yield up its extra oxygen in the pure state 

 ( being reduced ajjain to BaO ) by heating to a full 

 red heat, or, as is actually done in practice, by 

 greatly diminishing the gaseous pressure without 

 altering the temperature. It is estimated that 

 oxygen can be produced by this process at a cost 

 of from 5s. to 7s. 6d. per 1000 cubic feet. Oxygen 

 can now be obtained in practically any required 

 quantity in wronght-steel cylinders, in which it is 

 compressed up to a pressure of 120 atmospheres. 



Of the compounds of oxygen it is unnecessary to 

 speak here, as they are described in the articles on 

 the other chemical elements. 



Oxygen was discovered almost simultaneously, 

 in the year 1774, by Priestley and by Scheele, the 



Swedish chemist having, however, nearly completed 

 his discovery in 1772. Priestly called it Dcphloyisti- 

 cated Air ; Scheele termed it Empyreal Air; Con- 

 dorcet shortly afterwards suggested Vital Air, as 

 its most appropriate designation ; and in 1789 

 Lavoisier, who, by a series of carefully conducted 

 and very ingenious experiments, proved that the 

 combustion of bodies in the air consisted essentially 

 in their chemical combination with oxygen, and 

 thus overthrew the Phlogiston (q.v. ) theory, gave 

 it the name which it now retains (from oxys, 

 'acid,' and gennao, 'I produce'), in consequence 

 of his (erroneously) believing that it was a neces- 

 sary constituent of every acid. 



Oxyhydrogen. See LIME-LIGHT. 



Oyer and Tcrminer (Fr. ouir, 'to hear;' 

 terminer, ' to determine '). See ASSIZE. 



Oyster (Ostrea), a genus of bivalves, the mem- 

 bers of which are well known to be very passive 

 and very palatable. Structure. The fundamental 

 characteristics, as displayed by the favourite Euro- 

 pean species, Ostrea edulis, are those of other bivalve 

 Mollusca (q.v.), but the 'foot,' with which many 

 less sedentary forms move, is almost completely 

 degenerate, the two valves of the shell are unequal, 

 the hinge which unites them is without teeth, and 



Diagram of Internal Structure. 



The dorral surface is downwards, the anterior or head end to 

 the left, a, region where water enters and leaves the animal ; 

 the dark lines indicate where one mantle-flap lias been cut 

 away to expose the other structures ; 6, gills ; c, margin of 

 one of the mantle-folds ; d, anterior part of hinge ; e, hood 

 over mouth ; /, position of mouth ; g, ft, labial palps ; i, end 

 of intestine ; I, the closing muscle of the shell ; m, position of 

 the heart. 



the powerful closing muscle is almost median in 

 position. The left valve of the shell, that by which 

 the animal fixes itself, is hollowed out, while the 

 other is almost flat, and the whole animal is slightly 

 nnsymmetrical. On an opened oyster it is easy to 

 detect the fringed mantle which lines and makes 

 the shell, the ciliated gills or ' beard,' two some- 

 what similar flaps (labial palps) on each side of the 

 mouth, which, overhung by a hood, lies near one 

 end of the hinge, the brownish digestive gland, the 

 heart and the kidneys close beside the shell-shut- 

 ting muscle. ' I suppose,' says Professor Huxley, 

 ' that when the sapid and slippery morsel which 

 is and is gone like a flash of gustatory summer 

 lightning glides along the palate, few people 

 imagine that they are swallowing a piece of 

 machinery (and going machinery too) greatly more 

 complicated than a watch ' in fact a living organ- 

 ism of a high order. 



General Life. The oyster feeds on microscopic 

 organisms which are washed into the gaping shell 

 and on to the mouth by the ciliary activity of the 

 gills and palps ; and it may be noted that the 

 greenish tinge, regarded by epicures as one of the 

 highest credentials of an oyster, is probably due to 

 a copious diet of minute green algre. As every one 

 knows, oysters live gregariously in ' beds ' or 



