METHOD OF OBTAINING OXIGEN GAS.

IS 7

with nitrogen gas. The weight of the two gases when collectedwill be found to correspond very exactly with the weight of theacid which had been decomposed. The residue then left in theretort is potash.

4. Black oxid of manganese however is generally made use offor obtaining oxigen gas, on account of its cheapness. This na-tive oxid is reduced to a coarse powder; a stone or rather aniron retort is then charged with it and heated, As soon as theretort becomes ignited, oxigen gas is obtained plentifully.

Rationale. ..Black oxid of manganese is the metal calledmanganese fully saturated with oxigen, together with manyearthy impurities; on applying heat, part of the solid oxigenquits the metal and unites to caloric in order to form oxigen gas,the remainder of the oxigen remains united to the metal with aforcible affinity; the metal therefore re-approaches to the metal-lic state or is found in the state of a grey oxid of manganese.

One pound of the best manganese yields upwards of 1400 cu-bic inches of oxigen gas, nearly pure. If sulphuric acid be pre-viously added to the manganese, the gas is produced by a lessheat and in a larger quantity; a glass retort may then be used,and the heat of a lamp is sufficient.

5. Red oxid of mercury yields oxigen gas in a manner similarto that of manganese.

Rationale. ..This oxid consists likewise of solid oxigen andmercury, the combination of which takes place on exposing mer-cury to a heat of about 6lO° Fahr. At this degree it attractsoxigen and becomes converted into an oxid; but if the tempera-ture be increased to about 1000° the attraction of oxigen ischanged. The oxigen then attracts caloric stronger than it didthe mercury, it therefore abandons it, and forms oxigen gas.Ihe mercury then re-appears in its metallic state. (See p. 50.)

6. Red oxid of lead yields oxigen gas on the same principle.