Oxygen
in chemistry, symbol 0; at. wt. 16; a dyad element existing in the free state in the atmosphere, and in combination in the ocean. İt forms about one-fifth of the former and eightninths of the latter. It is also present in the great majority of substances forming the earth's crust, and is the most abundant of all the elements. It was discovered in 1774 by Scheele in Sweden and Priestley in England independently, but the name was given by Lavoisier some time after. It can be obtained pure by heating black oxide of manganese, or a mixture of this oxide with potassic chlorate in a retort, and collecting the gas over water. When