Oxygen was first liquefied in 1877 but was not obtained in the liquid form in considerable quantities until 1883. (See Liquefaction of Gases, p. 495). Liquid oxygen is a pale, steel blue, transparent liquid when in considerable masses. The density of liquid oxygen at 182.5° C. is 1.1181; the density of solid oxygen is 1.4256 at 252.5° C. The critical temperature of oxygen is C., and the critical pressure 50 atmospheres. At atmospheric pressure, the boiling point is 182.5° C.; the melting point is 218° C.
One volume of liquid oxygen at 182.5° C., if allowed to evaporate, furnishes an amount of gas which, at 0° C. and atmospheric pres sure, occupies a volume 782 times that of the original liquid taken.
In a Geissler tube, oxygen shows a luminous spectrum containing a bright band in the red, two in the green and one in the blue, but the spectrum is said to vary under varying condi tions.
Oxygen is slightly soluble in water. Its introduction into all animal life is by this means. Fish get the oxygen for their blood directly from the water in which it is dissolved. Among the air-breathing animals, including mankind, the process is not so obvious; the air is breathed into the lungs, oxygen is ab sorbed by the.moisture in the walls of the air sacs of the lungs and thus passes into the blood in the dissolved state.
While the oxygen of the air supports respira tion, and oxygen is the only gas suitable for this need of animal life, the proportion found in air is the one to which all life has become adjusted. Pure oxygen can be inhaled for a while with safety, but only relatively small amounts are required to produce a sense of narcotism in a normal, healthy person. A con tinued supply of pure oxygen furnished to a small rabbit caused it to eat voraciously but nevertheless it became very much emaciated from lack of ability to assimilate the food so rapidly. Under certain conditions, the animal became speedily narcotized, and the supply of Pure oxygen had to be discontinued to prevent its death. The cold-blooded animals were found to be very little affected, but most of the warm-blooded animals experimented upon speedily showed symptoms of great weakening. Oxygen, however; has been found very efficient in restoring persons overcome by noxious gases, and also, judiciously used, in the treat ment of certain affections of the lungs.
Oxygen forms oxides by direct combustion with nearly all other elements, the only excep tions being argon, bromine, chlorine, fluorine, gold, helium, iodine, neon, platinum and silver. Most of the non-metallic elements combined with oxygen form acid anhydrides; hydrogen forms a neutral oxide, water. A few of the metals in the molten state absorb oxygen which is given off again when the metal solidifies. Heated at 450° C., silver takes up about five times its volume of oxygen, gold 35 to 40, platinum 65 to 75 and- palladium about 500 times its volume. Finely divided platinum ab sorbs about 100 times its volume of oxygen and palladium sponge about 1,000 times. Cer
tain substances absorb increasing amounts of oxygen as the temperature is lowered, and give it off again as the temperature is increased; on the contrary, wood charcoal absorbs about 18 times its volume of oxygen at ordinary tem peratures, but as the temperature is lowered, it takes up increasing quantities of oxygen, so that at 185° C. one volume of charcoal ab sorbs 230 volumes of oxygen.
Liquid oxygen is a very perfect insulator and is a comparatively inert substance; phos phorus, sodium, potassium, etc., may be im mersed in it without any reaction taking place.
Oxygen is non-inflammable per se but its combination with other elements or materials with evolution of heat and light is commonly known as "combustion," and the material com bining thus with oxygen is said to be "in flammable"; examples are, burning of wood, coal, gas, etc.
Many combinations with oxygen, however, are not accompanied by light or extreme genera tion of heat, and these are classed, along with the combustion described above, under the term "oxidation," a familiar example being the rusting of iron.
All substances which burn in air burn with very much greater brilliancy in pure oxygen, and many which refuse to burn in air can be burned quite easily in oxygen. Thus, an iron wire, if previously heated to the ignition tem perature, burns with dazzling brilliancy in oxygen; even the diamond (dense form of car bon) if heated to redness and then plunged into oxygen burns readily without further appli cation of heat. Further, the temperature of a hydrogen or coal gas flame burning in oxygen is very much higher than that of the same gas burning in air. Some of the less obvious cases of slow oxidation are of particular interest. The bodily heat of all animals is furnished by the oxidation of certain constituents of the blood as it passes through the lungs, the oxy gen being furnished by the air. Slow oxidation of organic materials, which otherwise would cause discomfort or disease, takes place uni versally in nature so that the waters of streams automatically purify themselves by absorbing oxygen from the air; this process is imitated when large volumes of pure water are mixed with sewage, thus introducing dissolved oxygen which, aided by bacteria, rapidly renders the organic matter harmless. Not all slow oxida tions are beneficial, however. In cases where circulation of air through large masses of material is sufficient for slow oxidation but not for carrying off the heat generated, there may be sufficient accumulation of heat to bring about actual combustion; the result of such so called "spontaneous combustion* may be a burning hay-stack, a fire in the coal bunkers of a ship or a burning building set fire by a col lection of greasy waste rags or tow that has slowly heated itself up to the burning tempera ture. Insurance companies have very strict rules concerning the disposal of such waste as many serious conflagrations have resulted from such causes.