§ 2. Among the diatomic combinations of the three elements, hydrogen, nitrogen and oxygen, we find a molecular mobility much less than that of these elements themselves; at the same time that it is much greater than that of diatomic compounds in general. Of the two products formed by the union of oxygen with carbon, the first, called carbonic oxide, which contains one atom[3] of carbon to one of oxygen (expressed by the symbol CO) is a gas condensible only with great difficulty; and the second, carbonic acid, containing an additional atom of oxygen (CO_{2}) assumes a liquid form also only under a pressure of about forty atmospheres. The several compounds of oxygen with nitrogen, present us with an instructive gradation. Nitrous oxide (N_{2}O), is a gas condensible only under a pressure of some fifty atmospheres; nitric oxide (NO) is a gas which although it has been liquefied does not condense under a pressure of 270 atmospheres at 46.4° F. (8° C.): the molecular mobility remaining undiminished in consequence of the volume of the united gases remaining unchanged. Nitrogen trioxide (N_{2}O_{3}) is gaseous at ordinary temperatures, but condenses into a very volatile liquid at the zero of Fahrenheit; nitrogen tetroxide (N_{2}O_{4}) is liquid at ordinary temperatures and becomes solid at the zero of Fahrenheit; while nitrogen pentoxide (N_{2}O_{5}) may be obtained in crystals which melt at 85° and boil at 113°. In this series we see, though not with complete uniformity, a decrease of molecular mobility as the weights of the compound molecules are increased. The hydro-carbons illustrate the same general truth still better. One series of them will suffice. Marsh gas (CH_{4}) is gaseous except under great pressure and at very low temperatures. Olefiant gas (C_{2}H_{4}) and ethane (C_{2}H_{6}) may be readily liquefied by pressure. Propane (C_{3}H_{8}) becomes liquid without pressure at the zero of Fahrenheit. Hexane (C_{5}H_{12}) is a liquid which boils at 160°. And the successively higher multiples, heptane (C_{7}H_{16}), octane (C_{8}H_{18}), and nonane (C_{9}H_{20}) are liquids which boil respectively at 210°, 257°, and 302°. Pentadecan (C_{15}H_{32}) is a liquid which boils at 270°, while paraffin-wax, which contains the still higher multiples, is solid. There are three compounds of hydrogen and nitrogen that have been obtained in a free state--ammonia (NH_{3}) is gaseous, but liquefiable by pressure, or by reducing its temperature to -40° F., and it solidifies at -112° F.; hydrazine (NH_{2}--NH_{2}) is liquid at ordinary temperatures, but hydrozoic acid (N_{3}H) has so far only been obtained in the form of a highly explosive gas. In cyanogen, which is composed of carbon and nitrogen, (CN)_{2}, we have a gas that becomes liquid at a pressure of four atmospheres and solid at -30° F. And in paracyanogen, formed of the same proportions of these elements in higher multiples, we have a solid which does not fuse or volatilize at ordinary temperatures.