When hydrogen and oxygen are united by an electrical spark water is produced; certainly there is no parity between the liquid produced and the two gases. At 32° F., oxygen and hydrogen are elastic gaseous bodies, whose particles tend to fly away from one another; water at the same temperature is a strong though brittle solid. Such changes are called the properties of water. It is not assumed that a certain something called "acquosity" has entered into and taken possession of the oxide of hydrogen as soon as formed, and then guarded the particles in the facets of the crystal or amongst the leaflets of the hoar-frost. On the contrary, it is hoped molecular physics will in time explain the phenomena. "What better philosophical status," says Huxley,[10] "has vitality than acquosity. If the properties of water may be properly said to result from the nature and disposition of its molecules, I can find no intelligible ground for refusing to say that the properties of protoplasm result from the nature and disposition of its molecules." "To distinguish the living from the dead body," Herbert Spencer says, "the tree that puts out leaves when the spring brings change of temperature, the flower which opens and closes with the rising and setting of the sun, the plant that droops when the soil is dry and re-erects itself when watered, are considered alive because of these produced changes; in common with the zoophyte, which contracts on the passing of a cloud over the sun, the worm that comes to the ground when continually shaken, and the hedgehog which rolls itself up when attacked."