This section is from the book "Materia Medica And Therapeutics Inorganic Substances", by Charles D. F. Phillips. Also available from Amazon: Materia medica and therapeutics.
This metal does not occur native, but in various combinations is found extensively throughout all the kingdoms of nature; the chloride especially is abundant in the animal organism, also in sea-water, in many mineral springs and marine plants, as well as in mineral formations. The nitrate of soda occurs as an efflorescence on the soil in some countries.
Sodium, the metallic base of soda and its compounds, is of waxy consistence, and silver-white color. It has a great affinity for oxygen, and when placed upon water floats like potassium, producing effervescence from escape of hydrogen, and combining with the oxygen of the water to form soda: the sp. gr. is 0.972. Sodium is the only metal of which the ordinary salts are all soluble in water, and therefore do not furnish precipitation tests: we have, however, an excellent reaction in the flame-test, i.e., the communication of an intensely yellow color to a clear flame; so delicate is this test, and so universally diffused are the compounds of sodium, that it is difficult to obtain a flame perfectly free from all traces of them (Smith).