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 has not been found native, but its various compounds are very widely diffused. The nitrate occurs in various soils, and the chloride in mines, the tartrate in the juice of the grape and other fruit, and carbonates and chlorides are found in the ashes of all woods and plants; chloride of potassium abounds especially in the seeds of leguminosae (Berthier). From vegetables this salt passes into the animal organism, and hence the milk and the urine of herbivora contain much more of it than the same secretions of carnivora: the blood-globules and the contractile substance of muscle contain a comparatively large proportion of it.
The metal itself is soft and silvery white, so light (sp. gr. .865) that it floats on water, and with such affinity for oxygen that it abstracts that gas from the water, thus setting free hydrogen which ignites and burns with a violet purple flame, characteristic of the presence of potassium. Some liquid devoid of oxygen - like benzine - is therefore required for keeping the metal; if exposed to the air it rapidly oxidizes to potash.