Caesium (Cs.; at. wt. 132.7; sp. gr. 1.88), a metallic element closely allied to the alkali metals sodium, potassium, etc. It never occurs free, and its salts, though widely distributed, are only found in small quantities; amongst other sources, in mineral waters, saltpetre residues, ashes of plants - especially tobacco. It is silver white in colour, soft and ductile, and decomposes water very readily. It is best detected by the spectroscope, giving two fine lines in the blue.