there had been earlier studies of the characteristic colors of heated elements, but nothing systematic. in the summer of 1859, kirchhoff suggested to bunsen that he should try to form prismatic spectra of these colors. by october of that year the two scientists had invented an appropriate instrument, a prototype spectroscope. using it, they were able to identify the characteristic spectra of sodium, lithium, and potassium. after numerous laborious purifications, bunsen proved that highly pure samples gave unique spectra. in the course of this work, bunsen detected previously unknown new blue spectral emission lines in samples of mineral water from dürkheim. he guessed that these lines indicated the existence of an undiscovered chemical element. after careful distillation of forty tons of this water, in the spring of 1860 he was able to isolate 17 grams of a new element. he named the element "caesium", after the latin word for deep blue. the following year he discovered rubidium, by a similar process.