Principle of the method and definition of terms: When certain metal salts, such as table salt, are introduced into a Bunsen flame, the evaporating substance causes a distinct, in this example, intensely yellow glow in the flame. Other salts produce different colors in the flame; for example, strontium and lithium salts color the flame crimson red, calcium gives a brick red, barium a yellow-green, copper an emerald green, potassium a blue-violet, and arsenic, antimony, and lead a pale blue to pale green, etc. These flame colors are characteristic of certain cations, and sometimes also of certain compounds like CuOH. Therefore, these luminescent phenomena are often used as aids in qualitative chemical analyses. However, when multiple metal salts are introduced simultaneously, these simple methods fail because the eye cannot distinguish the mixture of several colors. Sometimes the characteristic colors also lie in a spectral range where the eye is no longer sensitive. When observing the mixed light emitted from a multi-colored flame in a spectroscope, one sees very specific spectral lines or bands (a collection of lines) that are characteristic of the individual elements or compounds. Using spectral line tables or charts, one can make some qualitative statements about the composition of the substances introduced into the flame. This well-known qualitative method will not be discussed further here. If one ensures through appropriate measures that the flame burns steadily for an extended period and that the supply of substances to the flame remains constant, the intensity of these observed spectral lines or bands serves as a measure of their concentration.