To state that a substance fails to obey Beer’s Law is an injustice, since it is the investigator who has failed to make proper use of the Law. If the substance becomes a mixture of substances when in solution, Beer’s Law must be applied to each component of the mixture. It does not seem to be generally realized that this can often be done, even when none of the components of the mixture can be isolated in the pure state.
ARTHUR C. HARDY and F. MANSFIELD YOUNG, "In Defense of Beer’s Law," J. Opt. Soc. Am. 38, 854-854 (1948)