A hot gas emits light with an emission spectrum that consists of discrete lines. This is because the energy levels of a gas atom is quantized, meaning it is only capable of emitting photons with energy corresponding to the energy gaps between a pair of allowed energy levels.
Since the energy structure of each element is unique, the emission spectrum for each element is also unique. This forms the basis for spectroscopy whereby we identify the elements in a hot gas by mapping its emission spectrum to characteristic line spectrum of known elements.
In this comic, the detective identifies the murderer to be Sodium gas, whose characteristic line spectrum contains the famous bright doublet at 588.9950 and 589.5924 nanometers. Haha.