The intensity of the color of the bromine vapor shows that the concentration of bromine, [Br_{2}]″, in the PCl_{3}Br_{2} tube, is ‹greater than the corresponding concentration›, [Br_{2}]′, in the PBr_{5} tube. As a molecule of pentahalide PX_{5} dissociates into one molecule of PX_{3} and one molecule of X_{2}, [PCl_{3}] equals [Br_{2}]″ and is ‹greater› than [PBr_{3}], which is equal to [Br_{2}]′. Further, more of the pentabromide than of the trichlordibromide must be left undecomposed, ‹i.e.› [PCl_{3}Br_{2}] is ‹smaller› than PBr_{5}. Since the factors in the ‹numerator› of the second equation are ‹both larger›, and the factor in the ‹denominator smaller, than the corresponding factors› in the first equation, ‹k›_{2} ‹must be greater than› ‹k›_{1}. These ‹constants› are thus seen to be a ‹measure› of the ‹chemical stability› of these pentahalides. It is evident, too, that in reactions which depend on the presence of free bromine, such as the bromination of many organic compounds, the trichlordibromide should be more ‹effective› than the equivalent quantity of the pentabromide. [p097] In the second place, if we were to introduce into either tube, for instance into the tube containing the phosphorus trichlordibromide, an ‹excess of one of the dissociation products›, say an excess of phosphorus trichloride, then the condition of equilibrium would necessarily be disturbed: ‹y› [PCl_{3}] × [Br_{2}]′ / [PCl_{3}Br_{2}] > ‹k›_{2}, in which the bracketed symbols represent the concentrations of the first experiment. The velocities of the two opposite reactions would be no longer equal, the combination of trichloride with bromine would be accelerated by the increased concentration of the former. Here, equilibrium would only be reëstablished when the trichloride and bromine had combined to a sufficient extent to make (‹y› [PCl_{3}] − ‹x›) × ([Br_{2}]′ − ‹x›) / ([PCl_{3}Br_{2}] + ‹x›) =  ‹k›_{2}, in which x represents the number of moles of additional phosphorus trichlordibromide formed in unit volume by the combination of bromine with phosphorus trichloride. The ‹net result› is seen to be that an ‹increase in the concentration of the one dissociation product eo ipso reduces the concentration of the other dissociation product›. EXP. A third tube charged with the same quantity of phosphorus trichlordibromide as the tube mentioned above, and with an added excess of phosphorus trichloride, is placed in the warm water next to the tube containing the trichlordibromide. Its color is much paler than that of the latter, owing to the suppression of free bromine.[179]