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and transmitted illumination. With the proper combination of lighting, the reaction spots for sulfuric acid appear as milky yellow, perfectly circular areas on a blue background. Most other acids will give yellow spots, but these will not contain the milky precipitate of barium sulfate.Phosphoric acid will also cause a milky precipitate within the reaction area; however, the barium phosphate formed is birefringent, in contrast to barium sulfate. The two can therefore be distinguished by means of crossed polars. Other sulfates and phosphates will give circular milky areas, but for the most part were not found to be sufficiently acidic to change the color of bromophenol blue.It was difficult to reproduce precisely the relationship between sulfuric acid droplet diameter and reaction spot di-ameter, presumably because of the sensitivity of the sulfuric acid droplet diameter to relative humidity. However, as a result of a large number of experiments, it is believed that the equation Dh = 2.5 Dd where Dh is the diameter of the reaction spot or "halo" and Du is the equivalent diameter of a droplet of pure sulfuric acid, represents the droplet size within ±30%. The equation is not valid above approximately 90% relative humidity. At extremely high humidities, the test fails because of the extreme dilution of the sulfuric acid drops.

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