This deposit is metallic silver, and is due to an effect called capillary rise in small diameter tubes. This dark lining or ‘ring’ shows the surface as a curve with the liquid rising along the capillary walls to form a very thin film on the capillary walls. This curve is called the meniscus. Since in this thin film the surface area to volume ratio of the solution is higher than anywhere else, evaporation will be fastest at this point. As the liquid evaporates the concentration of solids increases until they separate as a deposit. This deposit will always be found only in the dropper tube and slightly above the liquid level in the capillary.
