Acid test (gold) mark is tested by applying aqua fortis (nitric acid), which dissolves the mark of any item that is not gold. If the mark remains, it is tested by applying aqua regia (nitric acid and hydrochloric acid). If the mark is removed, then this test dissolves the gold, proving the item to be genuine gold. More accurate testing of the item for its fineness or purity can be done through the use of differing strengths of aqua regia and comparative testing of gold items of known fineness. The figurative meaning of the expression, where it is applied to tests of character,
Simultaneously extracted metals and Acid-volatile sulfide be determined. In initial versions of the SEM-AVS approach, six metals were measured: nickel, zinc, cadmium, copper, lead, and mercury. More current methods call for the measurement of silver in addition to these metals. In applying the SEM-AVS approach, two concentrations are determined: the total concentrations of all toxic heavy metals of interest, represented as [SEM], and the acid-volatile sulfide concentration, represented as [AVS]. From these concentrations, the [SEM]/[AVS] ratio can be obtained, summarizing the results in a single value. Based on the extremely low K values for heavy metal sulfides, if [SEM]/[AVS] is less than 1 ([SEM < [AVS]),