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,
Nervonic acid Recent studies show that cord nervonic acid levels were lower in women with preeclampsia delivering male babies as compared to normotensive control group. But this does not happen with girl babies. These results suggest that male babies born to mothers with preeclampsia may be at an increased risk of developing neurodevelopmental disorders as compared to female babies. This acid is present in the composition of aged eye lens, but it does not appear in normal eye lens. This data shows that we can use the presence of nervonic acid (together with heneicosylic acid and docosahexaenoic acid) as biomarkers of aging