Mercuric acid is simply mercury that has been mixed with acid to create a desired reaction. Mercury reacts with concentrated sulphuric acid and concentrated nitric acid to form mercury(II) nitrate or can be dissolved in nitric acid to form mercury(I) nitrate.
Mercury only reacts with oxidizing acids. There are a number of applications for mercuric acid. For example, mercury(I) nitrate is used to prepare other mercury(I) compounds. In the past, mercury(II) nitrate was used to make felt and treat fur. The use of mercuric acid to treat fur and make felt was banned in the United States in 1941, although mercury(II) continued to be used during war as a detonator.