[20] Mercuric nitrate, Hg(NO_{3})_{2},8H_{2}O, crystallises from a concentrated solution of mercury in an excess of boiling nitric acid. Water decomposes this salt; at the ordinary temperature crystals of a basic salt of the composition Hg(NO_{3})_{2},HgO,2H_{2}O are formed, and with an excess of water the insoluble yellow basic salt Hg(NO_{3})_{2},H_{2}O,2HgO. These three salts correspond with the type of ortho-nitric acid, (H_{3}NO_{4})_{2}, in which mercury is substituted for 1, 2 and 3 times H_{2}. As all these salts still contain water, it is possible that they correspond with the tetrahydrate = N_{2}O_{5} + 4H_{2}O = N_{2}O(OH)_{8} if ortho-nitric acid = N_{2}O_{5} + 3H_{2}O = 2NO(OH)_{3}. [21] To obtain the mercuric salt a large excess of strong sulphuric acid must be taken and strongly heated. With a small quantity of water colourless crystals of HgSO_{4},H_{2}O may be obtained. An excess of water, especially when heated, forms the basic salt (as in Note 20), HgSO_{4},2HgO, which corresponds with trihydrated sulphuric acid, SO_{3} + 3H_{2}O = S(OH)_{6}, with the substitution of H_{6} by 3Hg, which in mercuric salts is equivalent to H_{6}. Le Chatelier (1888) gives the following ratio between the amounts of equivalents per litre: HgSO_{4} 0·318 0·890 1·80 2·02 SO_{3} 0·752 1·42 2·10 2·40 --that is, the relative amount of free acid decreases as the strength of the solution increases.