According to the directions of the U. S. Pharmacopoeia, citrine ointment is made by dissolving mercury in nitric acid, adding the solution to a mixture of neat's foot oil and lard heated together to 200°, and stirring until effervescence ceases, and the ointment thickens. in the process, there is first obtained in solution a mixture of the nitrates of the protoxide and deutoxide, which are afterwards decomposed to some extent, and probably converted into the yellow subnitrate of the deutoxide of mercury; while the oils themselves are partially converted into the fatty acids, which probably combine with a portion of mercurial oxide, from which the nitric acid has been separated. The precise changes, however, have not been satisfactorily ascertained, and they differ somewhat with the circumstances of the operation. it is clear that the oxidation of the fatty matter, and the deutoxidation of the protoxide of mercury, are effected at the expense of a portion of the nitric acid, which, being thus decomposed, escapes in the form of nitrous fumes.

This ointment has a fine yellow colour when first prepared, which, however, gradually changes to a greenish hue on exposure. if well made, it retains its proper unctuous consistence for a long time. its odour is nitrous, but peculiar. i it is irritant, and probably alterative to the surface to which it is applied, and is one of the best local remedies in chronic cutaneous eruptions requiring stimulation. The particular affections in which it has been found most efficacious are porrigo and impetigo, especially the latter, affecting the scalp and the face, in the form of tinea capitis and crusta lactea. it has also proved serviceable in the advanced stage of eczema, when the inflammation has quite subsided, and in the scaly affections, psoriasis, lepra, and pityriasis. it is, in general, not applicable to the earlier stages of these affections, and, when first used, should be considerably diluted with lard; the strength being gradually increased as the case may seem to require. if it augment the irritation, it should be suspended, and resumed at a later stage of the disease. in the affection of the edges of the eyelids denominated Psorophthalmia, it is an excellent application. it may also be used, as a stimulant and alterative dressing, in foul phagedenic or indolent ulcers, particularly those of syphilitic origin.