246 Mr. Donovan on the [Oct. 



diately combines with the rest of the oxide, which changes to 

 yellow. This yellow powder I analyzed in the manner already 

 detailed (14), and obtained its elements almost precisely in the 

 same ratio as constitutes the red oxide • the difference of colour 

 being caused by the different state of aggregation. 



17. If the heat be about 212°, the above-mentioned change 

 takes place slowly ; but if the black oxide be boiled in water, 

 the colour changes more speedily ; the black colour changes to 

 olive-green, and black and red oxides with metallic mercury are 

 obtained. 



18. When red oxide of mercury is exposed merely to a red 

 heat, it becomes black, but I satisfied myself that there is no 

 change in its degree of oxidation, as will presently appear. 



19. When red oxide under water is exposed to light, bubbles 

 of oxygen appear, and black oxide may be detected in the 

 remaining powder. And if dry levigated red oxide be exposed 

 to light, it soon becomes coated with black oxide. 



20. Mercury, perhaps, presents an exception to the general 

 law, that the second dose of oxygen is retained by a metal with 

 less force than the first. If red oxide of mercury in grains be 

 exposed nearly to a red heat, it becomes black, but it is still 

 peroxide ; for if immersed in mercury or water, so as to exclude 

 oxygen, it comes out when cold even a brighter red than before. 

 The red oxide bears a much higher heat than the black. When 

 black oxide is exposed to a moderate heat, part is reduced, and 

 part is raised to the state of peroxide, which then bears even a 

 low red heat unaltered. It is true that light renders red oxide 

 black, but this is only in its progress to the metallic state. All 

 these facts appear to support the exception. 



Part II. 



It has been supposed by chemists, that beside the black and 

 red oxides of mercury, there are others, which, although not 

 known in the insulated form, nevertheless exist in the various 

 salts. By a careful examination of these salts, and by a compa- 

 rison of the salts formed by combination of acids with oxides of 

 known composition, with those salts produced in the common 

 way, this point may be ascertained. In the progress of this part 

 of the inquiry, I shall have to describe some new salts of mercury, 

 and to point out some new circumstances of the formation of 

 those already known. 



Sect. I. — Combination of the Nitric Acid with the Oxides of 



Mercury. 



21. When mercury is added by small quantities to very dilute 

 nitric acid, the fluid being kept continually cold, a solution is 

 obtained, the nature of which has been very differently stated. 

 It is generally supposed that the mercury is at the minimum of 

 oxidation ; but some have conceived that the oxide is interme- 



