1819.] Oxides and Salts of Mercury. 245 



Mercury. 96*04 



Oxygen 3-96 



10O00 



14. To discover the composition of the red oxide, I made use 

 of that prepared by heat alone, commonly called calcined 

 mercury : 50 grains of this were introduced into a green glass 

 tube, like the former, and the end of the tube was gradually 

 heated to redness, and maintained so until the decomposition 

 was complete. There was no trace of water, the mercury came 

 over easily, and was quite bright, and there was no need of 

 filling the tube with hydrogen, or of a second heating. The 

 mercury, when collected, weighed *-££ gr. ; there were minute 

 grains of some foreign body weighing ^ gr. which, being 

 deducted from the original oxide, gives its analysis as follows : 



Mercury. 92-75 



Oxygen 7*25 



100-00 



15. From these analyses, it would appear that 100 parts of 

 mercury take up to form black oxide, 4*12; and to form red 

 oxide, 7-82. My estimate of the red oxide agrees very nearly 

 with the analyses of Kirwan, Lavoisier, Fourcroy, Thenard, 

 Davy, and Sefstrom ; but the four latter rate the oxygen in the 

 black oxide lower, it being just half of what enters into the red. 

 These philosophers, however, used a black oxide which con- 

 tained a metallic mercury, and I separated that portion from 

 mine : hence the difference, probably. My analysis of the black 

 oxide, being not exactly coincident with the doctrine of definite 

 proportions, may be thought inaccurate. The difference, how- 

 ever, is not great : if the red consist of 7*82 for every 100 of 

 mercury, the black would consist of 3-91 ; whereas I rate it at 

 4-12— a difference of about -i-th of a grain; and notwithstanding 

 the smallness of this discordance, I could not, although the 

 experiments were often repeated, bring out any other result. 



III. On certain Properties of the Mercurial Oxides. 



Previously to entering on the constitution of the salts of mer- 

 cury, it will be expedient to state certain properties of the 

 oxides, a knowledge of which is necessary to avoid errors that 

 otherwise we might fall into with regard to their nature. 



16. When the black oxide of mercury is exposed to heat, it 

 is known to become yellow. The circumstances of this change 

 I find to be as follow. An additional dose of oxygen is absorbed, 

 but it is not from the atmosphere, as is commonly supposed. 

 Part of the oxide is reduced, the mercury is volatilized, or 

 remains, if the heat be Jow ; while the oxygen liberated imme- 



