244 Mr. Donovan on the [Oct, 



tion of the oxygen, although the absolute quantity may remain 

 the same. In the former case, the ratio of oxygen afforded by 

 analysis would be too small ; in the latter, unless the whole 

 product of the decomposition were employed, the ratio of oxygen 

 would be too large. 



10. By long continued agitation, in Boerhaave's mode, the 

 mercury is partly oxidized, but a great portion is only minutely 

 divided. From such a powder as this, Dr. Priestly obtained a. 

 large quantity of metallic mercury. Beside that, if the mercury 

 be pure, agitation has but little effect in oxidating it. This is, 

 therefore, a bad mode of forming this oxide. 



11. I endeavoured to obtain pure black oxide by triturating 

 mercury with syrup ; but after continued trituration of 60 gr. of 

 the metal for 40 hours, I obtained 54 gr. of unaltered metal : 

 hence there were but 6 gr. oxidized. 



12. After various attempts to procure black oxide fit for 

 analysis, I was obliged to proceed in the following manner. 

 About six grains of calomel were triturated well with a little 

 water, and a quantity of solution of pure potash was dashed at 

 once over it during brisk trituration. This process was repeated 

 several times, with new portions of calomel, until a sufficiency 

 of oxide was obtained. This powder did not contain a particle 

 of red oxide. It was extremely well dried in the shade, and 

 triturated until it would give out no more globules of mercury. 

 There might be, perhaps, a few grains of metal still concealed, 

 but the quantity must have been too small to influence the result 

 of the analysis. The globules were easily collected into one, 

 and separated. 



13. Fifty grains of this oxide were introduced into a green 

 glass tube, 10 inches long, and i diameter, sealed at one end. 

 The part of the tube which contained the oxide was gradually 

 heated to redness in a charcoal fire. A quantity of mercury 

 sublimed, but a portion of it at that high temperature, united 

 with the oxygen present, and formed red oxide. By means of- 

 a polished iron scraper nicely adapted to the cylindricity of the 

 tube, I scraped down to the bottom every thing that had sub- 

 limed, and introduced a capillary tube, by means of which the 

 large tube was filled with hydrogen. The end of the large tube 

 was again heated to redness, and when cold, the same process 

 was repeated with the scraper, and the tube again filled with 

 hydrogen. The third heating had reduced the oxide completely, 

 but a white powder remained which was not metallic ; it was 

 probably abraded from the mortar during the trituration. The 

 oxide employed in the analysis, I found, by a previous experi- 

 ment, to contain water. The result of the analysis was as 

 follows: Mercury, 47-48; oxygen, T96 ; water, 0-31; earthy 

 residue, 0-25. Therefore, deducting the two latter ingredients,, 

 the analysis of the black oxide will be 



