18 Historical Sketch of Improvements hi [July, 



Protoxide .... 100 mercury + 4*12 oxygen 

 Peroxide 100 ' + 7-82 



Mr. Donovan informs us, that though he repeated his experi- 

 ments several times, the results were precisely as above stated. 

 They do not accurately correspond with the atomic theory, and, 

 therefore, cannot be quite correct ; but if we take the mean of the 

 two, we obtain the composition of the two oxides of mercury as 

 follows : 



Protoxide .... 100 mercury + 3*98 oxygen 



Peroxide 100 + 7-96 



numbers which differ very little from those determined by former 

 experimenters, and which, I believe, to approach very near the 

 truth. 



Mr. Donovan's experiments on the oxides of mercury have led 

 him to suggest a very material improvement in the blue mercu- 

 rial pills and ointment used externally to excite salivation in 

 cases of the venereal disease. It has been long known that this 

 ointment consists in a great measure of metallic mercury merely 

 mechanically mixed with lard. He suspected that its efficacy 

 was owing entirely to the small quantity of black oxide which, 

 had been chemically united to the lard. This induced him to 

 try to form such a combination artificially. He found that when 

 an ounce of lard and 21 grs. of oxide are kept together at a tem- 

 perature of 320° for an hour, taking care to stir them constantly, 

 they combine chemically, and form an ointment much more 

 efficacious, much more cleanly, and much cheaper than the 

 common ointment. 



12. Gold. — Count Le Maistre has given a very simple process 

 by which a fine purple colour, fit for painting in oil, may be 

 obtained from the solution of gold. The colour I have myself 

 seen, and think it fine. The process is simple. I need only 

 refer the reader to the Annals of Philosop/ii/, xiv. 361, where the 

 method of proceeding is described by the Count himself. 



13. Flatiiium. — Mr. Fox has pointed out the strong affinity 

 which exists between platinum and tin ; and Dr. Clarke has 

 described the phenomena of the union of the two metals, when 

 exposed to the heat of the blow-pipe {Annals of Philosophy, xiv. 

 229). I have myself repeated the experiment, and find it very 

 similar to the appearance which takes place when tin is strongly 

 heated by the blow-pipe, and then dropped upon a table. The 

 metal burns with great brilliancy, and flies into a number of small 

 pieces. 



14. Fffect of Heat npon the Colours of Metals.— M. Chaudet 

 has published a set of experiments, which may have their 

 utility, to determine the appearances which different pure metals 

 exhibit when kept for some time in a strong heat on the cupel, 

 and how these appearances are modified when these various 



