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known that these oxides are admitted by chemists as having 

 been discovered by Thenard. The compounds obtained by 

 Thenard and to which he had ascribed the general formula 

 R. O2 were obtained by the action of peroxide of hydrogen 

 on the protoxides of the metals. But these bodies did not 

 possess any of the characters, which analogy would lead us 

 to ascribe to magnesian peroxides. In fact, they possess all 

 the properties of peroxide of hydrogen itself : they decom- 

 pose spontaneously and detonate with combustibles. Potash 

 accelerates their decomposition, while acids, under certain 

 circumstances, retard it. Their action with acids is opposed 

 to our conceptions of such unstable peroxides ; for they 

 dissolve in cold acids, without decomposition, which even 

 peroxide of Maganese, stable as it is, cannot do. All these 

 anomalous properties led Dr. Playfair to look for another 

 composition for M. Thenard's compounds. In the course 

 of his examination of the magnesian oxides, he found that 

 the peroxides had a great disposition to unite with the pro- 

 toxides, forming compounds of the general formula R O + 

 R2 O4. Now hydrogen itself is a magnesian metal, or, at 

 all events, possesses the characters of one, and its peroxide 

 should share the disposition to unite with protoxides. The 

 author therefore, drew the conclusion, that Thenard's com- 

 pounds were in reality similarly composed, having the 

 formula M O + H2 O4. And in confirmation of this view, 

 he stated, that Thenard^s own analyses coincide with it much 

 better than with his own view of their being peroxides. 

 This chemist obtained always too little oxygen to agree 

 with his theory of their composition. He ought to have 

 procured oxygen in the proportion of 3:6; whilst on Dr. 

 Playfair's view, the proportion would be 3 : 5, a result which 

 approximates closely to Thenard's analyses. From these 

 considerations, the author considered that he had a right 

 to affirm, that the peroxides of copper, zinc, and calcium. 



