of the Oxides of Manganese. 29 



found that on putting the product into dilute sulphuric acid, 

 which instantly dissolved all the protoxide, a portion of the 

 red oxide came into view. This affords a sure criterion of the 

 operation being complete; for the pure protoxide dissolves 

 without residue in dilute sulphuric acid, and yields with it a 

 perfectly colourless solution. There seems to be no risk of 

 decomposing the protoxide by the employment of a high tem- 

 perature. I have exposed the recently prepared protoxide a 

 second time to the action of hydrogen gas and a long con- 

 tinued bright red heat without the weight being changed in 

 the slightest degree ; and after exposure to the same gas and 

 a full white heat for an hour, it dissolves in dilute sulphuric 

 acid without the slightest effervescence. 



The protoxide of manganese is described by Forchhammer 

 as being of a beautiful light-green, and by Arfwedson as of a 

 pistachio-green colour. I have seen specimens with ' a tint 

 very near the pistachio-green, but these always contained an 

 admixture of red oxide. The colour of the pure protoxide is 

 very near the mountain-green. 



With respect to the action of air, my observations differ 

 from those of Forchhammer, who found that recently prepared 

 protoxide attracted oxygen from the atmosphere before he 

 could weigh it. The protoxide procured in my experiments 

 is far more permanent. I exposed fifteen grains of recently 

 prepared protoxide to the free action of the air during the 

 space of nineteen days, when it was found to have undergone 

 no change either in appearance or weight. If, therefore, it 

 does attract oxygen at all from the air, the operation must 

 proceed very tardily. It absorbs oxygen very slowly even at 

 a temperature of 400° F. ; for 7*269 grains of the protoxide, 

 after an hour's exposure to that degree of heat, did not gain 

 in weight more than 0*021 of a grain. At a temperature of 

 600° F. it absorbs oxygen much more rapidly ; and at a low 

 red heat it loses its green tint, and becomes almost black in 

 an instant. I have repeated this process frequently, but in no 

 case did the protoxide take fire, as occurred in the experi- 

 ments of Forchhammer and Arfwedson. I entirely agree with 

 M. Arfwedson, however, in the statement, that the protoxide 

 is converted, by simultaneous exposure to heat and air, into 

 the red oxide. This is the uniform result at whatever tem- 

 perature the oxidation is effected. 



I have already mentioned my opinion, that, of the oxides of 

 manganese, the protoxide is the only one which forms definite 

 compounds with acids. It unites readily with this class of 

 bodies, without effervescence, producing with them the same 

 salt which is formed when the same acids act on the carbonate 



of 



