Proceedings of the P olytechnic Association. 1025 



3. The black oxyd of manganese, when heated in an iron vessel, 

 will give off a portion of its oxygen, but the heat required is incon- 

 veniently high, and this method is rarely employed. 



4. A very neat laboratory method is to heat the bichromate of 

 potash and sulphuric acid together in a glass vessel ; considerable 

 oxygen is thus given off in a pure state. The proportions to be 

 emp loyed can be represented by the formula: 



KO, 2Cr03 + 4 HO, SO^ =Cr2 O^ 3 SO^ + KO, S0=^ + 4 HO + 3 O. 



Chrome alum is usually produced, and it is this mixture that is 

 employed in tlie bichromate of potash batteries. 



This method is valuable where the materials are cheap, but is not 

 available on a large scale. 



5. The action of sulphuric acid upon the black oxyd of manganese 

 is somewhat similar, and may be represented by the following 

 equation : 



2Mn02 + HO, SO«=Mn^O^ SO^+HO+O. 

 Tlie oxygen obtained is likely to be imjoure, owing to the foreign 

 matter in the black oxyd of manganese. 



6. For a long time nitrate of potash or saltpetre was employed in 

 the preparation of oxygen. The salt only requires to be heated 

 gently to disengage the gas, but the difficulty is to regulate the heat 

 so as to prevent the entire decomposition of the saltpetre. The 

 method is not available and is pretty much abandoned. 



7. The bi-sulphate of soda, the salt cake of commerce, when heated 

 with the black oxyd of manganese, yields sulphate of soda, sulphate 

 bf manganese and, oxygen gas. 



mO, 2 SO'^+MnO^^ISTaO, SO"^+MnO, SO'+O. 

 This method could be employed in England, where many thousand 

 tons of the salt cake are annually made, and where the consumption of 

 manganese is very great. It is not available in America, owing to 

 the cost of the materials. 



8. The decomposition of water by means of a powerful battery 

 gives both hydrogen and oxygen in a very pure state, but the cost of 

 running the battery makes this method very expensive. It is not 

 im]Dossible that we may some day be able to accomplish the decomj^o- 

 sition of water economically, and this result is highly to be desired, 

 as large sums are now annually expended in experiments in this 

 direction. 



9. It is said that if sand be heated with gypsum, the sulphuric acid 

 [Inst.] 65 



