The discovery of potassium led to that of the true nature of what had been long familiar to chemists by the name of _pure Potash_, but which ought to have been called the _hydrat_, for the _pure_ alkali was not known until after the discovery of Davy. The experiments of MM. Gay Lussac and Thénard have shown this substance to be a _Protoxide_. It is difficult of fusion; it has a grey colour and a vitreous fracture, and dissolves in water with much heat. The _Peroxide_ is procured by the combustion of potassium at a low temperature; it had been observed by Davy in October 1807, but at that time he supposed it to be the oxide containing the smallest proportion of oxygen: it has a yellow colour, and when thrown into water effervesces, and gives out oxygen gas.[81] When heated very strongly upon platina, oxygen is also expelled from it, and there remains the _protoxide_, or pure potash. [81] The 'Potassa Fusa' of our Pharmacopoeia generally contains a small proportion of the peroxide, and will therefore effervesce when thrown into water. It was a great object with Davy, to show that the product resulting from the combustion of potassium, was a pure oxide free from water; for it is evident that had potassium been a _Hydruret_, its combustion must have produced a _Hydrat_. This he accomplished by a series of experiments which he performed in the laboratory of Mr. Children, and which are published in his Bakerian Lecture of 1800. Having discovered the presence of oxygen in the fixed alkalies, he was naturally led by analogy to enquire whether ammonia might not also contain it. It was true that the chemical composition of that body had been considered as satisfactorily settled, and that the conversion of it into hydrogen and nitrogen, in the experiments of Scheele, Priestley, and Berthollet, had left nothing farther to be accomplished. All new facts, however, are necessarily accompanied by a new train of analogies; and Davy, in perusing the accounts of the various experiments to which ammonia had been submitted, tells us that he saw no reason for considering the presence of oxygen as impossible; for, supposing hydrogen and nitrogen to exist in combination with oxygen in low proportion, this latter principle might easily disappear in the analytical experiments by heat and electricity, in the form of water deposited upon the vessels employed, or dissolved in the gases produced.