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(in which 2.213 grammes of potassium gave.666 litre of hydrogen) the weight of the por-tion of potassium filling the cavity in the brasswas pretty exactly estimated at 0.0212 gramme.This quantity was therefore the standard of allthe subsequent experiments.

Potassium and Oxygen. If potassium isconfined in contact with oxygen at a commontemperature and kept at rest for some days, itgradually loses its metallic appearance, and isconverted entirely into a grey substance. Cut ifthe temperature is a little raised or the surface ofthe metal renewed, it takes fire and the com-bustion goes on rapidly.

Oxyds of Potassium. There are certainlytwo distinct oxyds of potassium, and probablythree, namely, the suboxyd, the alkali, and theperoxyd.

The suboxyd is a bluish-grey very brittlesubstance, softer than wax, easily fusible, soinflammable as to take fire in oxygen or evenin common air at about 70° Fahr. decompo-sable by water into alkali, and giving out hy-drogen like potassium but in smaller quantity.It is therefore in a lower state of oxydationthan the alkali. This suboxyd is produced byconfining potassium for some days in an emptyphial closed with a cork, the pores of whichadmit such a very gradual supply of air andmoisture as suits the slow and infer ior state ofoxydation required. It appears however doubt-ful whether this suboxyd may not be a mixtureof potassium and potash, and it has certainlynot been obtained sufficiently uniform to allowof its precise degree of oxydation to be wellascertained. 0

The second oxyd of potassium is the alkaliitself, the composition of which has alreadybeen mentioned. This oxyd, the alkali, is al-ways produced, whenever potassium or any ofits oxyds come in contact with water. Whenproduced from potassium or the suboxyd justdcsciibed, an evolution of hydrogen attends thealkalization of the metal, owing doubtless tothe decomposition of water: when producedby moistening the peroxyd, the generation ofthe alkali is attended with the evolution of thatportion of oxygen which constitutes the differ-ence between the alkali and the peroxyd, butin either case the contact of water and of potas-sium in any form determines the formation ofthe alkali. In this way of forming the alkaliit always unites with that portion of water ofcomposition (amounting to about 18 per cent.)which, as has been mentioned in the beginning

of this article, is inseparable from caustic potashprepared by watery solution, even when it hasbeen dried by fusion in a full red heat, andconstitutes not properly pure potash, but hydratof potash. The pure potash can only be ob-tained by heating the yellow peroxyd to be nowdescribed.

The orange-yellow peroxyd of potassium isprepared immediately by igniting potassium inoxygen, or even in common air. The discoveryofthe real nature of this oxyd, and of its contain-ing more oxygen than the alkali does, is due tothe eminent French chemists already mentioned.There is however some difficulty in performingthe combustion accurately. If the potassiumis burned upon glass or platina, these are botliattacked by the oxyd assisted by the great heatgenerated, and incorrect results follow. A sup-port of silver answers better, though with somerisk of its fusion, or as Sir H. Davy recom-mends, the potassium may be laid in a smalldish of platina, coated over with melted muriatof potash, which is not acted on by potassiumor any of its oxyds. The French chemists ob-serve that much of the inconvenience of aplatina support may be avoided by using potas-sium that lias been in oil, as it then burns at alower temperature, or still better by employ-ing the suboxyd which inflames spontaneously inoxygen. They give the results of eight differentexperiments on the combustion of the samequantity of potassium in a glass vessel full ofdry oxygen confined over mercury, the potas-sium being laid in some experiments on glass,in others on platina, in others on silver. Theyellow oxyd thence produced was then throwninto water, by which it passed to the state ofalkali, giving out all its excess of oxygen. Thereis so little agreement in these experiments as tothe entire quantity of oxygen absorbed by thepotassium, that the composition of the peroxydcannot be deduced from them. The largestquantity of oxygen absorbed (which is obviouslynearest the true estimation) was pretty exactlytwice as much as that portion required to con-vert the given weight of potassium into potash,and the experiments were so far satisfactory,that after the yellow oxyd was plunged intowater and had parted with its excess of oxygen,the remainder which entered into the constitu-tion of the alkali was nearly the same in eachexperiment. This peroxyd is fusible at a lowerheat than hydrat of potash, and crystallizes inlaminae by cooling. The colour is yellow whenpure but brownish when mixed with oxyd of