Faraday to Eleventh Duke of Somerset   28 January 1832

Royal Institution | Jany 28 1832

My Lord Duke

I hasten to reply to Your Grace's enquiry¹ relative to the prediction of Newton² and the combustibility of water³. Reasoning on the refractive power of water, Newton was led to believe that that fluid contained a combustible substance because this power was higher than it otherwise should be: and when water was found by Cavendish to be a compound of oxygen and hydrogen ⁴, Newton's conjecture was confirmed; for the hydrogen is a combustible body and indeed one of the most marked substances of that class. But water is not a combustible because it contains a combustible body; its burning part has already undergone combustion, and cannot re-exhibit the phenomena of combustion until the water has been decomposed, and its combustible element set free. If a flint & steel be struck together, particles of the steel (a combustible) are separated, which, being highly heated by the friction, burn into little black particles of oxide. If filings of the steel be dropped into a candle flame they burn; because the steel is combustible; but if these little black particles be dropped into the flame, they will not burn, being incombustible, though they contain in combination an inflammable body.

Water is to the combustible it contains, just what these incombustible particles are to the original particles of steel; and water cannot philosophically be considered a combustible substance[.]

There are however cases in which water seems to burn; thus if a little be thrown on a good fire, well red hot, a pale flame will arise, as if the water burnt. But this is because the vapour of the water is decomposed by the very hot coals with which it comes in contact, the oxygen of the water is abstracted by the carbon of the coals, the fluid is as it were unburnt, its hydrogen or combustible part is set free, and when it rises above the coals and gets air, it burns; producing water again which passes off as vapour.

In an analogous manner is produced all those effects in which water really presents the phenomena of combustion. Some preceding action has occurred which unmakes the water, & then the hydrogen presents the phenomena of combustion[.]

Effects of this kind are sometimes very striking; for when the action of the fire is very vigorous, even the compound formed by the oxygen of the water with the carbon of the coals contains so much of the latter principle, that, when it rises above the fire into free air, it takes more oxygen and itself exhibits the phenomena of combustion[.]

Hoping that in my endeavour to be clear in answer to Your Grace's question I have not been tiresome[.]

I remain | My Lord Duke | Your Grace's Very Obedient | Humble Servant | M. Faraday

His Grace | The Duke of Somerset | &c &c &c