Faraday to William Whewell   5 October 1846¹

Royal Institution | 5 Octr. 1846

My dear Sir

Many thanks for the name² - I think it is good.

Now as for the condition of the liquid when the state is assumed. Take (as I have done after Cagniard de la Tours instructions) a strong cylindrical glass tube put into it some ether & then seal it up hermetically so that the liquid ether shall at common temperatures occupy rather less than one half the space. Then gradually heat it. Part of the ether will rise in vapour forming compressed ether steam & the ether which remains fluid will expand. As the heat increases the fluid part will enlarge - partly because it expands by the heat & partly because the air or vapour space by such expansion of the liquid becomes less & less and consequently the vapour occupying the space gradually filled by liquid is condensed. If the proportion of liquid to space is well adjusted, the effect will be that in a tube 3 inches long the liquid will increase in volume until not more than 1/8 of an inch of space (containing vapour) remains above it & you may in fact consider the tube full of liquid (at that temperature). Now provided this state occurs at that temperature which may be called the Tourian point then the least addition of heat converts all this liquid into vapour & the tube is full of vapour. Since I wrote³ to you I have looked more carefully at this point of change & it is not the fact that the liquid & vapourous state pass insensibly into one another. The vapour does not occupy more bulk than the liquid or the liquid less than the vapour - and cooling the tube a little brings on the liquid state & heating it again the vapour state - but at the moment of change there is an alteration both ways of this kind. Consider the tube as almost filled with liquid & just below the point of change. There is then a faint line of demarcation between the liquid & the vapour space above & both spaces are clear. apply a little heat of a sudden the whole space within becomes obscured for a moment as if by a thick mist this quickly clears & the whole space is full of vapour in which rapid currents are seen (formed by the constant heat of the lamp applied) but no line of demarcation. Cool this & then again the thick mist state comes on which clears away & then there is the line of demarcation liquid & vapour lying in the tube together.

It seems to me that at the moment of the change & mistiness the cohesive attraction of the fluid comes on & goes off suddenly, which cohesion both Donny & Henry⁴ have shewn to be so considerable. Donny's recent & remarkable experiments including the explosion of water I have no doubt you know⁵.

As to types &c I really can say nothing[.] The theory of chemistry seems to me to be in a very backward state in comparison with that of many other branches of science.

Ever Truly Yours | M. Faraday

Rev. Dr. Whewell | &c &c &c