Faraday to William Whewell   9 November 1844

Royal Institution | 9 Novr. 1844.

My dear Sir

Cagniard de la Tour made an experiment¹ some years ago which give me occasion to want a new word will you help me?

If a glass tube sufficiently strong be nearly half filled with ether & hermetically sealed & then the whole be gradually heated the tension & density of the vapour in the upper part increases & at the same time the liquid ether below expands in volume. Gradually the surface of the liquid ether rises to different heights (1.2.3.4.5.6. &c); the difference between it & the vapour above becomes less & less & there is a point of temperature & pressure at which the liquid ether & the vapourous ether are identical in all their properties.

Now I am working on the same point and other fluids and want to express that point[.] Now what am I to call it? it differs of course both for pressure & temperature for other bodies than ether; but how am I to name this point at which the fluid & its vapour become one according to a law of continuity? Cagniard de la Tour has not named it; what shall I call it².

By having a tube long enough it is easy to have cold fluid below & pure elastic vapour above, yet no line of difference or demarcation any where in the tube[.]

Ever My dear Sir | Your Very Obliged Servant | M. Faraday

Rev. W. Whewell D.D | &c &c &c