From John Frederick William Herschel   April 12/62

Collingwood | Hawkhurst | Kent | April 12/62

Dear Sir

I have to thank you for two papers on radiant heat¹ both extremely interesting—you have made a grand step in meteorology in showing that the dry air is perfectly transcalescent² & that the invisible moisture is what stops the sun’s heat. In your 2^[n]d paper you express a wish to procure a good crystal of Rock Salt—I shall send you per railway a very fine specimen of which voici l’histoire.³

After the publication of my paper in the Phil Trans 1840⁴ I was very desirous to disengage myself from the influence of glass prisms & lenses & ascertain if possible whether in reality my insulated heat–spots α β γ δ ε in the specturm were of solar or terrestrial origin. Rock Salt was the obvious resource and after many & fruitless endeavours to obtain sufficiently large & pure specimens, as the late D^r Somerville⁵ was so good as to send me (as I understood from a friend in Cheshire) the very fine block which I now forward. It is however much cracked but, I have no doubt pieces large enough for lenses & prisms (especially if cemented? together) might be got from it.

But I was not prepared for the working it—evidently a very delicate & difficult process, (I proposed to dissolve off the corners &c and as it were lick it into shape) & though I have never quite lost sight of the matter I have not yet been able to do anything with it. Meanwhile I put it by. On looking at it a year or two after I was dismayed to find it had lost much by deliquescence. Accordingly I potted it up in salt in an earthen dish with iron rim—placed it on an upper shelf in a room with an Arnott Stove where it has remained ever since.

If you should find it of any use I would ask you if possible to repeat my expt. as described, & settle that point which has always struck me as a very important one.

Will you permit me a remark on your 1^st paper. You speak of air being dynamically heated by letting external air rush into a vaccuum (on the Joulian &c theory of vis viva about which I am very far from being satisfied). Now—when a receiver is exhausted & air let in till the indraft ceases & then the stopcock is closed for some minutes & then again opened, more air rushes in and so two or three times. Now certainly it may be said that in entering though latent heat at first disappears during the expansion, yet this is restored on the subsequent filling in & consequent compression, and that the heat developed by the shock is then placed in evidence & keeps the air expanded till an equilibrium (momentary) is attained.

But this ignores the vessel. Query, is not the true explanation that, the entering air being excessively chilled by expansion takes up heat from the vessel. As the elasticity approaches that of the external air this heat is given out & sustains part of the pressure & it is not till after closing the stop cock & letting the cold vessel again take up the lost heat that the deficiency of air becomes sensible and a partial vacuum results? Such has always at least been my view of this phenomenon.

I remain | dear Sir | yours very truly | JFW Herschel

RI MS JT/1/H/98