skip to content

Darwin Correspondence Project

From Robert Hunt   19 July 1855

London | Jermyn St

19 July 1855

Dear Sir,

many apologies are due from me, to you, for my apparent neglect— The fact, is, that shortly after seeing you I had to leave Town—and institute some enquiries connected with the production of Coal and Iron—1 This completely engaged my attention—and you were forgotten.— Pardon this I pray you.—

As I understood you, your object is, by the use of absorbent media to produce conditions—as it relates to the Solar radiations, similar to those which prevail in the countries, to which the plants under experiment are indigenous. 2

It is found that the three principles—or phenomena—of the Sunbeam Light, Heat and Actinism, vary according to some fixed law—not yet well defined—as we proceed from the poles towards the Equator3 To confine our attention to the Northern Hemisphere— The sunshine of Norway may be represented by 5A+5L+5H That of Italy to 3A+6L+6H—and as we advance nearer the Line the conditions will be well represented by 1A+7L+7H. I believe I have pretty clearly proved, that, the Chemical principle Actinism—is active in accelerating Germination and those operations in the growing plant which are strictly Chemical—4 Light acts by stimulating the vital forces of the plant—and thus accelerates the formation of wood.5 I distinguish this Living Chemistry—effecting the decomposition of Carbonic Acid—broadly from that process—which may take place in dead matter—producing Gum &c Heat evidently operates by its ordinary Calorific force—producing effects due merely to exaltation of temperature—and combined, as it were, with some Chemical force—in Herschel’s Parathermic rays 6—Heat produces the full developement of the Flower—carrying on indeed all the processes of reproduction. In the Geographical distribution of Plants, I think I perceive relations between the plant and the Sunshine which are explainable upon this view.

Now—we best imitate the conditions of a Norwegian Sky by a pale Blue Glass— Those of Southern Europe by a light Yellow one and those of Tropical Asia by a red glass. I enclose a few small pieces which may guide you in your Selection. After all, you will find it important, to try a few preliminary experiments—and determine from those the colours of the Media—under which your experiments shall be made. In any way it will afford me much pleasure to aid you— It is not probable that I shall ever be able to resume my investigations on these matters—and it would rejoice me much to find so able an observer as yourself—fairly working over this fertile field of research—

I am | Dear Sir | Yours very truly | Robert Hunt C. Darwin Esqe &c &c &c

CD annotations

crossed pencil
3.1 It is ] after square bracket added pencil
crossed pencil


Hunt was professor of experimental physics at the School of Mines in Jermyn Street.
In addition, CD hoped to induce the formation of monstrosities by ‘breaking’ their constitutions (see letter to J. D. Hooker, 14 [July 1855]).
Described in R. Hunt 1854.
R. Hunt 1854, pp. 221–8.
R. Hunt 1854, pp. 228–31.
John Frederick William Herschel had coined the term ‘parathermic’ to describe rays that lay between the red end of the visible spectrum and the calorific spectrum of radiant heat. He identified the rays by their action on photosensitive vegetable dyes. See Herschel 1843, p. 5.


Discusses how best to simulate the light at a particular point on the earth’s surface using coloured glass; considers sunlight as composed of three "principles", varying in proportion according to latitude, which affect germination, lignification, and floriation.

Letter details

Letter no.
Robert Hunt
Charles Robert Darwin
Sent from
London, Jermyn St
Source of text
DAR 261.11: 17
Physical description
3pp †

Please cite as

Darwin Correspondence Project, “Letter no. 1721,” accessed on 23 July 2019,

Also published in The Correspondence of Charles Darwin, vol. 5