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Darwin Correspondence Project

To Edward Frankland   22 July 1874

Down, | Beckenham, Kent.

July 22 1874

My dear Professor Frankland

Can you tell me what is the specific gravity of Phosphate of Ammonia? In Watt’s Dict. I find the Spec. grav. of several phosphates of Amm., but then I do not know which is the common phos.1

I bought a bottle from Hopkin & Williams2 labelled simply “phos. of Amm.”, containing a colourless chrystaline mass, & it is of this that I want to know the specific gravity. I enclose a card addressed, & if you will write the number on it, it will save you a little trouble.

Yours very sincerely | Ch. Darwin


CD’s annotated copy of Henry Watts’s Dictionary of chemistry is in the Darwin Library–Down (see Marginalia 1: 195–6). CD’s copy consists of the first three volumes of the second edition (Watts 1872–4), volumes four and five of an 1871 reprint of the first edition (Watts 1863–8), and a supplement to the first edition dated 1872. Specific gravity is a measurement of relative density of a substance in comparison to water. Different forms of phosphate of ammonia (now more commonly known as ammonium phosphate) have different numbers of ammonium ions and thus different specific gravities. The commonest solid forms are monoammonium phosphate and diammonium phosphate.
Hopkin & Williams were manufacturers of fine laboratory and photographic chemicals, with offices at 10 Cavendish Street, London, and a factory in Wandsworth, Surrey (Post Office London directory 1872). CD had obtained chemicals from them for his work on insectivorous plants (Correspondence vol. 21, letter to W. W. Baxter, 4 September 1873).


Correspondence: The correspondence of Charles Darwin. Edited by Frederick Burkhardt et al. 27 vols to date. Cambridge: Cambridge University Press. 1985–.

Marginalia: Charles Darwin’s marginalia. Edited by Mario A. Di Gregorio with the assistance of Nicholas W. Gill. Vol. 1. New York and London: Garland Publishing. 1990.

Post Office London directory: Post-Office annual directory. … A list of the principal merchants, traders of eminence, &c. in the cities of London and Westminster, the borough of Southwark, and parts adjacent … general and special information relating to the Post Office. Post Office London directory. London: His Majesty’s Postmaster-General [and others]. 1802–1967.

Watts, Henry. 1863–8. A dictionary of chemistry and the allied branches of other sciences. 5 vols. London: Longman, Green, & Co.

Watts, Henry. 1872–4. A dictionary of chemistry and the allied branches of other sciences. 2d edition. 5 vols. London: Longman, Green, & Co.


Asks for the specific gravity of common phosphate of ammonia.

Letter details

Letter no.
Charles Robert Darwin
Edward Frankland
Sent from
Source of text
The John Rylands Library, The University of Manchester
Physical description

Please cite as

Darwin Correspondence Project, “Letter no. 9559A,” accessed on 22 September 2021,

Also published in The Correspondence of Charles Darwin, vol. 22