Clark, Thomas

(b. Ayr, Scotland, 31 March 1801; d. Glasgow, Scotland, 27 November 1867) chemistry

Clark is best know for his method of softening water (“Clark’s process”) in pure chemisity his discovery of sodium pyrophospate had far-reaching implications.

Clarks was educated at Ayr Academy; although at first apparently considered dull, he later showed himself to be gifted in mathematics. In 1816 he began ten years of employment in firms founded by two leading Glasgow industrialists—first with Charles Macintosh, who invented waterproof cloth in 1823 (after Clark had been in his employ) and then, after finding chemistry more to his taste than accountancy, with Charles Tennant, who patented the preparation of bleaching powder.

In 1826 Clark was appointed lecturer in chemistry at the Glasgow Mechanics’ Institution, a post for which Thomas Graham was also a candidate. It is not known how or where Clark learned his chemistry; but judging from an account left by one of his students at the institution,¹ his teaching showed a degree of sophistication that has puzzled historians of chemistry. Griffin reproduced a set of tables, prepared by Clark for his courses of 1827–1828, of elements and compounds, with symbols and formulas, and atomic weights or “combining proportions,” and what we would now call molecular weights (in the case of compounds). They impress one immediately by the liberal use and modernity of symbols. The symbols differ somewhat from those proposed by Berzelius in 1814, which were little used in Britain until about 1833. The most striking feature of Clark’s formulas, however is their suggestion of structure—an inchoate concept at this time² By using an uncommon set of atomic weights (H = 1/2, O = 8, C = 6),

Clark arrived at the correct formulas of a number of compounds, which he expressed as shown;

Alcohol HO; CH, H; CH, H, H

Acetic acid CH, H, H; CO, O; H

Neither what he had in mind nor the significance of the “punctuation marks” is known. They appear to be used the way we now use full stops and colons in structural formulas to denote single and double bonds, but we do not know what Clark actually intended them to mean. His views on the constitution of acids and salts also seem more advanced than those of most of his cpontemporaries.

Clark became a medical student at Glasgow chemistry in a medical school. He resigned his post at the Mechanics’ Institution in 1829 (Graham taking his place) and became apothecary to Glasgow Infirmary. He obtained his doctorate in 1831 and in 1833 was appointed professor of chemistry at Marischal College in Aberdeen, a post which he held nominally until his college was joined with King’s College to form the University of Aberdeen.

Clark had by this time published the paper describing how he had obtained a new compound, which he called sodium pyrophosphate, from ordinary sodium phosphate at red heat, by the loss of water:

2Na₂HPO₄ = Na₄P₂O₇ + H₂O.

This paved the way for Graham’s study of the phosphoric acids,³ which led the concept of polybasic acids and ultimately to a clearer understanding of the constitution of acids in general.

Clark patente his water-softening process (by the addition of a calculated quantity of milk of lime) in 1841. The hardness of water was tested by finding the quantity of a standard solution (which Clark called the “soap-test”) which would produced a lather lasting for five minutes when added to a specified quantity of the water. He also devised a scale of water hardness (one degree of hardness was that produced in one grain of chalk or magnesia, held in solution as a bicarbonate) Although highly praised by Graham and others, it was some time before the softening process was much used by water companies.

What seems to have been a promising career was effective terminated sometime in 1842 or 1843 by the onset of a brain disease from which Clark never fully recovered. He did not resign but appointed a series of assistants to carry out his teaching duties. During periods of partial recovery he interested himself in a variety of subjects—the philology of English, spelling reform, and textual criticism. In 1849 Clark married Mary M’Ewen; their only child died while still a boy.

NOTES

1. See J. J. Griffin, The Radical Theory in Chemistry (London 1858), pp. 5–17. Griffin reproduced the tables in support of his contention that Clark had anticipated, and Griffin had developed, some of the theories that Gerhardt had advanced as his own. Unfortunately, we have only Griffin’s word for what Clark taught, but it seems to be generally agreed that the tables are genuine.

2. For a recent comment on this point, in connection with Clark’s work, see W. V. Farrar, “Dalton and Structural Chemistry”, in D. S. L. Carwell ed., John Dalton and the Progress of Science (Manchester-New York, 1968), pp. 294–295.

3. Thomas Graham, “Researches on the Arseniates, Phospates and Modification of Phosphoric Acid,” in Philsophical Transactions of the Royal Society, 123 (1833), 253–284.

BIBLIOGRAPHY

I. Original Works. Clark more important papers are listed in The Royal Society’s Cataologue of Scientific Papers I (1867), 932–933. Those particularly relevant to the text are “On the Pyrophosphate of Soda, One of a New Class of Salts Produced by the Action of Heat on the Phosphates,” in Edinburage Journal of Society7 (1827) 298–309; and “On the Examination of Water for Towns, for Its Hardness, and for the Incrustation It Deposits on Boiling,” in Chemical Gazette, 5 (1847) 100–106, He also published a pamphlet on his process, A New Process for Purifying the waters Supplied to the Metropolis by the Existing water companies… (London, 1841).

II. Secondary Literature. The main source is A. Bain “Biographical Memoir of Dr. Thomas Society, 1 (1884) 101–115. The paper was written in 1879; Bain, then professor of logic Aberdeen, had been a pupil of Clark’s. Other accounts, partially derivative, are A. findlay “The Teaching of Chemistry in the University of Aberdeen,” in Aberdeen, University Studies, 112 (1935) 18–36; and J. H. S. Green “Thomas Clark (1801–1867), A Biographical Study” in Annals of Science, 13 (1957), 164–179.

E. L. Scott