Thomas Clark

Background

Clark was born on March 31, 1801, in Ayr, Scotland, the son of a shipmaster and a needleworker.

Education

Clark was educated at Ayr Academy; although at first apparently considered dull, he later showed himself to be gifted in mathematics.

He became a medical student at Glasgow University in 1827, with a view solely to teaching chemistry in a medical school. He obtained his doctorate in 1831.

Career

In 1816 Clark 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, 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 time.

By using an uncommon set of atomic weights, Clark arrived at the correct formulas of a number of compounds. 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 contemporaries.

He resigned his post at the Mechanics’ Institution in 1829 (Graham taking his place) and became apothecary to Glasgow Infirmary, 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. This paved the way for Graham’s study of the phosphoric acids,3 which led to the concept of polybasic acids and ultimately to a clearer understanding of the constitution of acids in general. Clark patented 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 produce 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 gallon of water by 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 effectively 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.

Achievements

• Clark is best known by his water tests and by his process for softening chalk waters. He discovered the phosphate of soda, and the process of lime softening of hard water was called after him - the 'Clark process'.

Personality

During periods of partial recovery from his illness, Clark interested himself in a variety of subjects - the philology of English, spelling reform, and textual criticism.

Connections

In 1849 Clark married Mary M'Ewen; their only child died while still a boy.

Spouse:

Mary M'Ewen

References

• Dictionary of Scientific Biography Volume 3
  1971