Paul Daniel Hahn was the youngest son of the Rhenish missionary Johannes Samuel Hahn and his wife Helene Langenbeck, and a brother of J. Theophilus Hahn*. In 1852 his parents returned to Germany, where he received his schooling at the gymnasium at Soest, Westphalia. He continued his studies at the University of Halle in 1870, studying chemistry, physics, mineralogy and mathematics, and graduated as Master of Arts (MA) and Doctor of Philosophy (PhD) in March 1874. His doctoral thesis consisted of two papers which dealt with the phosphorescence of minerals and the chemical constitution of natural silicates respectively. Both were published in the proceedings of the Naturwissenschaftlichen Gesellschaft of Halle in 1875. In October 1874 he furthermore passed the "Staats Examen, pro facultate docendi", which entitled him to teach at any German university. On 28 June 1875 at Gotha he married Marie Aneck, born Black, with whom he had six children.
After spending some time as a researcher in London and Edinburgh, Hahn came to the Cape Colony in September 1875. He was appointed Jamison professor of experimental physics and practical chemistry at the South African College, Cape Town, in January 1876, the new chair being named after a generous benefactor of the College, Mrs Jamison. At the same time Dr John Shaw*, headmaster of the South African College School, was inexplicably appointed as part-time professor of physical science. It was Hahn, however, who drew the students in steadily increasing numbers. From 1893 his post was changed to professor of chemistry (which later included metallurgy), a position he retained with distinction to his death in 1918. From 1876 he also served on the council of the University of the Cape of Good Hope (of which the South African College was a constituent college) for an uninterupted period of 42 years and was one of its examiners in chemistry in 1907 and 1913-1916. He played an important part in the college's striving for university status and it eventually became the University of Cape Town a few weeks after his death.
When Hahn took up his post, chemistry was an almost unknown subject in South Africa and the college's accommodation and facilities for teaching it were barely adequate. However, a new chemistry laboratory was completed in 1881, planned by Hahn and funded by Mrs Jamison. Early in 1886 he proposed to senate that women students be admitted to the college. This resulted in the admission of three women students, to Hahn's department only, on an experimental basis in 1887. His report on the success of the experiment led to the opening up of the rest of the college to women.
In December 1890 Hahn published a lengthy article in the South African College Annual urging the establishment of a School of Mines in South Africa and described the advantages of such an institution for the country. Thus he played a key role in the foundation of the South African School of Mines at the college, and taught its students chemistry and metallurgy until the institution moved to Johannesburg in 1903. In 1900 he and Professor J.C. Beattie* were recognised by the Scottish universities as extra-academical lecturers and their courses in chemistry and physics respectively accepted for admission to the Scottish medical schools. Among the assistants that worked for him in the Chemistry Department, selected from among former pupils, were J.C. Watermeyer*, W. Versfeld* and H. Tietz*. In 1906 the college granted him a year's paid leave, in view of his long and outstanding service. He spent the time in Europe, where he attended the International Congress of Applied Chemistry in Rome and represented the college at the four hundredth anniversary of the founding of Aberdeen University.
For many years Hahn was the leading analytical chemist in the Cape Colony and his help or advice was sought by both the government and the private sector on a variety of industrial and scientific problems, including toxicological analyses in medico-legal cases, the identification and analysis of mineral deposits, and the analysis of agricultural soils. Many of his observations and analyses were included in various parliamentary reports. As a result of his leading position in the field he has been called the Father of Applied Chemistry in South Africa (Rose, 1960). On his recommendation a government chemical laboratory was established, directed by his former pupil, Dr C.F. Juritz*, which took over most work for the government. During his holidays Hahn travelled regularly in the colony, often accompanied by students, and was widely recognised as an expert. He assembled a very fine private collection of minerals, including South African minerals, during his long scientific career and in 1891 donated a collection of 39 different South African minerals to the Albany Museum, Grahamstown, and in 1911 presented rare minerals from the Cape Colony and German South West Africa (now Namibia) to the McGregor Museum in Kimberley.
Around 1900 the German government accorded him the title of Koniglicher Hofrat (Royal Privy Councillor) in recognition of his contributions to science and education. However, he published only a limited number of papers and pamphlets, most of minor importance. Early on, in 1879, he compiled comparative hydrometer tables which were published in Cape Town. At the joint meeting of the British and South African Associations for the Advancement of Science in South Africa in 1905 he read a paper on the chemistry of the hot spring at Caledon, which was published in the Addresses and papers... of the meeting (Vol. 1, pp. 247-259). Other publications dealt with the mineral springs at Malmesbury (1897) and Machadodorp (1913). At the 1910 meeting of the South African Association for the Advancement of Science he spoke on "A geyser in South Africa" (Report, pp. 240-241), and described "A rare copper mineral" (p. 286-288). At the annual meeting of the society in 1915 he read a paper which dealt with the question, "Can lithia be a constituent of plant food?" (Report, pp. 227-), and another on "Radioactive minerals in South Africa" (pp. 449-452).
Hahn had a strong interest in the practical application of science, one of his major interests being viticulture. He served on two government commissions to inquire into the improvement of colonial viticulture, the Vine Diseases Commission of 1880, and the Phylloxera Commission (1890-1893), the latter dealing with root rot in Cape vineyards. In association with J.H. Hofmeyr, a member of the college council, he urged the Cape government in 1882 to acquire and develop Groot Constantia as a model wine farm, and as a nursery for American vines with which to reconstitute the Cape vineyards after the ravages of phylloxera. In 1886 he contributed an article on "Viticulture of the Colony" to the Official handbook of the Cape of Good Hope (pp. 265-280). Years later he delivered a lecture under the auspices of the Cape Cambrian Society on "The chemistry of fermentation" (1903), and contributed a chapter on "Viticulture in Cape Colony" to the handbook Science in South Africa, published in preparation for the visit of the British Association to South Africa in 1905. In 1909 he read a paper on "The chemistry of Vitis capensis and of the wine made from its fruit" before the Cape Chemical Society. Three years later he published a paper on "Temperature treatment of wine" in the Agricultural Journal of the Cape of Good Hope.
Other aspects of agriculture, and the role that chemistry could play in them, also drew his interest. At the First South African Irrigation Congress in 1909 he spoke on "Physical relations of water to different kinds of soil". With D.S. Stevenson as co-author he read a paper on "Karroo soil, lucerne and the ostrich feather" at the 1910 meeting of the South African Association for the Advancement of Science (Report pp. 122-128), followed in 1915 by his "Contributions to the chemistry of the soya bean" (Report, pp. 124-127). He supported the formation of a forestry school and played an active role in the introduction of tuition in agriculture at Victoria College, Stellenbosch, and later in the establishment of the Department of Agriculture of the Cape Colony.
Hahn served on the first council of the South African Philosophical Society from 1877 to 1879 and in 1908 was elected a Fellow of its successor, the Royal Society of South Africa. In 1902/3 he was the first president of Section A of the South African Association for the Advancement of Science, and in 1911 was president of the whole Association. He served as president of the Cape Chemical Society for 1907-1908 and 1910-1911, and as vice-president for several years thereafter. He joined the British Association for the Advancement of Science in 1905, was a member of the South African Association of Analytical Chemists, and an honorary member of the Chemical, Metallurgical and Mining Society of South Africa.
Hahn was a passionate lover of music and during the 1890's founded the College Orchestral Society. Though a strict disciplinarian he was well-liked by his students and affectionately known as "Oom Daantjie". He was tall and broadly built, with a strong personality and boundless energy, meticulous and thorough in his work, a patient and inspiring lecturer despite his accented English. During 1917 he became seriously ill and in spite of being able to return to work in October that year he died five months later. During his long career he played a leading role in promoting the systematic study of not only chemistry, but also mining, metallurgy and agriculture.