Let me begin with an appraisal of this biography. I can recommend it to you if your library or you can afford $55.00 get it, and you can put a week aside and read it. However, I wonder what readership is addressed in the text. As an erstwhile physicist I expected to learn something of the trajectory of Helmholtz's science but while there is something of this expectation present it is thin in detail. For example, Helmholtz wrote theoretical physics and he has an equation named after him, but in this biography I could locate just one page of mathematics – a photograph of Helmholtz in front of a blackboard – and this was related to the principle of least action. A principle that Helmholtz hoped could be used as a unifying idea for all science.

David Cahan's study of Helmholtz is the best biography of a scientist I have read: it is long but beautifully written. Cahan's style could be a primer for a budding academic writer, his prose is without pretensions, he avoids obscure literary devices and his views and interpretations are clearly linked with his sources. Insofar as there are historical facts Cahan leads us to them, and his style gives a word picture of Helmholtz's life in all its complexity. The bibliographical and archival pages from which Cahan has crafted his excellent book are full and well ordered. Cahan's knowledge of the social and political history of Germany from 1750 to the present is encyclopedic. This history is the backcloth to Helmholtz's life and work.

By all accounts Helmholtz was a leading scientist in the nineteenth century; perhaps the most significant natural philosopher in the period. In Cahan's book some 270 works are credited to Helmholtz, beginning with a study of the nervous system in vertebrates in 1842 to a consideration of the principle of least action in 1894, the year of his death. Helmholtz was interested in everything, as is indicated by his papers and books. I would argue that this property is common to all physicists. The subject is related to everything and this claim underlies much of Cahan's biography. From an early age Helmholtz wished to become a physicist and he became famous for his scientific work and many honours were showered on him.

Cahan has chosen a chronological episodic form for this biography: ‘The making of a scientist’, covering the period from his birth (1821) to his marriage and time in Königsberg (c.1849); ‘Gaining scientific renown’, focusing on his work in Bonn and Heidelberg (1849–1871); and ‘Scientific grandee’, encompassing his later career and death in Berlin (1871–1894).

Helmholtz's philosopher father taught in the Potsdam Gymnasium but the son had his own philosophy. The family could not to afford the fees for physics, but in Berlin – where Helmholtz studied medicine and learnt physics – the Prussian state covered the costs and in return Helmholtz became an army doctor. In Berlin he lectured in anatomy for art students, gained scientific friends and garnered some influential contacts, Humboldt, for example. By 1849 Helmholtz could become an academic, and was called to teach physiology and pathology in Königsberg.

Helmholtz's studies of muscle contractions led him to accept the conservation of energy but his first paper on that subject was not a success. In Königsberg his research blossomed and his assiduous networking paid off. He continued developing optical instruments and he travelled widely in the mainland of Europe and in Britain. There he met many engineers and physicists. Later William Thomson (Lord Kelvin) became a friend. In particular his monograph on energy was accepted in Britain; in the BAAS 1853 Hull meeting he promulgated his ideas. His travels allowed him to observe people, cities and governance; perhaps these excursions helped him to become a leader in Berlin.

In the 1850s the character of science was changing. The importance of religion was declining, empiricism and materialism were becoming ascendant, but in Helmholtz's scientific work Kantian doctrines were important. By contrast, Helmholtz disliked Hegel's ‘speculative’ ideas; perhaps British pragmatic philosophy was more useful for science.

Königsberg had been kind to Helmholtz, but the area, rural, suffered from its geography, situated near the eastern border of Prussia. Many scholars were associated with the university, but Helmholtz considered that his ambitions and talents would be better served away from Königsberg; in July 1855 he took a post in Bonn. Science was not well supported, and he missed his friends, and Bonn underlined the importance of philosophy in German science. Politics defeated Helmholtz's attempts to modernize science in Bonn. However, in 1858 he moved to Heidelberg, where he was comfortable, for it was among the best universities in the German-speaking world. Heidelberg widened his knowledge and interests.

However, when his wife died late in 1859, depression attacked; by next March work had cured him. He married again and witnessed the birth of a new professional role, ‘scientist’. With his Irish friend John Tyndall, prestige helped to popularize science as both Britain and Germany recognized its utility. Helmholtz's wide interest in physics was evident in his Tonempfindungen (1863). His researches in physiology and in the eye and perception continued. In 1867 other subjects attracted him, for example electromagnetism, Greek geometry and non-Euclidean space, and thermodynamics. By 1871 Helmholtz was perhaps the leading physicist in Germany, with friends in the top tier of European science. Armed with his scientific and management skills he was ready to move to Berlin.

Berlin was now the capital of the German Empire and the university was to become the most significant centre of scientific research in the country. Founded in 1810, Helmholtz joined a university that was ripe for change and modernizing. To his mind the quality of the staff trumped other things and he sought out the best for Berlin. Hertz and Planck were two of his choices. A glance at Helmholtz's publications shows a man interested in everything; he wrote on most areas of physics and his output was large and lifelong. When he faced the task of building physics in Berlin his genius in management and administration allowed the family to move among the significant elite in Germany. He took a part in developing and defining standards and was a leader in the foundation of national physical laboratories. He was well paid for his time and his work. Theoretical and pragmatic problems were his life. He died from a stroke in Berlin in 1894.

Helmholtz's influence on physics continued at least until the 1960s. In Britain in the 1950s physics for undergraduates at UCL was modelled on Helmholtz's idea of physics education. During the first two years of my physics course (1952–1955) we were taught ‘classical physics’: essentially due to Helmholtz's ideas. Only in the third year did we hear of ‘modern physics’ and even that could have been entitled ‘From Planck to J.J. Thomson’.

Helmholtz could have taught most of the courses.