That Alexander von Humboldt was not dead by the age of thirty-five was a minor miracle. In 1794 he nearly suffocated while testing his miner’s lamp in a subterranean tunnel. The next year he subjected his body to such an extreme series of galvanic experiments that his doctor felt compelled to intervene. In 1800, among the ceiba trees beside the Apure River in Venezuela, he disturbed a resting jaguar (‘never had a tiger appeared to me so enormous’). On that occasion, Humboldt tiptoed to safety, but weeks later he almost paralysed himself while pulling on a sock contaminated with curare, the lethal arrow poison.

Humboldt’s narrowest escape of all, perhaps, came in 1802, during one of his series of high-altitude ascents in the mountains of Ecuador. Nearing a summit, he glanced down to see a bluish light glowing through the snow. He smelled sulphur. ‘He realised with a shudder,’ writes Maren Meinhardt in this evocative and perceptive biography, that he and his companion ‘were on top of the crater itself’. The only thing separating them from the volcano was a ‘thin bridge of compacted snow’.

Resolute, death-defying, highly attuned to his environment: this is the picture of Alexander von Humboldt many people retain. He symbolises better than almost anyone else the lengths to which the Romantics were willing to go to experience nature and establish scientific truth. But this is far from the full picture. As Meinhardt explains, there was nothing of early brilliance about Humboldt. As a boy he was constantly outshone by his elder brother, Wilhelm. ‘Doubts were voiced whether he would ever achieve even average educational standards.’ Humboldt himself conceded it was not until ‘much later in my boyhood that all of a sudden the light in my head got switched on’.

The first third of A Longing for Wide and Unknown Things shows how this unpromising boy with a fondness for butterflies began the process of metamorphosis that would eventually turn him into Humboldt the scientific colossus. Meinhardt, editor for German literature and natural history at the Times Literary Supplement, is the ideal guide to the intellectual salons of 1780s Berlin. She shows how, at eighteen, Humboldt was admitted into the fashionable salon run by Markus and Henriette Herz at their home on Spandauer Strasse. Here the dull rote of his boyhood classes was supplanted by something much richer. He watched scientific demonstrations and learned the steps to the Minuet de la Reine. These salons – strongly Jewish and female in character – had a lasting effect on Humboldt. Henriette Herz’s circle (known as the Tugendbund or ‘League of Virtue’) generated, Meinhardt explains, ‘a close weave of friendships and relationships of various gradations and intensities, one to which Alexander would feel bound for the rest of his life’.

It was on the sofas at Spandauer Strasse that the first flicker of a Humboldtian sensibility started to show. This was different from the high-flown, all-out urge to collect, classify and obtain that had guided the likes of Joseph Banks a generation before. Instead, Meinhardt writes intriguingly, it was ‘in the nature of the project of finding new and better forms of living that the goal should never be quite reached’. The pursuit was the important thing, along with the change the pursuit wrought in the pursuer. The achievement itself was secondary. With this in mind, Meinhardt identifies a pattern in Humboldt’s life of ‘the unfinished and the fragmentary’ that accords with the preoccupations of German Romanticism. The iconic aborted ascent of the Andean peak Chimborazo – then thought to be the highest mountain in the world – in 1802 is ‘emblematic’ of this. Humboldt did not reach the top. But by the time he turned around, the experience had already changed him.

The literature on Humboldt already stretches across library shelves. His own voluble letters, lectures and travels are sufficient alone to keep a reader at work for several months. Meinhardt brings to this biography a brisk, fluent and erudite approach. Humboldt’s almost ninety years are distilled into twenty-five nimble chapters, with the prime focus being the thirty years after his birth in 1769 and the period of his travels (1799–1804).

Richly anecdotal, the book contains a lot of close detail to be absorbed, but there is also plenty of wit and insight to brighten the pages. Meinhardt writes, for instance, that Christian Kunth, the tutor to the Humboldt boys, gave such dense historical lessons that Wilhelm once quipped, ‘one should wish to be Adam, when history was still quite short.’ One learns (with a cheer) that Humboldt never sat a formal examination of any sort in his entire life. Also to be savoured is Humboldt’s culinary review of a meal of deer (Cervus mexicanus) in South America that catches the zeitgeist beautifully: ‘delicious … the muscle fibre less charred, more oxygenated’ than the deer back home in Europe.

The action roams from the mines of Germany to the peaks of South America, but Meinhardt tries to keep some salient questions in view. What was motivating Humboldt? What was the exact nature of his personal relationships? These simple questions prove stubbornly difficult to answer. Born in the same year that James Cook reached New Zealand, Humboldt always felt a yearning to follow the great explorers of the recent past. ‘Things that we only know from the vivid descriptions of travellers have a special attraction for us,’ he wrote, ‘shapes, rendered hazy through distance, prove themselves irresistible to the imagination.’

Like these hazy shapes, Humboldt is particularly hard to pin down. Meinhardt shows that he was a shrewd operator, constantly smoothing the path before him with flattering letters and complimentary copies of his books. This suggests ambition, but Meinhardt also writes of his ‘calm yet persistent refusal to submit to definitions’. He refused well-paid jobs and did not marry. Later in life, he wrote to his friend Goethe that he longed for ‘the wide and the blue’.

The wilds of South America, ‘nature, direct and unmediated’, perhaps came closest to satisfying this nebulous urge. ‘Not only did nature seem magnified,’ Meinhardt writes, ‘but human fates did too.’ Humboldt’s five years of travels among the crocodiles and jaguars, up mountain peaks and down rivers, would pass into history as some of the great exploration feats of the Romantic age. His body responded as well to travel as his mind did. During all his voyaging, he never suffered seasickness. He arrived in Bordeaux in 1804 ‘healthier, stronger and more keen to work than ever’. It seems revealing that when Humboldt returned to conventional life in Berlin, ‘his almost immediate response was to develop a skin complaint, a measles-like outbreak’.

Still the restless force nagged at Humboldt, encapsulated by what he called his ‘longing for wide and unknown things’, a line Meinhardt has chosen for her title. In the heroic view of Humboldt’s life, this might be interpreted as wanderlust. But Meinhardt suggests that it was a feeling that went beyond the desire for travel. ‘Could it be’, Meinhardt wonders, that ‘this aimless, but increasingly undeniable, feeling … was not confined to the geographical realm, but rather permeated his whole personality?’