The knowledge that Humboldt, who had more than once been reported dead, had actually returned to his native land, bringing with him his valuable collections, created an immense excitement all over Europe, and his name soon became a household word.

For the first time Europeans had an accurate and life-like picture presented to them of the New World, which had always possessed such a mysterious charm, and as they read the fascinating descriptions of Humboldt, they followed him in imagination through all his wonderful journey. With him they sailed up the Orinoco, traversed the llanos, crossed the snow-fields of the Andes, and visited the tropical forests; and the popular fancy, not content with actualities, threw over the adventures of Humboldt even a more magic spell. It was said that in his western tour he had fought and conquered giants, visited the tombs of dead nations and learned their buried secrets, had his courage tested by encounters with races that were but half-human, and had learned of Nature in her great solitudes the secret which governed the life of man, and the wisdom of all the ages. And though this view of Humboldt was but the result of that love for the marvelous which is ever seeking something new, it did not lessen the fascination which was attached to his mane, and he was regarded as a second Marco Polo, whose adventures were more romantic and exciting than any tale of the "Arabian Nights."

For many years after his return Humboldt spent his time in preparing the complete history of this travels in America, giving public lectures, and perfecting his great theory as to the nature and development of the plan of the universe.

But during this period of twenty-five years he had ever before his mind a great journey to Central Asia. With this in view he studied the languages, geography, and history of the East, as well as the existing descriptions of the physical formation of the country, and in 1829, having completed all his arrangements fro an extensive tour, set out for India. Humboldt was accompanied by a number of scientists, and the expenses of the journey were to be defrayed by the Russian Government, which was desirous of obtaining accurate scientific reports of the mineral wealth of its dominions.

The expedition left St. Petersburg in May, 1829, and was absent eight months, during which time Humboldt's industry was indefatigable. During this time he explored the Ural formations, and gained an important insight into the gold and platinum deposits, besides discovering several new minerals; visited the Altai Mountains, and made an important expedition to the Caspian Sea for the purpose of analyzing its waters and obtaining specimens of fish, besides making many observation on the climate, soil, and geological formation of the mountains.

The journey was a most important one for science. Previous to this there existed only the vaguest ideas of the geography of Central Asia, the connection of the mountain-chains, and the productions of the soil; but Humboldt's accurate survey of the hitherto unknown territory put geographical knowledge on a firmer foundation, and gave to the world a clear idea of that interesting land which possessed a charm for all nations as being the supposed first home of the human race. Humboldt proved that Central Asia was neither a broad plateau nor an immense cluster of mountains, as had been supposed, but that it was crossed by the four mountain-systems which have exercised an immense influence on the migration of nations, and helped for form the history of the world.

Humboldt also made important observations on the boundary of eternal snow, and, in his study of the winds and tides and their relation to climate, and to the forms of the continents, deduced a theory of the different temperatures of places in the same latitude but at great distances from one another.

The entire result of Humboldt's travels and studies was incorporated in his great work called "Cosmos," which was complied from his notes, and in the composition of which he was aided buy Cuvier and other naturalists. "Cosmos" contains a theory of the formation of the universe, and embraces observations on the heavens, mountains, earthquakes, the sea, the earth's crust, the atmosphere, the geography of plants and animals, the races of men, the form, density, latent heat, and magnetic power of the earth, and the aurora borealis.

It is the most exhaustive work ever by a single mind, and shows, as nothing else could have done, the extent and originality of Humboldt's powers. Besides the valuable contributions to geographical and geological knowledge, Humboldt's theories of the distribution of heat and magnetism were of special importance. His observations on the magnetic needle and the aurora borealis were of the greatest service to science, and it is largely due to him that observatories have been erected all over the world, from Canada to the Cape of Good Hope, and from Paris to Pekin, with special reference to the study of the earth's magnetism.

Humboldt also devoted much time to the study of the isothermal lines, or lines of equal temperature which connect different places, and likewise the sciences of climatology and geognosy may be said to date from his time.