Many discoveries and inventions are touted as history - changing. But as Thomas Hager admirably proves in his new book, The Alchemy of Air, Fritz Haber and Carl Bosch not only changed history, they made much of recent human history possible. As Hager solemnly notes in his introduction, "the discovery described in this book is keeping alive nearly half the people on earth." That discovery, known as the Haber - Bosch system, makes it possible to fertilize enough agricultural land to feed our population at much more than subsistence level. The human race is now so well fed that obesity is a problem in regions that a mere 40 years ago were plagued with famine. For this plenty, the world may thank - or curse, depending on one's views - the German chemical industry of the early 20th century.

Hager discovered the story of Haber and Bosch while writing The Demon Under the Microscope, an account of the discovery of the first effective antibiotics. But while sulfa drugs prevented the deaths of millions, the survivors would have nothing to eat unless nitrogen, one of the pillars of life on earth, could be mass produced. Natural fixed - nitrogen fertilizer is quite limited in supply, creating a ceiling on the amount of food humans could grow. Haber and Bosch, who both died broken and disillusioned, found a way to transform a portion of the vast reservoir of atmospheric nitrogen into usable nitrogen - based fertilizer, thereby lifting the food ceiling to heights not yet discovered.

As with almost all technological advancement, however, there is a downside. The synthetic Haber - Bosch nitrogen, which now makes up about half the nitrogen in every human body, also fueled the weapons of the world wars and created a nitrogen - rich environment that is having a huge impact on Earth, from lush vegetative growth to dead zones in the oceans. Thanks to two visionary and troubled scientists, we are all now, in Hager's words, "creatures of the air," dependent for our very existence on a process whose consequences we don't completely understand.

Chris Scott writes, and eats, in Nashville.

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