By HAROLD McGEE

What do garlic and onions have in common with gunpowder? A lot. They’re incendiary. They can do harm and they delight. Sulfur is central to their powers.

And they helped inspire the work of a chemist who has just published a welcome treatise on the smelly yet indispensable allium family. For over four decades Eric Block has worked on allium chemistry at the State University at Albany in New York. Dr. Block’s book “Garlic and Other Alliums: The Lore and the Science” was published earlier this year by the Royal Society of Chemistry.

In his book, Dr. Block carefully evaluates the mixed evidence for allium efficacy in folk and modern medicine, and explicates the chemistry and treatment of garlic breath. (It can emanate from deep within for a day and more; raw kiwi, eggplant, mushrooms or parsley can help.) Most helpfully for the cook, he sorts out the different kinds of allium flavors and how they evolve on the cutting board and stove.

And he gives an intriguing preview of new alliums just over the horizon. “It’s still astounding to me what happens when you cut or bite into an onion or a garlic clove,” Dr. Block said recently. “These plants originated in a very tough neighborhood, in Central Asia , and they evolved some serious chemical weapons to defend themselves.” Their sulfur-based defense systems give the alliums their distinctive flavors. The plants deploy them when their tissues are breached by biting, crushing or cutting.

The chemicals are highly irritating, and discourage most creatures from coming back . They kill microbes and repel insects, and they damage the red blood cells of dogs and cats. Any cook knows that chopping alliums releases chemicals that sting. Dr. Block explains that different alliums stockpile different chemicals to make their weapons, and this accounts for their varying flavors.

Onions, shallots, scallions and leeks produce a small sulfur molecule that launches itself from the damaged tissue and attacks our eyes and nasal passages. This longdistance weapon is called the lachrymatory factor because it makes people’s eyes water. “Use your nose, follow the changes,” Dr. Block said, “and you’ll discover new and delicious things.”