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Imagine a substance so potent that depending on how you use it, it could blow you to pieces, or it could save your life. This stuff exists, and you've probably heard of it. It's called nitroglycerin.

Nitroglycerin was first discovered in 1847 by an Italian chemist named Ascanio Sobrero, who was experimenting with acids derived from plants. This brand new compound he synthesized was an oily liquid, with some unusual characteristics. When he tasted a small amount he found that it was sweet, but gave him a violent headache that lasted for hours. Apparently he never got the memo that you're not supposed to just put a mysterious chemical in your mouth to see what it tasted like.

Sobrero quickly discovered that the substance had another more dangerous property. Even a small physical knock could cause it to detonate, producing a powerful explosion. He showed off nitroglycerin's amazing power by detonating a small amount in front of a packed lecture hall. Nitroglycerin explosions were hard to control though to the point that Sobrero himself was eventually badly scarred by a later explosion in his lab.

The explosive force lies in nitroglycerine's molecular structure. Each molecule is made up of three nitrate groups bound to a chain of carbons. In general, you need two main things for combustion to happen: fuel, and an oxidizing agent, which removes electrons from molecules of fuel to keep the reaction going. The most common oxidizing agent is oxygen which is why fires normally need air to keep going, but those three nitrate groups, and each nitroglycerin molecule, are powerful oxidizing agents themselves, so once an explosive reaction is started, it doesn't need oxygen to sustain itself. Everything the reaction needs is right there in the molecules.

Because nitroglycerin is so unstable, it's not actually very useful as an explosive in its pure form. It's just too hard to control. But in 1864 the Swedish chemist Alfred Nobel patented a process for making nitroglycerine more stable, so that it could be set off on demand, with a detonator. Nobel named his creation dynamite, and it turned out that the ingredient that made a deadly explosive had another life-saving property, hinted at when workers in dynamite factories noticed that coming into contact with nitroglycerine seemed to ease chest pains.

Around the same time that Nobel was inventing dynamite, scientists and doctors began noting the effects of nitrate compounds on the heart, and experimenting with using them to treat chest pain. At first they used a different chemical called amyl nitrite, a compound that we now know has a nitrite group that dilates blood vessels and lowers blood pressure. This increases the flow of blood and oxygen to the heart and keeps it from having to work so hard. But at the time no one knew why amyl nitrite worked only that it did. They didn't think to try nitroglycerin for a couple more decades.

The first attempt at a medicinal use for nitroglycerin was as a headache treatment by people who believed in homeopathy. Homeopathy has no scientific basis but its main principle is that "like cures like", that is when a lot of something causes a symptom, a small amount of that same thing can cure the symptoms. Since nitroglycerin caused violent headaches, early homeopaths thought that small doses of it could relieve them. It didn't really work. But by around 1880, doctors began to realize that nitroglycerine was even more effective and faster acting for heart problems than amyl nitrite.

Even after it became an accepted treatment for angina, the technical term for chest pain, what exactly nitroglycerine does to the circulatory system remained a mystery for a long time. It wasn't until the 20th century that scientists finally figured out the mechanism at work. Your body converts nitroglycerin to nitric oxide, which relaxes blood vessels, and lowers blood pressure. So nitroglycerine works a lot like amyl nitrite once it's inside the human body. If you or someone you know has been prescribed nitroglycerin tablets though, you won't see any warning labels about possible explosion. The pills only contain a very small amount of nitroglycerin diluted with inert material to make it safe. But they can still set off those bomb-sniffing machines at airport security.

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