In 1912 a chemists by the name of Wilbur Scoville, working for the Parke-Davis pharmaceutical company, developed a method to measure the heat level of chile peppers. The test is named after him, the "Scoville Organoleptic Test". It is a subjective dilution-taste procedure. In the original test, Wilbur blended pure ground Chiles with sugar-water and a panel of "testers" then sipped the solution, in increasingly diluted concentrations, until they reached the point that the liquid no longer burned their mouths. A number was then assigned to each chile pepper based on how much it needed to be diluted before they could no longer taste (feel) the heat.
The pungency (or heat factor) of chile peppers is measured in multiples of 100 units. The sweet bell peppers at zero Scoville units to the mighty Naga Jolokia (Ghost Pepper) at over 1,000,000 Scoville units! One part of chile "heat" per 1,000,000 drops of water is rated at only 1.5 Scoville Units. The substance that makes a chile so hot is called Capsaicin
(cap-say-ah-sin). Pure Capsaicin rates between 15,000,000 and 16,000,000 Scoville Units! Today more scientific and accurate methods like Electrochemistry and High Performance Liquid Chromatography (HPLC) are used to determine capsaicin levels. In honor of Dr. Wilbur the unit of measure is still named Scoville.