Urey's previous research had concluded that the atmosphere consisted of water, hydrogen, ammonia and methane. He thought that these chemicals were reducing so set about doing an experiment to prove it. With the assistance of Miller, a continuous 60,000-volt electrical charge was passed through a container that contained the four constituents. The experiment concluded that after about seven days the ammonia and methane had reduced considerably, but carbon monoxide and nitrogen were present. The water also contained a dark substance.
Examination of the dark substance in the water proved difficult, but resulted in an important discovery. It contained hydroxy acids, amino acids and fatty acids -- the molecules keys to living organisms. The Miller-Urey experiment was considered a major breakthrough and led to substantial investigative work on the subject of prebiotic chemistry.
Prebiotic chemistry deals with spontaneous chemicals reactions that in some way may have led to the creation of life on Earth. The Miller-Urey experiment concluded at the time that under certain conditions, primitive living organisms may form. The importance of this discovery led to substantial investment in prebiotic chemistry, resulting in today's knowledge base about molecular formation.
Many of the results from the original Miller-Urey experiment have been subsequently questioned, particularly in relation to the atmosphere reducing. However, without the important experiment, it is possible that today's, still somewhat limited, understanding of the origins of life, would have been severely affected. It is still widely accepted that prebiotic chemistry does have a place in events that took place on Earth millions of years ago.