In later life, Urey helped develop the field of 'cosmochemistry' and is credited with coining the term. His work on oxygen-18 led him to develop theories about the abundance of the chemical elements on Earth and of their abundance and evolution in the stars. This work was among the pioneering paleoclimatic research. Urey summarised his work in the book "The planets: their origin and development" (1952). Urey speculated that the early terrestrial atmosphere was probably composed of ammonia, methane and hydrogen; it was one of his Chicago graduate students, Stanley L. Miller, who showed that, if such a mixture be exposed to electric sparks and to water, it can interact to produce amino acids, commonly called the "building blocks of life" ("see Miller-Urey experiment").