Max Ferdinand Perutz (1914-2002) was awarded half the 1962 Nobel Prize in Chemistry for his X-ray diffraction analysis of the structure of hemoglobin. Perutz started working on the structure of hemoglobin in 1937 and, a year later, published a paper on X-ray diffraction from crystals of hemoglobin and chymotrypsin, a pancreatic enzyme that catalyze the breakdown of proteins in the small intestine. The chymotrypsin crystals were twinned and therefore were difficult to work with, so Perutz worked on hemoglobin instead. In 1953 he showed that X-rays diffracted from protein crystals could be phased by comparing the diffraction patterns from crystals with and without heavy atoms attached. Using this technique, he determined the molecular structure of hemoglobin and showed that it is composed of four separate polypeptide chains that form a tetrameric structure, with four heme groups near the molecule's surface. Later, Pertuz studied the flow of glaciers. Using crystallography, he examined how snow becomes ice and was the first to measure the velocity distribution of a glacier.