Important discoveries were made in the 20th century. These include applying statistical mechanics to chemical systems and Irving Langmuir's work on colloids and surface chemistry. In the 1930's, Linus Pauling and others applied quantum mechanics to develop quantum chemistry. Chemical theories have grown with new experimental discoveries. New forms of spectroscopy started in the 20th century including: infrared spectroscopy, microwave spectroscopy, EPR spectroscopy and NMR spectroscopy.