The mathematical physicist and engineer William Thomson, 1st Baron Kelvin (1824-1904) is best known for devising the Kelvin scale of absolute temperature and for his work on the first and second laws of thermodynamics. The lectures in this collection demonstrate an attempt by Baron Kelvin to formulate a physical model for the existence of ether. This concept of a medium for light propagation became prominent in the late nineteenth century, arising from the combination of Maxwell’s equations stating that light is an electromagnetic wave with the demands of Newtonian physics that light must move in a unique reference frame. First published in 1904, Kelvin’s lectures describe the difficulties inherent in this model. These problems with the concept of ether are credited for inspiring Einstein to devise the theory of special relativity and the photoelectric effect, both of which are central to modern physics.