"Man, as part of a multitude, is a very different being from the same man as an isolated individual. His conscious individuality vanishes in the unconscious personality of the crowd," observed author Gustave Le Bon. In his 1896 classic of social psychology, The Crowd, Le Bon conducted a landmark investigation of mob mentality. His subsequent work, The Psychology of Revolution, takes a closer look at his observations on the subject and his analogy of crowd behavior as an infectious disease -- a sickness that passes from person to person, obliterating individuality and inciting irrationality and mindless destructiveness. Le Bon examines the psychology of revolutions in general, both religious and political, in addition to the mental and emotional qualities of the movements' leaders. Most of his examples are drawn from French history, with a particular emphasis on the personalities and events of the French Revolution: its origins; its rational, affective, mystic, and collective influences; and the conflict between ancestral influences and revolutionary principles. Le Bon concludes with an enlightening survey of revolutionary principles in the early nineteenth century, tracing the progress and evolution of democratic beliefs.