The relationship between liquids and gases engaged the attention of a number of distinguished scientists in the mid 19th Century. In a definitive paper published in 1869, Thomas Andrews described experiments he performed on carbon dioxide and from which he concluded that a critical temperature exists below which liquids and gases are distinct phases of matter, but above which they merge into a single fluid phase. During the years which followed, other natural phenomena were discovered to which the same critical point description can be applied - such as ferromagnetism and solutions. This book provides an historical account of theoretical explanations of critical phenomena which ultimately led to a major triumph of statistical mechanics in the 20th Century - with the award of the Nobel Prize for Physics