This book has been written strictly according to the guidelines of the revised syllabus (CBCS Pattern) of B.Sc. I Physics (Heat and Thermodymics) prescribed by Board of Studies in Physics, Punyashlok Ahilyadevi Holkar Solapur University, Solapur. It is our humble belief that the text books are among the invaluable resources for successful teaching - learning process, provided they are written within the framework of the aims and objectives laid down and hence this book is really the honest efforts in that direction.