One of recent topics in informatics is "Information Retrieval for historical documents." The topics for history of science, also for scientists, searching for old scientific documents is a key demand in the world of digital documents. This study reveals how different among Clausius's four papers classified under the same title of the theory of electricity, using statistical methods. We, Dr. Eri YAGI (Prof.Emeritus of Toyo University in Japan) and I have studied on R. Clausius (1822-1888) through both quantitative and qualitative approaches over twenty years. In the way of these approaches, we can obtain better understandings how different Clausius's theory of electricity and his "mechanical theory of heat" are. I will mainly discuss on the difference between Clausius's XIIth paper (1853) and his other three papers by some statistical analyses on self-made digital documents for Clausius's theory of electricity, which were originally written in nineteenth century.