|Reading for Philosophical Inquiry: A Brief Introduction to Philosophical Thinking ver. 0.21; An Open Source Reader
|Chapter 27. "Science of Natural Processes" by Frederick Engels
Marxists Internet Archive . Marxist Writers and History. Comprehensive reference and sources for the philosophy of Marxism—useful for many online sources not available elsewhere.
Cosmology Today. A series of accessible articles by scientists on the present and future state of science including present concerns of "a theory of everything"
Mechanical Equivalent of Heat, from Denison Olmsted, An Introduction to Natural Philosophy, 1844, 341.