|Opening page of William Thomson's 1849 “An Account of Carnot’s Theory of the Motive Power of Heat”, wherein he spearheads the introduction of thermodynamics in outline or aims.|
Of great effect, Thomson’s “An Account of Carnot’s Theory” was referenced heavily in the introduction to Clausius’ founding 1850 paper "On the Moving Force of Heat and the Laws of Heat which may be Deduced Therefrom", the paper which acted to lay out the initial foundations of the mechanical theory of heat.  In particular, Clausius quotes a section of a footnote from Thomson’s paper (paragraph 12), a digression on the term “thermal agency” in regards the discovery by English physicist James Joule of the generation of heat by internal friction in a fluid body, as this might related to the criterion of a perfect thermo-dynamic engine. The specific quote of Thomson that Clausius repeats is: 
“If we abandon this principle, we meet with innumerable other difficulties—insuperable without further experimental investigation—and an entire reconstruction of the theory of heat from its foundation.”
Clausius concludes, on this excerpt, “I believe that we should not be daunted by these difficulties”; and proceeds over the next fifteen years to lay out the mechanical theory of heat in a set of nine memoirs, culminating with the binding of these memoirs into the 1865 textbook The Mechanical Theory of Heat, the bedrock of thermodynamics.
1. Thomson, William. (1849). “An Account of Carnot’s Theory of the Motive Power of Heat – with Numerical Results Deduced from Regnault’s Experiments on Steam”, Transactions of the Edinburgh Royal Society, xvi.; In: Annales de Chime, xxxv. (1852); In: Mathematical and Physical Papers, Vol. 1 (pgs. 113-155) (1882); In: Reflections on the Motive Power of Heat and on Machines Fitted to Develop that Power (pgs. 127-204), Robert Thurston, Engl. Trans. Ed. (1890).
2. Clausius, Rudolf. (1850). "On the Motive Power of Heat, and on the Laws which may be deduced from it for the Theory of Heat", Communicated in the Academy of Berlin, Feb.; Published in Poggendorff's Annalen der Physick, March-April. LXXIX, 368, 500.
3. ibid, Thomson (1849). Math. and Phys. Papers, Vol. i, pg. 119, note.