Nicolas Léonard Sadi Carnot (] ; 1 June 1796 – 24 August 1832) was a French military engineer and physicist, often described as the "father of thermodynamics".
 In his only publication, the 1824 monograph "Reflections on the Motive Power of Fire", Carnot gave the first successful theory of the maximum efficiency of heat engines. Carnot's work attracted little attention during his lifetime, but it was later used by Rudolf Clausius and Lord Kelvin to formalize the second law of thermodynamics and define the concept of entropy.
In the philosophy of thermal and statistical physics, Maxwell's demon is a thought experiment created by the physicist James Clerk Maxwell in which he suggested how the second law of thermodynamics might hypothetically be violated.
 In the thought experiment, a demon controls a small door between two chambers of gas.
 As individual gas molecules approach the door, the demon quickly opens and shuts the door so that fast molecules pass into the other chamber, while slow molecules remain in the first chamber.
 Because faster molecules are hotter, the demon's behavior causes one chamber to warm up as the other cools, thus decreasing entropy and violating the second law of thermodynamics.
In thermodynamics, a thermodynamicist is one who studies thermodynamic processes and phenomena, i.e. the physics that deals with mechanical action and relations of heat.
 Among the well-known number of , include Sadi Carnot, Rudolf Clausius, Willard Gibbs, Hermann von Helmholtz, and Max Planck.
In classical statistical mechanics, the "H"-theorem, introduced by Ludwig Boltzmann in 1872, describes the tendency to decrease in the quantity "H" (defined below) in a nearly-ideal gas of molecules.
 As this quantity "H" was meant to represent the entropy of thermodynamics, the "H"-theorem was an early demonstration of the power of statistical mechanics as it claimed to derive the second law of thermodynamics—a statement about fundamentally irreversible processes—from reversible microscopic mechanics.
 It is thought to prove the second law of thermodynamics.
In the history of science, the theory of heat or mechanical theory of heat was a theory, introduced in 1798 by Sir Benjamin Thompson (better known as 'Count Rumford'), and developed more thoroughly in 1824 by the French physicist Sadi Carnot, that heat and mechanical work are equivalent.
 It is related to the mechanical equivalent of heat.
 Over the next century, with the introduction of the second law of thermodynamics in 1850 by Rudolf Clausius, this theory evolved into the science of thermodynamics.
 In 1851, in his "On the Dynamical Theory of Heat", William Thomson outlined the view, as based on recent experiments by those such as James Joule, that “heat is not a substance, but a dynamical form of mechanical effect, we perceive that there must be an equivalence between mechanical work and heat, as between cause and effect.”
In the history of thermodynamics, disgregation was defined in 1862 by Rudolf Clausius as the magnitude of the degree in which the molecules of a body are separated from each other.
 This term was modeled on certain passages in French physicist Sadi Carnot's 1824 paper "On the Motive Power of Fire" that characterized the "transformations" of "working substances" (particles of a thermodynamic system) of an engine cycle, namely "mode of aggregation", which was a precursor to the concept of entropy, which Clausius coined in 1865.
 It was also a precursor to that of Ludwig Boltzmann's 1870s theories of entropy and order and disorder.
The Clausius–Clapeyron relation, named after Rudolf Clausius and Benoît Paul Émile Clapeyron, is a way of characterizing a discontinuous phase transition between two phases of matter of a single constituent.
 On a pressure–temperature (P–T) diagram, the line separating the two phases is known as the coexistence curve.
 The Clausius–Clapeyron relation gives the slope of the tangents to this curve.
 Mathematically,
In thermodynamics, entropy is commonly associated with the amount of order, disorder, or chaos in a thermodynamic system.
 This stems from Rudolf Clausius' 1862 assertion that any thermodynamic process always "admits to being reduced [reduction] to the alteration in some way or another of the "arrangement" of the constituent parts of the working body" and that internal work associated with these alterations is quantified energetically by a measure of "entropy" change, according to the following differential expression:
William John Macquorn Rankine, {'1': ", '2': ", '3': 'FRSE FRS', '4': "} ( ; 5 July 1820 – 24 December 1872) was a Scottish mechanical engineer who also contributed to civil engineering, physics and mathematics.
 He was a founding contributor, with Rudolf Clausius and William Thomson (Lord Kelvin), to the science of thermodynamics, particularly focusing on the first of the three thermodynamic laws.
Rudolf Julius Emanuel Clausius (2 January 1822 – 24 August 1888) was a German physicist and mathematician and is considered one of the central founders of the science of thermodynamics.
 By his restatement of Sadi Carnot's principle known as the Carnot cycle, he gave the theory of heat a truer and sounder basis.
 His most important paper, "On the Moving Force of Heat", published in 1850, first stated the basic ideas of the second law of thermodynamics.
 In 1865 he introduced the concept of entropy.
 In 1870 he introduced the virial theorem which applied to heat.

Question: When did Rudolf Clausius introduce the second law of thermodynamics?
Answer: 1850