the system itself along any virtual displacement consistent with the constraints of the system, is zero. Named after Jean le Rond d'Alembert. Dahl's law, a sound rule of Northeast Bantu languages, a case of voicing dissimilation. Dale's principle, in neuroscience, states that a neuron is capable of producing and secreting only one neurotransmitter from its axon terminals. Named after Henry Hallett Dale but more recent data suggests it to be false. A more common interpretation of the original statement made by Dale is that neurons release the same set of transmitters at all of their synapses. Dalton's law, in chemistry and physics, states that the total pressure exerted by a gaseous mixture is equal to the sum of the partial pressures of each individual component in a gas mixture. Also called Dalton's law of partial pressure, and related to the ideal gas laws, this empirical law was observed by John Dalton in 1801. Darcy's law, in hydrogeology, describes the flow of a fluid (such as water) through a porous medium (such as an aquifer). Davis's law, in anatomy, describes how soft tissue models along imposed demands. Corollary to Wolff's law. De Morgan's laws apply to formal logic regarding the negation of pairs of logical operators. Dermott's law: The sidereal period of major satellites tends to follow a geometric series. Named after Stanley Dermott. De Vaucouleurs' law, in astronomy, describes how the surface brightness of an elliptical galaxy varies as a function of apparent distance from the center. Named after Gérard de Vaucouleurs. Dilbert principle: "the most ineffective workers are systematically moved to the place where they can do the least damage: management." Coined by Scott Adams. Doctorow's law: "Anytime someone puts a lock on something you own, against your wishes, and doesn't give you the key, they're not doing it for your benefit." Dolbear's law is an empirical relationship between temperature and the rate of cricket chirping. Dollo's law: "An organism is unable to return, even partially, to a previous stage already realized in the ranks of its ancestors." Simply put this law states that evolution is not reversible; the "law" is regarded as a generalisation as exceptions may exist. Dulong–Petit law states the classical expression for the specific heat capacity of a crystal due to its lattice vibrations. Named for Pierre Louis Dulong and Alexis Thérèse Petit. Dunbar's number is a theoretical cognitive limit to the number of people