Helium was not discovered on Earth itself until 1895 when the Swedish chemists Per Teodor Cleve and Nils Abraham Langlet found a novel gas being produced from the ore cleveite. The discovery of helium is now credited to Janssen and Lockyer 1. Consequently the name helium was derived from “Helios”, the Greek sun god, by Lockyer and his colleague, the English chemist Edward Frankland. He realized that it could not be sodium as the line was located between the two known sodium lines and therefore represented a new element. Later that year, in October, Norman Lockyer also saw this line. Helium is important as a cryogenic coolant for MRI systemsĭuring the solar eclipse of the summer of 1868, the French astronomer Jules Janssen identified a yellow spectral line which he assumed was sodium. Hyperpolarized helium-3 pulmonary functional MRI 4 Loss of consciousness rapidly ensues when oxygen concentrations become even lower (~6-10%), with convulsions and agonal breathing as hypoxic-ischemic brain injury occurs 3. normal atmospheric concentration of 21%): The initial symptoms and signs of hypoxia are seen when oxygen levels decrease to 12-16% (cf. Due to its low density, helium rapidly replaces oxygen in the lungs when it is inhaled in a closed atmosphere and because there is no accumulation of carbon dioxide, no breathing reflex supervenes. Like any gas, helium can produce death by asphyxiation, if it prevents the inhalation of sufficient oxygen. The other isotopes are all radioactive, helium-5 to helium-10 have been generated artificially but they all have very short half-lives 2. Most of the remaining terrestrial hydrogen is helium-3, which is the only other stable isotope 1,2. The nucleus of helium-4 is identical to an alpha particle. More than 99.999% of helium on earth is of the helium-4 form. It is the least reactive of all the chemical elements 1. It has a melting point of -272☌ and a boiling point of -269☌, which are the lowest of any element. It has an atomic number 2 with a relative atomic weight of 4.0026. A consistent set of sentences that contains a contradictionġ0.Helium is an odorless colourless non-flammable gas. Two logically equivalent sentences that together are an inconsistent setĩ. Two logically equivalent sentences, one of which is a tautology and one of which is contingentĨ. Two logically equivalent sentences, both of which are tautologiesħ. An invalid argument, the conclusion of which is a tautologyĦ. A valid argument, the conclusion of which is a contradictionĤ. A valid argument that has a false conclusionģ. A valid argument that has one false premise and one true premiseĢ. * Part D Which of the following is possible? If it is possible, give an example. Which are consistent? Which are inconsistent? 11, and consider each of the following sets of sentences. * Part C Look back at the sentences G1–G4 on p. If anyone has ever crossed the Rubicon, it was Caesar. Even though Caesar crossed the Rubicon, no one has ever crossed the Rubicon.Ħ. If Caesar crossed the Rubicon, then someone has.ĥ. Part B For each of the following: Is it a tautology, a contradiction, or a contingent sentence?Ĥ. Part A Which of the following are ‘sentences’ in the logical sense? The answers to some of the problems are provided at the end of the book in appendix B the problems that are solved in the appendix are marked with a *. There is no substitute for actually working through some problems, because logic is more about a way of thinking than it is about memorizing facts. \)Īt the end of each chapter, you will find a series of practice problems that review and explore the material covered in the chapter.
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