_Ascendant._ Rising toward the zenith. _Asexual._ Without sex. _Assay._ A sort of analysis, especially of metals. _Association._ Mental connection. The word has also three or four meanings in chemistry, which cannot be explained briefly. _Asteroid._ A minor planet. _Astronomy._ The science of the heavenly bodies. _Astrophysics._ The science which deals with the constitution of the heavenly bodies. It is a branch of astronomy. _Atmosphere._ The gaseous envelope about a heavenly body. Especially the air which surrounds the earth. As a measurement, a pressure of fifteen pounds per square inch. _Atom._ The smallest particle of an element that enters into combination. According to old definitions, it is indivisible. _Aurora._ A luminous atmospheric phenomenon. The aurora borealis radiates from the north magnetic pole, the aurora australis from the south magnetic pole of the earth. _Avogadro’s Hypothesis._ The assumption that equal volumes of gases contain equal numbers of molecules if conditions of temperature and pressure are the same. It appears to be close to the truth. _Axiom._ A truth which is considered self-evident. _Bacillus._ One of a sort of rod-shaped bacteria. Loosely, any bacterium. Plural: bacilli. The Oxford English Dictionary distinguishes bacillus from bacterium, “with which it agrees in its rodlike form, and characterized by its larger size and mode of reproduction.” _Bacterium._ More familiar in plural: bacteria. A group of vegetable microörganisms. Not all cause disease. _Barbarian._ Of a civilization intermediate between one which is called savage and the most complex forms. _Barium._ A silver-white element, classed as a metal. Symbol: Ba. _Barometer._ An instrument used to measure atmospheric pressure, and thus to predict storms or determine altitudes. _Base._ A substance which can neutralize acids to form a salt. The term includes alkalis. Part of an organ (of a plant or animal) attached to another organ which is more central. _Batrachia._ An order of amphibians which have no tails in the adult stage. _Battery._ A system of cells (sometimes used for a single cell) which changes chemical energy into electricity. _Becquerel rays._ Rays emitted by radium and other substances which are susceptible to deflection by electricity and which electrically charge other bodies. _Behaviorism._ Watson’s psychological system. Opposed to introspective psychology, and it emphasizes the physiology of human behavior. _Bessemer steel._ Steel made from cast iron by a process which burns out carbon, sulphur, and phosphorus. _Beta rays._ Becquerel rays which are apparently identical with cathode rays. _Bichloride._ A compound in which there are two atoms of chlorine in the molecule. _Biennial._ A plant that rises one year and flowers, fructifies, and dies the next. _Binary._ Composed of two elements. _Biochemistry._ The chemical study of living organisms and their products. _Biology._ The science of living organisms.