_Allotropy._ Variation of physical properties without change of substance. Carbon, for example, may exist as lampblack, graphite, or the diamond. _Alloy._ A mixture (not a compound) of metals. _Alluvium._ The deposit of earth and other materials by the mechanical action of running water. More familiar is the adjective: alluvial. _Alpha rays._ Becquerel rays which have less power of penetration than beta rays but greater power of inducing conductivity in gases. _Alpine._ Relating to a mountain district so high that it has arctic climate. _Alum._ A double sulphate of aluminium and potassium. Other sulphates of similar crystalline form are also known as alums. _Aluminium._ A metallic element. The spelling given is the usual one in Great Britain, and it is also the one employed by American chemists. In ordinary use in the United States: aluminum. The symbol: Al. _Amalgam._ An alloy containing mercury. To amalgamate is to form an amalgam. _Ammeter._ An instrument for measuring the strength of electric currents in amperes. _Amorphous._ Uncrystallized. _Ampere._ Sometimes spelled: ampère. The unit of electric current, the current that one volt can send through one ohm. _Amphibia._ A division of Vertebrata, intermediate between reptiles and fishes, including the frog. The study of Amphibia is known as amphibiology. _Amplexicaul._ Encircling or clasping a stem. _Anabolism._ Constructive metabolism. _Analogy._ Functional correspondence between parts of different origin and structure. _Analysis._ Resolution into simple elements, as by chemical processes. In mathematics, algebraic investigation. _Anandrous._ Without stamens. _Anaphylaxis._ Excessive susceptibility to an albuminous substance. _Anatomy._ The science of bodily structure. _Anemograph._ An instrument for recording the direction and force of wind. _Aneroid barometer._ A barometer in which there is no liquid, the pressure of the air being measured by its action on the elastic lid of a box in which a partial vacuum has been created. _Angiosperm._ A plant of a class which has its seeds in a closed ovary. _Anhydrous._ Without water of crystallization or water in general. _Animal._ A living being distinguished from plants in that it is capable of voluntary motion and sensation. Some recent investigations appear to lead to the conclusion that other differentia will have to be adopted. The human being, incidentally, is an animal, although popular language sometimes distinguishes man from the animals. _Animalcule._ A minute animal, usually one which cannot be seen with the naked eye. _Anion._ A negative ion. _Annual._ A plant that lives only for a year. _Annulate_, _annulated_. Ringed. _Anode._ A positive electrode. _Antenna._ Feeler, sensory organ found in pairs on heads of insects and crustacea. Irritable processes found in some male flowers. An elevated conductor for receiving or transmitting electric waves. Plural: antennæ.