_Poly-._ In compounds, many-. For example, polyadelphous: having the stamens united in three or more bundles. Polyandrous: having at one time more than one husband. Polygynous: having at one time more than one wife. _Postglacial._ Belonging to a later period than that of the glaciers. Recent. _Potassium._ A metallic element. Symbol: K. _Potential energy._ Energy which is latent, not employed or in motion. Energy which a body has because of its position. _Poundal._ The force which can act upon a pound mass for a second to give it a velocity of one foot per second, or which will give a mass of one pound an acceleration of one foot per second each second. _Practice._ What is true in theory works out in practice, all due allowances for the influence of other theoretic laws having been made. _Praseodymium._ A metallic element which occurs in rare minerals. Symbol: Pr. _Pre-Cambrian._ Relating to the geological eras before the Cambrian period. _Precipitate._ To change into liquid form. To cause a substance to be deposited in solid form from a solution. To make a dissolved substance insoluble. That which has been precipitated. _Prehensile._ Used for grasping, suitable for grasping. _Pressure._ The amount of exertion of force, expressed by the weight upon a unit area. _Primate._ A member of the highest order (Primates) of mammals, including man and the monkeys. _Property._ That which belongs to a whole class but is not used to distinguish it from others. Characteristic. A legal right to wealth. _Protean._ Variable, readily changing form. _Proteid._ Protein, or member of a special class of proteins. _Protein._ One of a class of complex organic substances consisting of carbon, hydrogen, nitrogen, oxygen, and, in some cases, other elements. All living cells contain proteins. _Protoplasm._ The living matter of which all animal and plant cells are formed. _Protozoan._ A member of the phylum (Protozoa) of animals of the simplest type. A one-celled animal. _Pseudo-._ In many compounds, false, seeming, similar to. _Psychiatry._ The treatment of mental disease. _Psychical research._ The study of phenomena which are not considered by scientists as realities, and which are explained by some persons as due not to natural forces but rather to spirit as opposed to matter. If such phenomena are explained by means of the laws of physics and psychology, there is no need for special “psychical research.” There seems to be no reason why we should postulate spiritual or vital force, even if our knowledge is not yet sufficient to explain all phenomena scientifically. _Psychology._ The study of the mind; in practice, the study of behavior. Literally: the science of the soul. Perhaps psychology is not yet a true science, but this branch of knowledge is undoubtedly becoming more scientific, in spite of strong philosophical and mystical tendencies.