_Psychophysics._ The study of the relation between the mental and the physical, between psychology and physics. _Psychosis._ Mental derangement, especially when there is no apparent brain or nerve injury. Consciousness, a conscious process. _Pteridophyte._ One of a phylum (Pteridophyta) of flowerless plants, including the ferns. _Pure._ Unmixed, abstract. Pure science deals with general principles, not with particular applications. _Pyro-._ In compounds, fire-, fever-. _Quantel._ An elementary entity of matter consisting of positive and negative parts, moving in all directions with the velocity of light, capable of passing through solids. (According to the theory of Langmuir.) _Quantity._ Anything which can be represented by means of a number. In chemistry, the number of gram molecules. _Quantum._ The unit of energy transmitted by radioactive bodies. _Quaternary._ Belonging to the most recent geological period. _Radical._ Pertaining to a root, growing out of or from a point close to the root. An atomic group which remains unchanged during the ordinary reactions of the compound of which it is a part. _Radicle._ That part of the embryo of a plant which develops into the main root. A rootlike subdivision of a nerve or vein. Sometimes a mere spelling variation of _radical_. _Radioactive._ Emitting invisible rays that penetrate opaque matter and produce electrical effects. Radium and some other substances are especially radioactive, but radioactivity is supposed to be characteristic of all substances. _Radium._ A metallic element. Symbol: Ra. _Rarefaction._ Making less dense. _Ray._ A line at right angles to the wave front in which various forms of energy may be transmitted. _React._ Tend in the reverse direction, enter into chemical combination. Act because of a stimulus. _Reaction._ An opposing force. A chemical change. The usual nervous response. _Real number._ A number representing a quantity and one of two opposite directions or senses. _Réaumur._ A thermometric scale with the boiling point of water at 80 degrees and the freezing point at zero. _Recapitulation Theory._ The theory which was at one time generally favored by evolutionists, still accepted by many biologists, that ontogeny recapitulates phylogeny; i. e., that the development of the individual is an abridgement of the natural history of the species to which it belongs. _Recent._ In geology, pertaining to the epoch still in existence. _Rectify._ Purify, as by repeated distillation. (Important in the chemistry of bootlegging.) _Reduce._ Remove oxygen, which combines with another substance. _Refract._ Deflect (light, for example) at an angle. Refraction is due to varying velocity in two mediums.