_Krypton._ A gaseous element. Symbol: Kr. _Labile._ Unstable. _Lanthanum._ A metallic element. Symbol: La. _Latent heat._ The equivalent of the work performed in changing the state of a substance from solid to liquid or liquid to gaseous. _Law._ A generalization. A statement of the way things invariably behave under given conditions. A scientific law does not direct or impose penalties or state that things ought to behave in a certain way. It is a correct statement of invariable sequence. If the correctness of a statement has not been proved beyond a reasonable doubt, it is not a law. _Lead._ A metallic element. Symbol: Pb. _Leaf._ One of the expanded organs, usually green, springing from the stem, a branch, or sometimes a root, of a plant. _Lens._ A portion of glass or other transparent substance with one or both sides curved, used to alter the direction of light rays. _Leucocyte._ A colorless corpuscle of the blood. _Lever._ A rigid structure used to modify force and motion while transmitting it. _Life._ The quality which differentiates the organic from the inorganic. The ability to reproduce itself seems to be the differentiating characteristic of a living organism. _Light._ A form of energy which is apprehended by the eye. It is supposed to move at a velocity of about 186,300 miles per second. _Limb._ In astronomy, an edge or border. This is not the same word etymologically as the one which means an arm, a leg, or a wing. _Lithium._ A metallic element. Symbol: Li. _Litmus._ A substance which is turned red by acids and made blue again by bases. Paper stained with litmus is usually employed for such tests. _Logic._ The science of thought. Scientific method. _Lutecium._ A metallic element. Symbol: Lu. _Magnesium._ A metallic element. Symbol: Mg. _Magnetize._ To change into a magnet, give magnetic quality to, make capable of attracting iron. _Male._ Of the sex or the organs that fecundate. Of a reproductive organ that fertilizes. _Mammal._ One of the class of animals (Mammalia) which are vertebrate and which are distinguished by suckling their young. _Mandible._ Jaw, especially the lower jaw. Part of a beak. _Manganese._ A metallic element. Symbol: Mn. _Marsupial._ One of an order of animals (Marsupialia) which are low mammals, and most of whose females carry the young in abdominal pouches. _Mass._ Quantity of matter. This remains constant, but weight varies a little with geographical location. _Materia medica._ The study of medicinal substances. _Mathematics._ The science of space and quantity as such. _Matter._ That which takes up space. _Mean._ Equally removed from two extremes, average. _Mechanics._ The science of motion. The study of the action of force on bodies. May be considered a branch of mathematics or of physics. _Medicine._ The art of maintaining and restoring health. Surgery is sometimes excluded from the conception of medicine. The basis of medicine is only partially scientific.