Questões Militares Comentadas para cirurgião-dentista - odontopediatria

                           How things work: 100 scientific explanations

      Preservation property has always been important to humans. Historians in the ninth century were the first to record the use of varnish, a protective liquid composed of resins, natural oils, and alcohol, among other ingredients. Furniture makers learned that if they painted coats of the stuff on a piece of furniture or wood floor, it made it impervious to liquids and their damage and shielded it from normal wear and tear. And the makers liked the shine furniture had after it was varnished.

      Varnish comes in many formulations. Some are oil based, using linseed or tung oil, while others are water-based. Varnish cures, or dries, on what it’s painted, creating a glossy, clear film on the surface. Some oil-based varnishes can turn to a yellowish color; water-based varieties do not. Wood must be carefully cleaned and sanded before any varnish is applied. Several coats are often required. All varnishes contain resins — terpenes with five-carbon molecules called isoprenes and also include a drying oil or solvent to reduce drying time.

     Although furniture and flooors are still the most frequently varnished items, the wooden hulls of boats are also often varnished. Clear nail polish, a type of varnish, can be used for countless small fixes: dab small amounts to keep splintered wood from snagging, prevent ink from running, and stop buttons on clothing or screws on sunglasses from coming loose.

           (National Geographic Special Publication “How Things Work” Adapted.)

      American jazz is a trove of sounds borrowed from such varied sources as American and African folk music, European classical music, and Christian gospel songs. One of the recognizable traits of jazz is its use of improvisation: certain parts of the music are written out and played the same way by various performers, and other improvised parts are created spontaneously during a performance and vary widely from performer to performer.

      The root form of jazz was ragtime, lively songs or rags performed on the piano, and the best-known of the ragtime performers and composers was Scott Joplin. Born in the 1868 to former slaves, Scott Joplin earned his living from a very early age playing the piano in bars around the Mississippi. One of these regular jobs was in the Maple Leaf Club in Sedalia, Missouri. It was there that he began writing the more than 500 compositions that he was to produce, the most famous of which was “The Maple Leaf Rag”.

        (Longman Preparation Course for the TOEFL, Deborah Philips Adapted.)