4. Write a short composition ‘Concrete’ (70 words). Text 5. Special Purpose Glass

Match the following words and translations.

Most people over the age of 50 would have a hard time reading this page with the naked eye. But they can read even smaller print easily with the help of lenses. Spectacles were first made in the late 13th century; and with the invention of printing their use became widespread. We now have many uses for lenses that are made of a hard, clear glass, called optical glass; we find them in microscopes, for instance, and in telescopes and cameras.

The development of optical glass is closely connected with advances in chemistry; and the story of optical technology actually began only when scientists found that they could alter the amount of refraction by adding small amounts of other compounds to plain glass.

Chemists, working hand in hand with glass makers, have also found ways to make glass that will absorb certain light rays while letting others pass through. For instance, we now have glass that allows the passage of very short invisible rays such as X-rays; and we also have glass that does the very opposite: it absorbs X-rays and the shorter gamma rays from nuclear reactions. Still another type of glass transmits invisible infrared rays, which can penetrate haze; used as a camera lens, it greatly extends the range of long-distance photography.

Another important result of applying chemistry to glass manufacture was the American invention, in 1916, of Pyrex, a strong glass containing a high percentage of boric oxide. Because it can withstand sharp temperature changes without cracking, Pyrex is used in baking dishes and frying pans that can be put directly over a naked flame. But it has an important industrial use as well: Large-diameter Pyrex pipes are used for conveying chemicals through series of continuous processes. Pyrex is resistant to attack by corrosive chemicals; because it is transparent, the processes can be watched through all the stages of certain kinds of manufacture.

There are many other types of modern glass, of course. Two that are worth mentioning here because they are vital to present-day motoring and flying are both safety glasses. The first is Triplex, so called because the French inventor – E. Benedictus – produced in 1909 a three-part ‘sandwich’ consisting of two sheets of glass glued to an in-between layer of celluloid. Used as a windshield in place of plate glass, Triplex has saved many lives because, in the event of an accident, the glass does not break up into sharp-edged fragments but sticks to the celluloid. On the Triplex principle, modern aircraft carry windshields up to 1.5 inches thick, with as many as nine layers of alternate glass and plastic sheet.

The other safety glass is called toughened glass. It is made by heating glass sheet up to the softening point; then both sides of the sheet are suddenly cooled by a blast of compressed air. The outside solidifies at once; and the sudden colding and consequent contraction pulls the inner film into a state of high compression. This has the effect of making the whole sheet immensely strong. Furthermore, because of the intense internal stresses, such glass is so very brittle that, if it does get broken, it breaks into tiny fragments – too tiny to cause injury. That is why it has come into common use for car windshields.