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CORNING GLASS WORKS     Most of this superiority is inherent in the glass itself. It is uniform all the way through. Chip or break a PYREX insulator and the surface you reveal is exactly the same as the original outer surface of the insulator. All PYREX insulators are made in one piece, without cement or glaze.     The mechanical ruggedness of PYREX insulators is again inherent in the glass of which they are made. All PYREX insulators will withstand cantilever pull sufficient to bend standard pins.     An important safeguard provided by PYREX insulators is their high puncture voltage in air. Every PYREX insulator rated 13 kv. or more is subjected to a continuous five minute test at approximately 25% more than the normal dry flashover potential. Artificial lightning surge tests have demonstrated their ability to withstand steep wave front voltages of 1,500,000 volts--the limit of the equipment on which the tests were made. Power arcs of 2,000 amperes or more maintained for several seconds result only in crazing the surface of PYREX insulators to a depth of about 1/16 inch. Their insulating value remains practically unimpaired and the line will go back into service immediately.     The transparency of PYREX insulators-- greater than the finest plate glass--has permitted more rapid progress in their development than could possibly have been made with an opaque substance. At every step in their manufacture they are visually inspected clear to the pinhole. It would be almost impossible for any defect to go unnoticed--a fact that is well demonstrated by the small number of failures on electrical tests; less than one fortieth of one per cent fail to pass.     On your lines the transparency of PYREX insulators results in the transmission of sunlight to such a degree that even the hottest sun does not raise the temperature of the insulator appreciably above that of the surrounding atmosphere. Destructive stresses from sun heat and consequent line failure are eliminated when you equip your lines with PYREX insulators.     In 44 states, Alaska, Canada, Brazil, Mexico, Venezuela, South Africa, India, Australia and other foreign countries, PYREX insulators are improving operating efficiency and lowering maintenance costs.