Detroit, and elsewhere, serious breaks in water-mains have been repaired after a freezing process has solidified the stream. Polarized Light and X-Rays. Light, as well as heat and cold, is to-day bidden to perform new duties. It was long ago observed that polarized light as it takes its way through transparent crystal or glass clearly reveals in areas of variegation, any strains to which the crystal or glass may be subjected. Of late this fact has been applied with new skill to investigating strains in engineering structures. A model in glass, carefully annealed, is placed in the path of a beam of polarized light. By shifting the points of application and of support, by loading the structure more or less, and here or there, the distribution of stresses and strains is directly shown to the eye. In this way curved shapes of various kinds have been investigated, as well as bodies in which Hooke’s law of the strict proportionality of strain to stress does not apply. Photographs taken by this method show the distribution of stresses in rings subjected to external compression, crank shafts, and car-coupler hooks. It would be interesting thus to compare standard types of girders, trusses, and bridges, as well as arches of various forms, both regular and skew. [Illustration: Polarized light showing strains in glass.] Polarized light, which when first discovered seemed nothing more than a singular and quite sterile phenomenon, has other uses of great importance. It tells the chemist how much sugar a given solution contains; it displays the inner architecture of rocks when these are sawn into thin sections. Even more valuable than polarized light are the X-rays discovered by Professor Röntgen. One of their latest uses is to reveal impurities and air bubbles in electric cables, affording a procedure much simpler and easier than to employ electrical instruments. In the production of X-rays and similar rays a tube as nearly vacuous as possible is employed. As an aid in removing air Professor James Dewar, of Cambridge University, has recently adopted cocoanut charcoal with remarkable success. He subjects it to the intense cold of liquid air, then establishing communication between a receptacle filled with this charcoal and a bulb exhausted to one fourth of the ordinary atmospheric pressure, he has air so tenuous that an electric spark passes through it with difficulty. So much for developing the long known affinity of charcoal for gases, a property which increases in degree as temperatures fall. CHAPTER XXII AUTOMATICITY AND INITIATION Self-acting devices abridge labor . . . Trigger effects in the laboratory, the studio, and the workshop . . . Automatic telephones . . . Equilibrium of the atmosphere may be easily upset.