The next step, and the one which a great many scientists had declared impossible, was the manufacture of a tungsten wire through a regular process of drawing it out through dies to the desired length, and in the desired thickness. The investigators had declared that in spite of all they could do, tungsten was too brittle ever to be drawn into wire. In the latest methods this is accomplished with such perfection that tungsten wire of 0.0015 of an inch in diameter is produced. "With the invention of a method for drawing out tungsten wire," continued the scientist, "an almost ideal lamp was practically accomplished. The wire simply was strung on the spiders or cross pieces, and a filament of almost any length giving almost any desired candlepower light could be used. "You see in an incandescent light the higher the melting point of the filament the greater the quantity of light for the amount of electricity used. Also tungsten has a low vapour tension, which prevents discolouration of the globe by the evaporation of the filament. It also has other advantages which are too technical for us to go into. "Of course, tungsten lamps still have the drawback of being rather delicate. When not in use, and when the filament is cold, it is apt to break with rough treatment, but when lighted the filament, being at a white heat, is more durable. This delicacy of the tungsten lamp is the reason the fixtures for most of them are placed in stationary positions, rather than on swinging drop cords, as is the case with so many carbon incandescent lights. "While the tungsten lamp is far from perfect, it is a great advance over other forms, and an advance in the right direction, for it gives a better light with a smaller consumption of electricity than other types. I think your father will agree with me that anything that will help ever so little to reduce the high cost of living is a benefit." "But," answered the boy, "there are other new kinds of electric lights besides tungsten, aren't there?" "Oh, of course, but they are hardly as generally used as the tungsten light. There is the mercury light about which you read in 'The Second Boys' Book of Inventions,' several new kinds of arc lights, the Nernst light, the tantalum lamp (which we know is much like the tungsten lamp with the exception that in the latter each loop of the wire can be made longer), and the new carbon dioxide gas electric light, which is a very good imitation of daylight. "From all our little scientific journeys you have doubtless formed the idea that light is not the simple thing it seems, and that the rays of different kinds of light will bear a limitless amount of study. Now some of the greatest scientists the world ever has known have spent the best part of their lives trying to produce a light that would duplicate the beautiful health-giving rays of the sun. This light we are speaking of comes as near to it as any."