252 Master Minds of Modern Science 



their life was increased from one hundred to as much as 

 one thousand hours, but still they were not satisfactory, 

 for a carbon filament gradually breaks up, and after a 

 time these lamps lost their illuminating power. 



The first improvement was the so-called ' squirted ' 

 filament. We have mentioned how Topham made a fila- 

 ment by the use of liquid viscose. It was found that by 

 dissolving cotton-wool in zinc chloride the material called 

 cellulose could be made. This was forced through a die 

 and ' squirted ' into a very fine thread ; after washing 

 and drying the thread was carbonized in a closed box in 

 a furnace. Loops of this material were treated by 

 depositing fresh carbon upon them, and yielded a more 

 uniform filament than could be made of bamboo or woven 

 thread. The melting-point of these carbon threads was 

 about seventeen hundred degrees Fahrenheit. 



Many inventors were busy making substitutes for car- 

 bon, but the difficulty was to find a material which could 

 be raised to a higher temperature than seventeen hundred 

 degrees without melting. Platinum was tried, but found 

 to be useless. It was known that certain of the rarer 

 metals, such as tungsten, tantalum, and molybdenum, 

 had melting-points higher than platinum, but in those 

 days there was no supply of these metals, which were 

 merely curiosities of the laboratory. 



In 1897 Nernst brought out his new lamp; this con- 

 tained a rod of oxide of magnesium mixed with oxides of 

 other rare metals, heated by a white-hot platinum spiral. 

 It was a good lamp, it gave a fine light, and lasted better 

 than the carbon, but the worst of it was that it took about 

 fifteen seconds, after the turning on of the current, to 

 give its full light. However, it served well until replaced 

 by the metallic filament lamp. 



The first metalled lamp was the tantalum made by 

 Siemens Brothers, but this, in its turn, gave way to the 

 tungsten lamp, which is still in use. Tungsten is a metal 



