LIST OF ILLUSTRATIONS Xlll 



PAGE 



This very unsociability of helium, however, renders it the ideal 

 gas for balloons and avoids all the risk of fire or explosion attendant 

 upon the use of hydrogen. Helium exists in small quantity in 

 many natural gases (see p. 345), and is obtained pure by liquefying 

 out the other components. 



CHEMICAL SMOKE-SCREENS AGAINST SUBMARINES. A VESSEL USING 



SILICON TETRACHLORIDE 360 



The silicon tetrachloride is contained in a cylinder fixed on the deck 

 at the stern of the ship, and is forced out into a long funnel, in the 

 form of a fine spray, by the pressure of liquid carbon dioxide. Am- 

 monia gas issues from a second cylinder, and rapid mixture is effected 

 in the presence of excess of moist air by means of an electric fan 

 placed at one end of the funnel. The current of air produced 

 drives the smoke out at the other end of the funnel. 



CHEMICAL SMOKE-SCREENS AGAINST SUBMARINES. A VESSEL USING 



OLEUM 361 



Oleum (fuming sulphuric acid) was also employed to conceal ships 

 against submarines during the Great War. A finely-divided spray 

 of oleum was injected into the smoke-stacks of the vessel and 

 carried by the hot gases issuing therefrom into the atmosphere in 

 the form of vapor. In contact with the cool moist air over the 

 ocean, this vapor condensed to give tiny droplets of sulphuric 

 acid, which hung over the surface as a very persistent mist (compare 

 pp. 263-4), extremely difficult to distinguish from a natural fog. 



KOPPERS BENZOL RECOVERY PLANT, RIVER FURNACE Co., CLEVE- 

 LAND, OHIO 424 



The recovery of valuable hydrocarbons of the aromatic series 

 (benzene, toluene, naphthalene, etc.) from coal gas is a feature of 

 every modern coke and gas plant. The bulk of the peace-time 

 demand for "benzols" as the light oils extracted by washing 

 the gas are called in industry is for employment as motor fuels. 

 From one ton of coal approximately 3 gallons of " benzol " is 

 recovered, giving from 20 to 30 per cent more power and mileage 

 than high-grade gasoline, and causing less carbon trouble. 



OIL SHALE CLIFF, UTAH 425 



Enormous deposits of oil shale (see p. 345) have recently been 

 opened up in the United States, notably hi Colorado, Utah, and 

 Nevada. Though at first sight it would appear that the cost of 

 producing oil from shale would prohibit competition with well 

 petroleum, yet the value of the by-products offsets this disadvantage 



