244 CHEMICAL DISCOVERY AND INVENTION 



The difference of density, however, is so great that gas of this 

 quality does not satisfy the modern requirements. Not that it 

 is by any means necessary to secure chemically pure hydrogen, 

 as the proportions of the ordinary impurities, such as sulphuretted 

 hydrogen, arsenetted hydrogen, and the vapours of volatile 

 hydrocarbons, or a little carbon monoxide are so small as to 

 exercise but little influence on the density, and hence the lifting 

 power of the gas. The chief objection to the presence of such 

 impurities as sulphuretted hydrogen is that they slowly oxidise 

 into acids which are apt to destroy the fabric of the balloon. 



The largest Zeppelins are said to be 600 feet long by 65 feet in 

 diameter. Suppose we assume an envelope 500 feet long by 

 60 feet in diameter has to be filled with hydrogen gas. The 

 volume of gas required is approximately 40 million litres, or 

 nearly 1J million cubic feet, which, at freezing-point and under 

 the pressure of 1 atmosphere (or 15 pounds per square inch of 

 surface), would weigh 3584 kilograms or about 3| tons. Air is 

 nearly 14 J times heavier than hydrogen, consequently the same 

 bulk of air, under the same conditions, weighs just over 50 tons. 

 The lifting power of such a volume of hydrogen gas would there- 

 fore be about 46-47 tons. When the frame of the balloon, the 

 car, the engines, the fuel, and the guns, if any, are allowed for, 

 it is obvious that the machine is still capable of carrying a 

 considerable number of men and bombs or other cargo. But it 

 must be remembered that when the airship leaves the ground it 

 rapidly rises into regions in which the pressure is very much less 

 than 1 atmosphere, and the gas expands, and if closely confined 

 would probably burst the envelope. This effect is at high 

 elevations counteracted to some extent by the reduction of 

 temperature. In order to provide for expansion it is customary 

 not to inflate the gas-bag fully, but to allow for an increase of 

 -gV of the original volume for every 1000 feet of height to be 

 attained. The material of the envelope consists of a cotton 

 cloth, rubbered on one side and varnished on both sides with a 

 kind of collodion. This is not wholly impervious to hydrogen, 

 and a better material is cotton rubbered and covered with gold- 

 beater's skin, the whole being coated with a boiled oil varnish. 

 The permeability of this by hydrogen under an excess of pressure 

 equal to 15 mm. of water above atmospheric pressure is said to 

 be only | litre per square metre in twenty-four hours. 



The non-rigid fish-shaped balloon has inside it two air bags the 



