GAS 



2401 



GAS 



has since been found that for each gas there 

 is a similar critical temperature, above which 

 it' cannot be liquefied, however great the pres- 

 sure applied. This accounts for the failure of 

 Faraday and others to liquefy oxygen, nitro- 

 gen, hydrogen and carbon monoxide. All gases 

 have since been liquefied (see LIQUID AIR), and 

 this suggests that gas is more like a liquid 

 than a solid, which is true. Both are known 

 to the scientist as fluids; that is, both flow and 

 are essentially formless, since they will assume 

 any form. 



Unlike a liquid, however, gas expands read- 

 ily to occupy fully any space, however large. 

 To get an idea of the habit of a gas, one may 

 think of its ultimate particles, or molecules, 

 as exceedingly small rubber balls in constant 

 and violent motion. These molecules, so infi- 

 nitely small that a volume of gas as large as 

 the head of a pin would contain thirty million 

 times as many molecules as there are people 

 on the globe, fly hither and thither in space, 

 jostling one another and striking and rebound- 

 ing from the sides of any vessel or room that 

 contains them. They deliver a bombardment 

 of tiny blows, which results in the phenom- 

 enon we call gaseous pressure. This is the 

 conception of the nature of gases which is com- 

 monly accepted among scientists. It is called 

 "kinetic theory" of gases. 



Boyle's Law. A seventeenth-century nat- 

 ural philosopher, Robert Boyle, noticed that 

 when gas was compressed into a smaller space 

 the pressure was increased. He announced 

 that if the temperature of a gas remains con- 

 stant, the smaller the space the greater the 

 pressure. The pressure, moreover, is in di- 

 rect proportion to the density; that is, when 

 the external pressure is increased four times, 

 the gas is forced to occupy one-fourth of its 

 original space. It is easy to see why this is 

 so, since the same number of molecules, con- 

 fined in a narrower space, would deliver their 

 blows against the sides more frequently. 



Heat has a surprising effect on gas. It im- 

 parts energy to the molecules and causes 

 them to deliver their blows with greater rapid- 

 ity. This naturally results in a greater pres- 

 sure in a vessel of the same size; if the walls 

 are movable, the gas will push them out. Thus 

 we say gas expands with heat, just as do nearly 

 all other substances. 



Charles's Law. A Frenchman named Charles 

 reduced this truth to law by announcing that 

 as long as the pressure remains the same the 

 space filled by the gas will be increased by a 

 constant fraction of its original volume for 

 each degree of rise in temperature. 



All gases obey these two laws with a fair 

 degree of accuracy. 



Illuminating Gas 



Since coal in burning gives off flames, it 

 hardly seems surprising that it should be made 

 to yield a luminous gas. It was not, however, 

 until the close of the eighteenth century that 

 a Scotchman, William Murdock, showed in the 

 most conclusive way that an inflammable gas 

 could be extracted from coal. He succeeded 

 in lighting his home and his office in Redruth, 

 Cornwall, England, with such gas. After that 

 it was not long until coal gas began to be 

 manufactured in large quantities for lighting 

 streets and houses. At the present time it is 

 manufactured chiefly from coal but sometimes 

 from oils derived from petroleum. 



Coal Gas. Coal gas is formed by the distil- 

 lation of coal. This is a rather complicated 

 process, and it is carried out in huge gas 

 plants. It may not be easy to understand 

 at first how a solid can be distilled, and indeed 

 the process is not exactly distillation in the 

 same sense as we speak of the distillation of 

 water or whisky. What is really done is to 

 decompose the coal into gas and coke, a form 

 151 



of carbon. It is easy to imitate the manufac- 

 ture of coal gas. Take a clay pipe and fill the 

 bowl with soft coal; stop the mouth of the 

 bowl with clay and allow it to dry well. Blow 

 gently into the pipe to make sure that it is 

 tightly closed by the clay. Then thrust the 

 bowl into a fire. In a short time gas will 

 issue from the stem, and if a match is touched 

 to this gas, it will burn. Such crudely-manu- 

 factured gas is full of impurities, but still it 

 is gas. When freed from impurities, it be- 

 comes the gas we burn in jet or fireplace. 



The first process in making gas is to heat 

 coal in fire-clay retorts. The retorts are oval 

 or D-shaped in cross section. They are about 

 nine feet long and 16x26 inches across the end, 

 and they will hold from 250 to 350 pounds of 

 coal. After charging they are tightly closed 

 and heated in a furnace. In these retorts the 

 carbon compounds of the coal are decomposed. 

 The coke left in the retorts is used as a fuel or 

 for the manufacture of carbons for electric arc 

 lights. The gas passes off through a pipe, the 



