The Romance of Modern Chemistry

INVISIBLE SUBSTANCES transferred to one holding twenty gallons, the gas will occupy every corner of the latter. There is, of course, less of it at any particular point, and its total weight remains the same, but its distribution is carried to the utmost limits of the containing vessel, however large that may be. A moment’s thought will show how different this is from the case of a liquid such as water, len gallons of water remain ten gallons whether the containing cistern holds ten, twenty, or a hundred gallons. Gases, then, are distinguished from liquids by their re- markable expansibility and compressibility. The space or volume which a given quantity of gas occupies depends altogether on the pressure to which it is subjected, and the very simple law has been discovered that the volume of a gas diminishes in the same proportion as the pressure increases; that is, when the pressure is doubled, the volume is halved; when the pressure is trebled, the volume is one-third of what it was origin- ally, and so on. The volume of a gas is further very sensitive to changes of temperature, and it has been found that a gas which occupies ten gallons at 32° Fahrenheit will occupy between thirteen and fourteen gallons at 212° fahrenheit, provided the pressure has remained the same. This behaviour, again, is quite different from that ex- hibited by liquids. Everybody knows that a pint of water does not become noticeably more bulky when it is raised to boiling. As a matter of fact, it does expand, but the expansion is too slight to be detected by the eye. These striking differences between gases and liquids were, of course, obvious to our scientific forefathers, and some generations ago they adopted the explanation which 47