304 LIQUIDS AND GASES. 



Generally speakiug, a gas, when compressed, decreases in volume 

 to an amount erpial to tliat by wliicli its pressure is raised, provided 

 its temperature be kept constant. This was discovered by Eobert 

 Boyle in 1000; in 1001 he presented to the Eoyal Society a Latin trans- 

 lation of his book, " Touching the Spring of the Air and its Effects." 

 His words are : 



"It is evident, that as common air, when reduced to half its natural 

 extent, obtained a spring about twice as forcible as it had before; so 

 the air, being thus compressed, being further crowded into half this 

 narrow room, obtained a spring as strong again as that it last had, and 

 consequently four times as strong as that of common air." 



To illustrate this, and to show how such relations may be expressed 

 by a curve, I will ask your attention to this model. We have a 

 piston, fitting a long horizontal glass tube. It confines air under the 

 pressure of the atmosphere — that is, some 15 pounds on each s(]uare 

 inch of area of the x)iston. The pressure is supposed to be registered 

 by the height of the liquid in the vertical tube. On increasing tlie 

 volume of the air, so as to double it, the pressure is decreased to half 

 its original amount. On decreasing the volume to half its original 

 amount, the i)ressure is d^mbled. On again halving, the i)ressuie is 

 again doubled. Tims you see a curve may be traced, in which the 

 relation of V(»lume to pressure is exhibited. Such a curve, it may be 

 remarked incidentally, is termed an hypeibola. 



We can repeat Boyle's experiment by pouring mercury into the open 

 limb of tliis tube containing a measured amount of air; on causing 

 the level of the mercury in the open limb to stand oO inches (that is, 

 the height of the barometer) higher in the 0])en lind) than the closed 

 limb, the pressure ot the atmosphere is doubled, and the volume is 

 halved. And on trebling the pressure of the atmosphere the volume 

 is reduced to one-third of its original amount; and on adding another 

 30 inches of mercury, the volume of the air is now one-quarter of that 

 which it originally occupietl. 



It must be remembered that here the temperature is kept constant; 

 that it is the temperature of the surrounding atmosphere. 



Let us next examine the behavior of a gas when its temperature is 

 altered, when it becomes hotter. T'his tube contains a gas— air— con- 

 fined at atmospheric pressure by mercury, in a tube surrounded by a 

 jacket or mantle of glass, and the vapor of boiling water can be blown 

 into the space between the mantle and the tube containing the air, so as 

 to heat the tube to 100° C, the temperature of the steam. The tempera- 

 ture of the room is 17° C, and the gas occupies 290 divisions of the scale. 

 On blowing in steam, the gas expands, and on again equalizing pres- 

 sure, it stands at 373 divisions of the scale. The gas has thus expanded 

 from 200 to 373 divisions, i. e., its volume has increased by 83 divisions; 

 and the temi)erature has risen from 17° to 100c>, i. ^., through 83*^0. 

 This law of the expansion of gases was discovered almost simultane- 



