366 Professor W, Hamsay [May 8, 



Generally speaking, a gas, when comjDressed, decreases in volume 

 to an amount equal to that by which its pressure is raised, provided 

 its temperature be kept constant. This was discovered by Eobert 

 Boyle in 1660. (In 1661 he presented to the Eoyal Society a Latin 

 translation of his book ' Touching the Spring of the Air and its 

 Effects.') His words are : — 



" 'Tis evident that as common air, when reduced to half its natural 

 extent, obtained a sj^riug 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 again 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 glass tube. It confines air under the 

 pressure of the atmosphere, that is, some 15 lb. on each square inch 

 of area of the piston. The pressure is supposed to be registered by 

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

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

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

 amount the pressure is doubled. On again halving, the j)ressure is 

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

 relation of volume to pressure is exhibited. Such a curve, it may be 

 remarked incidentally, is termed a hyperbola. 



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

 open limb of this tube containing a measured amount of air. On 

 causing the level of the mercury in the open limb to stand 30 inches 

 (that is the height of the barometer) higher in the open limb than 

 the closed limb, the pressure of 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 other 30 inches of mercury the volume of the air is now one 

 quarter of that which it originally occupied. 



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 behaviour of a gas when its temperature 

 is altered ; when it becomes hotter. This tube contains a gas, air, 

 confined by mercury, in a tube surrounded by a jacket or mantle of 

 glass, and the vapour of boiling w^ater can be blown into the space 

 between the mantle and the tube containing the air, so as to heat the 

 tube to 100^, the temperature of the steam. The temperature 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 equalising pressure 

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

 from 290 to 373 divisions, i. e. its volume has increased by 83 

 divisions, and the temperature has risen from 17° to 100°, i. e. through 

 83 degrees. This law of the expansion of gases was discovered 

 almost simultaneously by Dalton and Gay-Lussac in 1801. It usually 



