LIQUIDS AND GA8ES. 305 



ously by Dalton and (ray-Lussac in isoi; it usually ,i;oes by the uame 

 of Gay-Lussac's law. Now, if we do not allow the volume of the gas to 

 increase, we shall find that the pressnre Avill increase in the same pro- 

 portion that the volume would have increased had the gas been allowed 

 to expand, the i^ressnre having l)een kept constant. To decrease the 

 volume of the gas, then, according to IJoyle's law, will require a higher 

 initial pressure; and if we were to represent the results by a curve, 

 we should get an hyperbola, as before, but one lying higher as regards 

 pressures. And so we should get a set of hyperbolas for higher iiud 

 higher temiieratures. 



We have experimented up to the present with air — a. mixture of two 

 gases, oxygen and nitrogen ; and the boiling points of both of these ele- 

 ments lie at very low temperatures: —184° C. and — r.l.i'^.l C, respect- 

 ively. The ordinary atmospheric temi)erature lies a long way above the 

 boiling points of li<piid oxygen and licpiid nitrogen at the ordiiuxry at- 

 mospheric i^ressure. But it is oi)en to us to study a gas, which, at the 

 ordinary atmospheric temperature and pressure, exists in th(^ liquid 

 state; and for this purpose I shall choose \Aater gas. In order that it 

 may be a gas at ordinary atmospheric pressure, however, we must h(;at 

 it to a temperature above 100'^ C, its boiling point. This tube contains 

 water gas at a temperature of lOoO C; it is under ordinary j>ressure, 

 for the mercury columns are at the same level in both the tubes and 

 in this reservoir, which communicates with the lower end of the tube 

 by means of the india-rubber tubing. Tlie temperature 105° is main- 

 tained by the vapor of chloro-benzene, boiling in the bulb sealed to the 

 Jacket, at a j)ressure lower than that of the atmosphere. 



Let us now examine the effect of increasing pressure. On raising 

 the reservoir tlie volume of the gas is diminished, as usual, and nearly 

 in the ratio given by Boyle's law; that is, tlie volume decreases in the 

 same proportion as the pressure increases. But a change is soon ob- 

 served; the pressure soon ceases to rise; the distance between the 

 mercury in the reservoir and that in the tube renniins constant, and 

 the gas is now condensing to liquid. The pressure continues constant 

 during this change, and it is only when all the water gas has condensed to 

 liquid water that the pressure again rises. After all the gas is condensed 

 an enormous increase of pressure is necessary to cause any measurable 

 decrease in volume, for liquid water scarc'cly yields to pressure, and in 

 such a tube as this no measurements could b(^ attempted with success- 



Kepresenting this diagrammatically, the right-hand ])art of the curve 

 represents the compression of the gas, and the curve is, as before, 

 nearly a hyperbola. Then comes a break, and great decrease in 

 volume occurs without rise of pressure, represented l)y a horizontal 

 line; the substance in the tube here consists of water gas in presence 

 of water; the vertical, or nearly vertical line represents the sudden 

 and great rise of pri^ssure, where li(iuid water is being slightly com- 

 pressed. The ])ressure registered by the horizontal line is termed the 

 II. Mis. lU 20 



