306 LIQUIDS AND GASES. 



"vapor- pressure" of water. If now the temperature were raised to 

 110° C, we should liave a greater initial volume for the water gas; it is 

 compressible by rise of the mercury as before, the relation of pressure 

 to volume being, as before, represented on the diagram as an approxi- 

 mate hyperbola; and as before, condensation occurs when volume is 

 suflBcieutly reduced, but this time at a higher pressure. AVe have 

 again a horizontal portion, representing the pressure of water gas at 

 110° 0. in contact with liijuid water; again, a sliarp angle where all 

 gaseeus water is condensed, and again a very steep curve, almost a 

 straight line, representing the slight decreases of volume of water pro- 

 duced by a great increase of pressure. And we should have similar 

 lines for 12()o, 130°, 140°, ir>Oo C, and for all temperatures within certain 

 limits. kSuoh lines are called isothermal lines, or shortly "isothermals," 

 or lines of equal temperature, and represent the relations of pressure 

 to volume for different temperatures. 



Dr. Andrews made similar measurements of the relations between 

 the pressures and volumes of carbon dioxide, at pressures much higher 

 than tliose 1 ha^•e shown you for water. But I prefer to si^eak to you 

 about similar results obtained by Prof. Sydney Young and myself with 

 ether, because Dr. Aiulrews was unable to work with carbon dioxide 

 free from air, and that influenced his results. For example, you see 

 that the meeting points of his hyperbolic curves with the straight lines 

 of vapor i^ressures are curves, and not angles; that is caused by the 

 presence of about 1 part of air in 500 parts of carbon dioxide; also the 

 condensation of gas was not perfect, for he obtained curves at the 

 points of change from a mixture of liquid and gas to liquid. We 

 however were more easily able to fill a tube with ether free from air, 

 and you will notice that the points I have referred to are angles, not 

 curves. 



Let me first direct your attention to the shapes of the curves in the 

 diagram. As the temperature rises the vapor-pressure lines lie at 

 higher and higher pressures, and the lines themselves become shorter 

 and shorter. And finally, at the temperature of 31° C. for carbon di- 

 oxide, and at 195° C. for ether, there ceases to be a horizontal portion 

 at all; or rather the curve touches the horizontal at one point in its 

 course. That point corresponds to a definite temperature, 195° C. for 

 ether; to a definite pressure, 27 meters of mercury, or 35.0 atmos- 

 pheres; and to a definite volume, 4.00 cubic centimeters per gram of 

 ether. At that point the ether is not liquid, aud it is not gas; it is a 

 homogeneous substance. At that temperature ether has the appear- 

 ance of a blue mist; the striie mentioned by Dr. Andrews and by 

 other observers are the result of unequal heating, one portion of the 

 substance being liquid and another gas. You see the appearance of 

 this state on the screen. 



^Vhen a gas is compressed it is heated. Work is done on the gas, 

 and its temperature rises. If I compress the air in this syringe forci- 



