186 HEAT. 



volume, but keeping the temperature at 0, the steam will be represented 

 by successive points along a curve al>, which is nearly a hyperbola, since 

 i;he pressure multiplied by the volume is nearly constant. But when the 

 pressure reaches 4'6 mm., and the volume is 200,000 c.c.,the state being 

 represented by b, condensation normally begins and goes on at the same 

 pressure till all is condensed to water at 0. The curve, therefore, 

 changes at b into a horizontal straight line be, points on this line repre- 

 senting different proportions of the mixture of steam and water ; c will 

 represent the volume of the condensed water, just over 1 c.c. The 

 pressure may now be increased, but the water diminishes only very 

 slightly in volume, so that subsequent points lie along a line cd, only 

 slightly leaning towards OP. In the figure it is impossible to represent 

 the curve on proper scale. It is, therefore, only drawn so as to show its 

 general nature. If we now start again with 1 gramme of steam at 50 

 and 1 mm. pressure, its volume will be about 1,100,000 c.c., represented 

 by the point a'. As the pressure increases, the temperature remaining 

 the same, the volume diminishes, the relation between volume and 

 pressure being represented by the nearly hyperbolic curve a'b', till at 

 92 mm. pressure and 12,000 c.c. volume condensation begins and goes on 

 at constant pressure till all is water. This change is represented by the 

 line b'c. The point c will represent about 1*01 c.c., and is, therefore, 

 very near the line cd. Subsequent increase of pressure corresponds to 

 the line c'd', this again only slightly leaning towards OP. Starting anew 

 with the steam at 100 and at 1 mm. pressure, the initial volume is 

 about 1,300,000. The subsequent changes are represented by a" if" c" d", 

 b" corresponding to 760 mm. pressure and 1700 c.c. volume, c" represent- 

 ing about 1'04 c.c. volume. The successive curves for each temperature 

 are termed isothermal s. 



We may represent on the diagram any change of volume and 

 pressure occurring in a quantity of water substance. For instance, if 1 

 gramme of water at 0, and, say, 92 mm., be gradually heated at 

 constant pressure in a closed extensible vessel, it will be represented 

 by a horizontal line starting from the point on cd level with c, and 

 cutting all the water-isothermals till the temperature reaches 50, when 

 it will have reached c'. It now normally begins to turn into steam, and 

 its course is represented by the line c'b', all at 50. After b' is reached 

 the temperature rises again, and successive steam-isothermals are cut 

 along b'f. But if suitable precautions as to vessel and freedom from air 

 bubbles are taken, the water may be heated above 50, still remaining 

 water. This implies that the water-isothermals do not end in their down- 

 ward course at the points cc'c", but may be prolonged, as represented in 

 Fig. 109 to ee. Or, again, we have seen that water may easily be heated 

 in a very clean glass vessel at the atmospheric pressure to 106 without 

 boiling. This would be represented by the point g, where the 106 

 water-isothermal produced cuts the line of 760 mm. pressure. Dufour's 

 experiment shows that even the 178 isothermal may be prolonged down 

 thus far. 



Another case is afforded by a phenomenon sometimes observed in 

 barometer-tubes. If the tube is very clean and the mercury free from 

 air, it is possible after filling and inversion, to raise the top of the tube 

 far above 30 inches without detachment of the mercury from it. This 



