1891.] on Liquids and Gases. 371 



temperatures 175°, 180°, 185°, 190°, and so on, at certain definite 

 pressures, which may be read from a properly constructed diagram. 

 AVe can map the course of lines of equal volume, of which the instance 

 given is one, using temperatures as ordinates and pressures as abscissaB. 

 We can thus find the relations of temperature to pressure for certain 

 definite volumes, which we may select to suit our convenience ; say, 

 2 c.c. per gram, 3, 4, 5, 6, and so on. Now all such lines are 

 straight. That is, the relation of pressure to temperature, at constant 

 volume, is one of the simplest ; pressure is a linear function of tem- 

 perature measured on the absolute scale. Expressed mathematically, 



p = h t — a, 



where h and a are constants, depending on the volume chosen, and 

 varying with each volume. But a straight line may be extrapolated 

 without error, and so having found values for a and h for such a 

 volume as 6 c.c. per gram, by help of experiments at temperatures 

 higher than 195°, it is possible by extrapolation to obtain the pres- 

 sures corresponding to temperatures below the critical point 195°, in a 

 simple manner. But below that temperature the substance at volume 6 

 is in practice partly liquid and partly gas. Yet it is possible by such 

 means to ascertain the relations of pressure to temperature for the 

 unrealisahle portion of the state of a liquid, that is, we can deduce the 

 pressure and temperature corresponding to a continuous change from 

 liquid to gas. And in this manner the sinuous lines on the figure 

 have been constructed. 



It is possible to realise experimentally certain portions of such 

 continuous curves. If we condense all gaseous ether, and, when the 

 tube is completely filled with liquid, carefully reduce pressure, the 

 pressure may be lowered considerably below the vapour pressure 

 corresponding to the temperature of ebullition without any change, 

 further than the slight expansion of the liquid resulting from the 

 reduction of pressure — an expansion too small to be seen with this 

 apparatus. But on still further reducing pressure sudden ebullition 

 occurs, and a portion of the liquid suddenly changes into gas, while 

 the pressure rises quickly to the vapour-pressure corresponding to 

 the temperature. If we are successful in expelling all air or gas 

 from the ether in filling the tube, a considerable portion of this curve 

 can be experimentally realised. 



The first notice of this appearance, or rather, of one owing its 

 existence to a precisely similar cause, is due to Mr. Hooke, the cele- 

 brated contemporary of Boyle. It is noted in the account of the 

 Proceedings of the Royal Society, on November 6th, 1672, that 

 " Mr. Hooke read a discourse of his, containing his thoughts of the 

 experiment of the quicksilver's standing top-full, and far above the 

 29 inches ; together with some experiments made by him, in order to 

 determine the cause of this strange phenomenon. He was ordered to 

 prepare those experiments for the view of the Society." And on 



