LIQUIDS AND GASES. 309 



simplest: pressure is a linear fauction ol" tempcratiiie. Exi^ressed 

 iiiathematieally — 



p = bt — a, 



•where h iiiid a iire eoiistants, depeiiding- on the volume cliosen, aud 

 varying' with each volume. IJut a^ strai.ij;ht line uiay be extrapolated 

 without error; and so, havinji' found values for <( and h for sueh a 

 vohnue as (> e.e. i)er gram, by help of experinu'uts at temperatures 

 higher than 195^, it is possible by e.\tra])olation to obtain the pressures 

 corresponding to temperatures 1)elow the critical point 19.")'3 by simple 

 means. But below that temperature the substance at volume is in 

 practice partly li(juid and partly gas. Yet it is possible by such means 

 to ascertain the relations of pressure to temperature for the unrealiz- 

 able portion of the state of a liquid — that is, we cau deduce the pressure 

 and temperature corresponding to a continous (-hauge from liquid to 

 gas. And in this nmuner the sinuous lines on the figure have been 

 constructed. 



It is i)ossible to realize ex))crimeutally certain ])ortions of such con- 

 tinuous curves. If we condense all gaseous ethei- and, when the tube 

 is completely tilled with liijuid, carefully reduce pressure, the pressure 

 may be lowered considerably below the vapor pressure corresponding 

 to the temperature of ebullition, without any change further than the 

 slight expansion of the li(]uid resulting tVom the reduction of pres- 

 sure — an expansion too small to be seen with this api)aratus. But on 

 still further reducing pressure, vSudden ebullition occurs, and a ])ortion 

 of the liquid suddenly changes into gas, while the pressure rises 

 quickly to the vapor pressure corresiionding to the teuq)erature. If we 

 are successful in expelling all air or gas from the ether in filling the 

 tube, a considerable i)ortion of this curve can be ex])erimentally realized. 



The first notice of this appearaiu'e, or rather of one owing- its exist- 

 ence to a precisely similar cause, is due to Ifooke, the celebrated con- 

 temporary of Boyle. It is noted in the account of the Proeeedni(/s of 

 the Royal kSovletii on ]Srovend)er 0, I(i72, that "Mr. Ilooke read a dis- 

 course of his, containing his th<mghts of the cx])erinuMit of the <jnick- 

 silver's standing- to]) full, and far above the height of 2t» inches, to- 

 gether with some exi)erinu'uts made by hiju, in order to determiiK^ the 

 cause of this strange phenomenon, lie was ordeied to prepare those 

 experiments for the view of the Society." Aiid on November 13 ''the 

 exi)eriment for the high suspension of (iui<'ksilver being called for, it 

 was found that it had failcMl. It was oi-dered tliat thicker glasses 

 slumld be provided iov the next meeting."' 



There can be no doubt tliat this behax lor is caused hy the attrac- 

 tion of the nu)lecul<^s of the li<|uid for each other. An«l if the tenq)era- 

 ture be sufiicientlylow, the pressure maybe so reduced that it becomes 

 negative — that is, until the liquid is exposed to a strain or pull, as is 

 the mercury. This lias been experimentally realized by M. ]^)erthelot 

 and by Mr. AVorthington, the latter of whom has succeeded in strain- 



