292 Dr. H. Herwig's Investigations on the Conformity 



The size of the air-bubble which was present amounted to 

 *064 cubic centim. for 0° and a pressure of 0*760 millim. The 

 weight of the alcohol examined was *0248 grin. Hence are cal- 

 culated, for the following different temperatures, the final vapour- 

 densities which correspond to the mean value of the constant pv 

 for each temperature. 



Temperature ... 



23 



30-5 



36-4 



41-9 



47-8 



57-8 



629 



69-9 



Vapour-density . 



1550 



1-555 



1-555 



1-550 



1-552 



1-551 1-552 



1-548 



That these densities are all too small is due simply to this — 

 that the alcohol that was used was not entirely free from water, 

 but had been allowed to stand in the air for a considerable time 

 in a flask closed by only a cork. On this the first filling of the 

 apparatus, it was my intention only to test its accuracy. How- 

 ever, as it immediately proved itself to be reliable, I then carried 

 on this first investigation to the end. But even as regards the 

 object in view, it is of small consequence whether the alcohol 

 were perfectly pure or contained some water ; it is only necessary 

 to keep in mind that the numbers obtained above refer to alcohol 

 not entirely free from water. 



A comparison of the vapour- densities obtained at different 

 temperatures shows clearly that the vapour-densities are con- 

 stant. It therefore exhibits the simultaneous appearance of 

 Gay-Lussac's law and that of Mariotte ; and, indeed, nothing 

 different could have been expected a priori. At the same time 

 it is shown experimentally that by means of the apparatus here 

 employed the vapour-densities can be accurately determined even 

 at low temperatures (much below the boiling-point of the bodies 

 examined), which is worth noting, by reason of the difficulty 

 encountered in the determination of the vapour-densities of 

 several bodies when at a high temperature according to the 

 usual methods. 



A further comparison of the figures entered in column pv, 

 the particulars of which exhibit the magnitude of the deviation 

 of the vapour from Mariotte's law at different temperatures, 

 shows us that at each approach to condensation the deviation 

 increases with ascending temperatures. That it does so in the 

 case of water, at least, Clausius tells us in his first memoir*. 



If the volume and density of perfectly saturated vapour, which 

 thus has absorbed the last drop of liquid, be denoted by v } and 

 p lf while V and P are the corresponding quantities for a condition 

 of the vapour in which it already obeys Mariotte's law at the 



PV 



specified temperature, then the quotient will increase with 



increasing temperatures. 



P\ v \ 



* The Mechanical Theory of Heat. London, 1867. Van Voorst. 



