203 



after Le Vkrrikr that this metal is capable of existing in more tlian 

 one allotropie moditieation and he found a tmnsitiüii temperature 

 between 500 and 600°. I hope shortly lo report on this point, in 

 connexion also with a question whicii is important from a tecimical 

 standpoint i.e. tlie disintegration of abiminiuni objects at room tem- 

 perature, a disease which is tlie cause of a good many comphiints in 

 industrial circles as well as in daily life. 



9. That others had never observed the i)henomena described by 

 Le Vkkhier may be explained by the fact that they had not heated their 

 l»reparations repeakdhj to high temperatures, as he did. We have 

 also observed during our dilatometric researches that such a transition 

 point can be overpassed several hundreds of degrees without any effect. 

 If on the contrary the n>etal is repeatedly cooled and iieated the 

 transition is "set going". As the means of overcoming these retard- 

 ations are now known we are able to avoid them. A systematic 

 research in this direction is now possible and I hope to report 

 shortly on it. 



Utrecht, June 1914. van 't Wow- Laboratory. 



Physics. — ''Measurements of isotherms of hi/droqen at 20° C. and 

 15°.5 6V' By Prof. Ph. Kohnstamm and Dr. K. W. Walstra. 

 Van der Waals fund rescavches N". 7. (Communicated by 

 Prof. J. D. VAN DER Waals). 



(Gommuuicatcd in the meeting of April 24, 1914). 



§ 1. Choice of the substance and the temperature. 



With the apparatus described in N". 5 and 6 of this series we 

 have made measurements of hydrogen isotherm.s at 20'' C and 

 15°. 5 C. This choice was led by the following considerations. As 

 we already set forth in the beginning of Communication N". 5, one 

 of the motives of our research was the desire to be able to make 

 an accurate comparison with the results obtained by Amagat. Our 

 first intention was to determine anew Amagat's air isotherms ; then 

 we were, however, checked by peculiar difficulties. Every time, 

 namely, when a measuring tube was filled with air in the way 

 described in the previous Communication, and was then left for 



