576 
nearly similar, vary greatly from the curves of the two meta- 
derivatives, which are also nearly similar. 
The molecular refraction of the p-nitro-aniline, the absorption 
band of which is much deeper and lies nearer to the yellow, is 
larger than that of the m-nitro-aniline. 
The 3.5- and 2.6-dinitro-p-toluidines and their dimethyl-derivatives 
behave very irregularly. Here we have an example of the above 
mentioned exception. MR. M‚R-sÂ 
3.5. dinitro-toluidineand 3.5. dinitro-dimethy l-toluidine55,0and58,8 5,8 
A i ER Bess £ us 49,6 „ 59,8|10,2 
A look at the absorption curves (fig. ID) *) explains this behaviour. 
The curves of the two 2.6-dinitro-derivatives are almost similar, 
wheras those of the 3.5-dinitro-compounds diverge greatly. The 
molecular refraction of the 3.5-dinitro-p-toluidine, which possesses a 
deep absorption band, differs very little from that of its dimethyl 
derivative (3,8); this is in perfect accordance with the well-known 
fact that on the red side of a deep absorption band the refractive 
index is raised (anomalous dispersion). 
Utrecht. Org. Chem. Univ. Lab. 
Physics. — “On adiabatic changes of a system in connection with 
the quantum theory.’ By Prof. Dr. P. Enrenvest. (Communi- 
cated by Prof. H. A. Lorentz). 
(Communicated in the meeting of June 24, 1916). 
Introduction. In an increasing number of physical problems the 
foundations of classical mechanics (and electrodynamics) are used 
together with the quantum hypothesis, which is in contradiction 
with them. It remains of course desirable to come here to some 
general point of view from which each time the limit between the 
“classic?” and the “quantum” region may be drawn. 
Wier’s law has been found by an application of classic principles: 
the changes of the distribution of the energy over the spectrum and 
the work done by a reversible adiabatic compression are calculated 
quite according to classic electrodynamics. This law derived without 
the use of quanta stands unshaken amid the quantum theory. This 
fact now is worth our attention. 
Perhaps something similar holds in more general cases, when no 
longer harmonic vibrations take place, but more general motions : 
y MORGAN, JoBLiNG, and Barnett, J. Chem. Soc. 101, 1211 (1912). 
