200 M. J. A. Groshans on the 



On the Variability of Physical Properties. 



I have given the above name to the following phenomena: — 



One frequently finds differences (according to Kopp reaching 

 sometimes as much as 2 per cent.) in the density of the same 

 substance at the same temperature. Similar differences are 

 to be observed in the fusing- and boiling-points. Such dif- 

 ferences present themselves when these physical properties 

 have been determined by different persons or by the same 

 observer at different times. 



Space does not permit me to cite more than a very few 

 instances. 



I shall begin with crotonic acid, C 4 H 6 2 ; a=86 ; n=12. 

 According to the law of density-numbers, and in consonance 

 with analogy (which I need not further refer to), this acid 

 should have the value of Tn/a corresponding to # = 5, that is, 

 the same constant as that of water. 



There are three known isomers of this substance, which 

 differ in chemical properties, with which I need not trouble 

 my readers. One of these isomers, /3, or isocrotonic or 

 quartenylic acid, has the following physical properties ; it is 

 a liquid, not solidifying even at —15° C., and boiling at 171°- 

 173°; from S 0/ v^5 is calculated 172°*5. There is thus complete 

 concordance between the calculated and observed boiling- 

 point. 



With regard to the other isomers, their physical properties, 

 as given in Fehling, are as follows : — 



(1) a. acid — solid crotonic acid, tetracrylic acid : melting- 

 point 71°-72°, boiling-point 187° (corr.). 



(2) Metacrylic acid: melting-point +16°, boiling-point 

 160°-5. 



We have seen above that the /3 or iso-acid has #==5. For 

 its two isomers we find respectively # = 5'330 and # = 4'734; 

 the mean value is <r = 5'03 ; which is essentially the same as 

 #=5. 



The particular fact that suggested to me the idea of the 

 mean value for x in various isomers was as follows : — 



"The liquid acid, which does not solidify even at —15°. is 

 changed, by heating to 180° in a sealed tube, into the solid 

 modification melting at 71°-5 and boiling at 187°"*. 



We may infer from this that the physical properties depend, 

 among other things, on the treatment to which the compound 

 has been subjected. 



* Wurtz, Dipt. Sup pi. p. 551. 



