LUEDEKING PHYSICO-CHEM. STUDIES IN FATTY ACID SERIES. 643 



in our introduction, and say that the acids in the liquid form do 

 most certainly consist of i)i molecules (CnHonOs), and further that 

 this m must be different in value for the different acids. The ne- 

 gative heat observed in the formation of the first hydrates of acetic, 

 propionic and butyric acids is due to nothing else than the dis- 

 sociation of these complex molecules. As dilution of substance, 

 especially in the gaseous state, is known to produce dissociation 

 of complex molecules, so it is evidently with our fatty acids. Their 

 complex molecules, which are the cause of their differences in the 

 lower hydrates, become dissociated later on, and we then find 

 perfect relative coincidence. We should expect the dissociation of 

 the molecule of butyric acid to be attended with the greatest nega- 

 tive thermal effect, since it is so much nearer the solid state of mat- 

 ter, and in its case, therefore, the value of m is so much greater 

 than for the others. 



We should also expect that its actual heat of hydration, which 

 is positive of course, is very much less than for any of the other 

 acids, since its affinity for water must be less, becaure it lies so 

 much nearer that class of acids which are actually fatty, and 

 whose properties it therefore possesses in a degree superior to the 

 other acids. Consequently all conditions concur to make the ther- 

 mal effect more negative than for any of the others. 



Even formic acid can now be understood in is differences from 

 the others. It has the strongest affinity for water, and in its mo- 

 lecular complexity the value of ni is less than for any of the 

 others ; consequently we have all the conditions at hand for a 

 positive thermal effect in its first hydrations. 



Complexity of experimental conditions, therefore, is the cause 

 of our not observing absolute coincidence in the physical proper- 

 ties of the four acids that we have examined. 



My especial thanks are due Professors G. and E. Wiedemann, 

 at whose instigation these determinations were made, and who 

 kindly gave me the benefit of their advice during the progress 

 of this work. 



-4—7 [May 6, 1866. 



