HEAT EQUIVALENT OF NUTRIEMTS OF FOOD. 603 



Tbe mean of all the ratios is 98.39. We found a similar relation in the 

 case of albumens and fats. This bears out our opinion (see p. 593) that 

 the results ol)tained by us several years ago by the potassium chlorate 

 method were in faet relatively correct, and that the small variations from 

 the real values were due to small errors made in the determination of 

 the constants used. 



When the flgures in the table are conipared it will be seen that the 

 isomeric couipounds have dift'erent heat equivalents, though the differ- 

 ences are not great. Since calorimctric measurements were not as exact 

 as they now are, it was possible to explain these variations on the ground 

 of .accidents or errors of observation, as indeed was done by Berthelot. 

 This is, however, not the case. The variations have an imi^ortant mean- 

 ing, and are in part due to the interior structure of the molecule, but 

 especially to greater or lesser instability of the molecule. This insta- 

 bility of a molecule manifests itself in two ways; either the compound 

 is easily decomposed by the action of certain agents, or the atoms within 

 the molecule undergo a change in position. Such unstable bodies pos- 

 sess, without exception, a higher heat equivalent than the more stable 

 ones. Of the four hexoses, C6H12O6, there are two iiairs which are 

 certainly similar, glucose and galactose which are aldoses, and fructose 

 and sorbinose which are ketoses. Of the two first glucose is less stable 

 than galactose, since it is more easily broken uj) by the organisms which 

 produce fermentation. Glucose ferments more easily than galactose. 

 The heat equivalent of these two is: 



Calories. 



Ghicose 673. 7 



Galactose 669. 9 



The two ketoses behave in the same way. Fructose is less stable 

 than sorbinose. It ferments more readily. Sorbinose is stable. It can 

 not be decomposed by the yeast plant. The heat equivalents express 

 this difference in stability. 



Calories. 



Fructose 675. 9 



Sorbinose 668. 6 



In tlie same way, in the case of isomeric bodies, rearrangements of 

 the atoms or groups of atoms in the molecule can occur without any 

 change in elementary composition taking jilace. The unstable maleic 

 acid, 041140^, is readily changed into its geometrical isomer, the stable 

 fumaric acid. 



The heat equivalents of these acids are as follows: 



Calories. 



Maleic acid 326.3 



Fiunaric acid 319. 7 



Oleic acid and elaidic acid behave in the same way and erucic acid 

 and brassic acid also. 



Two isomeric bodies, one of which has the allyl group, GH2.CH:CH2, 

 the other the propyleu group, CH: CH. CH3, by the change in position 

 of one H atom become alike in structure. The change lakes place 



