152 CHEMICAL TRANSFORMATIONS IN 



proved experimentally for the reaction between gaseous hydrogen and 

 bromine vapour. Here a molecule of hydrogen and a molecule of bro- 

 mine unite to form two molecules of hydrogen bromide, and there is no 

 change in number of reacting molecules; thus H 2 -f Br 2 = HBr + HBr ; 

 accordingly we have P (H ,orBr) = K.P H *> and no matter how the 

 pressure of hydrogen, bromine, and gaseous hydrobromic acid are 

 altered, the percentage of the three reacting constituents ought to 

 have the same ratio that is, there ought always to be the same. per- 

 centage of bromine vapour converted into hydrobromic acid. This is 

 found to hold experimentally; within fairly wide limits, no matter 

 how the pressure is varied, the percentage of conversion remains 

 the same. 



III. Where a single compound splits up into two compounds, 

 or vice versa when the reaction is proceeding in the opposite direc- 

 tion. Under this class are also included those reactions in which, 

 although there are two substances on each side of the reaction, 

 one by passing out of solution or by blending with the solvent 

 with which it happens to be identical, develops no change in 

 osmotic pressure, and hence there is no work done on or by it, 

 and it does not come into the reaction. Such a reaction would 

 be included, for example, as a disaccharide splitting into its con- 

 stituent hexoses, or in the reverse direction, although the molecule of 

 water added to or taken away from the disaccharides in the reaction 

 makes the number of molecules apparently equal on the two sides 

 of the equation. The molecule of water in the reaction, however, 

 comes from or is returned to the great mass of water forming the 

 solvent for the disaccharide molecule, and hence has no osmotic 

 pressure before, during, or after the reaction. Accordingly, it 

 does not come into the equilibrium equation, for no change of 

 osmotic energy takes place in connection with it, and hence such 

 reaction is practically one in which one molecule forms two or 

 vice versa. 



Nearly all the reactions of digestion and metabolism also 

 belong to this' type of reaction, with the slight modification that 

 the number of simpler molecules formed is often three or more 

 instead of two, and all the conclusions deduced for the simpler 

 splitting into two molecules, or the reverse, can be applied to such 

 reactions with but slight modification. For example, the hydro- 

 lyses of neutral fats or triglycerides into fatty acids and of starches 

 into sugars belong to this category. 



