134 TlMEHRI. 
The bearing of this doctrine upon physiology was at 
once apparent. " Vital force" no longer stood out alone 
and distinct, but could be regarded as forming one of 
the family of " energies" and, therefore, capable of 
producing and being produced by any one of the other 
forms. 
The first views of any importance on the subject of 
the relationship of food to work were those of LlEBIG. 
Coming from the pen of the first chemist of the day, they 
may be said to have been universally received throughout 
the scientific world. LlEBIG divided foods into two 
classes, the nitrogenous, or those containing nitrogen, 
and the non-nitrogenous, or those which consisted only 
of carbon, hydrogen, and oxygen. The former class he 
termed "plastic," and attributed to them the func- 
tions of supplying the materials for the production of 
muscular and nervous force. To the other class he 
applied the title of " heat producers," or " calorifacient." 
Looked at in this light, the principal part of nutrition 
would be played by nitrogenous food. Not only would 
nitrogenous food, as LlEBIG imagined, renew the tissues, 
but it would also indirectly form the source of muscular 
work, since this latter form of energy would according to 
LlEBIG, be due to the transformation of the stored-up 
chemical energy of the tissues consequent on their oxida- 
tion and destruction within the system. 
Attractive and plausible though this theory was, as 
the test of experimental research came to be applied to 
it, doubts arose as to its validity. Were the tissues by 
their oxidation and destruction the source of muscular 
power, increased work would mean increased destruc- 
