THE CIRCULATION OF NITROGEN IN NATURE. 73 
Lastly, there are some theoretical considerations arising 
out of the chemical changes brought about by the nitri- 
fying organisms which are very interesting. 
Until the discovery of the nitrifying bacilli it was almost 
universally accepted that those plants only which contained 
chlorophyll could acquire the carbon necessary for their 
tissues from carbon dioxide. And this seemed consistent 
with both theory and observation. 
Chemical reactions are classed as exothermal or endo- 
thermal according as the combining bodies contain locked 
up in themselves more or less than sufficient energy for the 
process of combination. In the former case there is an 
excess of potential energy, the surplus generally taking the 
shape of sensible heat, hence exothermal. Endotliermal 
reactions cannot take place unless the necessary supplemen- 
tar}'- energy is supplied from an outside source. The com- 
bustion of carbon is an exothermal reaction which we daily 
employ in order to avail ourselves of the surplus energj^, 
with which we cook our food and warm our fingers. But to 
convert the carbon dioxide into any less oxidized substance 
requires a supply of energy from some outside source, which, 
in the case of the chlorophyll-containing plants, is the light 
of the sun. Hence plants devoid of chlorophyll were sup- 
posed to derive their carbon from organic matters only ; 
and, generally speaking, observation tended to support this 
view. 
When it became clear, however, that the nitrifying 
bacilli, devoid of chlorophyll, working only in the dark, yet 
obtained their carbon from carbon dioxide only, and were 
even prejudicially affected by the presence of organic matter, 
it became necessary to look about for some new source of 
energy. This is found in the oxidation of the nitrogen, in 
which reaction a large surplus of energy is liberated, much 
