REGULATION OF BREATHING vd 
changes colour from a dark purple to bright scarlet 
when it takes up oxygen. The oxygen is taken up in 
the form of a weak chemical combination, the com- 
pound having the property of being stable only in 
presence of a certain concentration of free oxygen, 
and dissociating rapidly as the concentration of oxygen 
falls. The function fulfilled by haemoglobin as a car- 
rier of oxygen from the lungs to the tissues is thus 
readily intelligible, as well as the difference in colour 
between arterial and venous blood. Substances in the 
blood combine to form similar readily dissociable com- 
pounds with carbon dioxide, but no change in colour 
is associated with this process. 
Both deficiency of oxygen and excess of carbon 
dioxide in the air were found to produce increase in 
the breathing, and till recently the respective parts 
played by oxygen and carbon dioxide in regulating 
the breathing were by no means clear, and opinions 
on the subject were divided. I was myself led to 
investigate the whole subject through observations on 
the effects of air vitiated by respiration or by the gases 
met with in coal-mines and other confined spaces. 
When air highly vitiated by respiration or combus- 
tion of carbonaceous material is breathed the amount 
of air inspired or expired is increased. The increase 
is due to the carbon dioxide in the air; for when this 
is removed there is no increase unless the deficiency of 
oxygen is extreme. The effect produced on the breath- 
ing by carbon dioxide in the inspired air increases out 
of proportion to increase in the percentage of the car- 
bon dioxide. This fact suggested that in ordinary 
