36 ORGANISM AND ENVIRONMENT 
required to keep up the work. These considerations 
led me to the conclusion that it is probably in virtue 
of its acidity that dissolved CO, (H,CO, or carbonic 
acid) affects the respiratory centre, and that other 
acids will therefore have a similar effect, and will thus 
help CO, to excite the centre. This theory explains 
why less CO, in the alveolar air is sufficient to excite 
breathing under the various conditions just referred 
to. 
At the time, however, there was no means available 
of accurately measuring the slight alkalinity of the 
blood. The old method of adding standard acid till 
an indicator changed colour was not only very rough, 
but also fallacious in principle. The blood is only very 
slightly alkaline, yet quite a large quantity of acid can 
be added to it before it becomes acid. It is full of so- 
called “buffer substances,” which are capable of com- 
bining with acids or alkalies, but are not themselves 
very definitely acid or alkaline. Thus the amount of 
acid which has to be added to blood to change its 
reaction is a measure of the buffer substances rather 
than of the alkalinity of the blood. According to 
modern ideas the acidity or alkalinity of a solution 
depends on the relative concentrations in it of hydro- 
gen and hydroxyl “ions.” This concentration can be 
measured directly by the electrometric method, but the 
difficulties in applying the method to blood were very 
great. 
In 1912, however, Hasselbalch of Copenhagen suc- 
ceeded in obtaining reliable results ; and he and Lunds- 
gaard published curves showing graphically the rela- 
