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 2 (H 2 CO 3 or carbonic 

 acid) affects the respiratory centre, and that other 

 acids will therefore have a similar effect, and will thus 

 help CO 2 to excite the centre. This theory explains 

 why less CO 2 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- 



