RESPIRATION 179 



from the true neutrality point where hydrogen and hydroxyl ions 

 are equal in concentration, as in pure water. 



The relative diffusion pressures, or (to use the incorrect lan- 

 guage of the still generally accepted van't HofTs theory of osmotic 

 pressure, etc.) the relative concentrations of any particular sort 

 of ion, in different solutions, can be measured by the differences 

 of potential communicated to a suitable electrode dipped in the 

 solutions. Thus with a hydrogen electrode hydrogen ion con- 

 centration can be measured directly; and this method was ap- 

 plied, soon after its discovery, to the measurement of the hydrogen 

 ion concentration (and therefore indirectly also of the hydroxyl 

 ion concentration) of blood. The earlier attempts gave the result 

 that the blood was neutral in reaction, and remained neutral even 

 in acidosis. The physiological signs of acidosis were, however, 

 very clear, as already explained. The electrometric method in its 

 earlier form was thus far too rough for physiological work. 



It was mentioned in Chapter I that the experiments of Geppert 

 and Zuntz on the hyperpnoea following- muscular contractions in 

 animals showed a great diminution in CO 2 and a slight excess of 

 oxygen in the arterial blood during the hyperpnoea. They there- 

 fore concluded that neither excess of CO 2 nor want of oxygen can 

 be the cause of the hyperpnoea ; and they sought for the cause in 

 some acid substance present in the blood, since acids were known 

 to stimulate the breathing. The search made for the acid substance 

 did not, however, lead to any definite result ; and the experiments 

 of Priestley and myself on man brought us back to CO 2 as the 

 stimulus to the increased breathing. The improbability of any 

 organic acid being the stimulus to the breathing seemed to us to 

 be in any case very great. No acid other than CO 2 is given off in 

 the expired air, and organic acids, etc., are not appreciably oxi- 

 dized in the blood itself. It did not therefore seem possible to un- 

 derstand how the air hunger of muscular exertion could be re- 

 lieved, as it undoubtedly is, by increased breathing. In any case 

 the diminished proportion of CO 2 in the arterial blood in these 

 experiments was entirely discounted by the fact that this dimin- 

 ished proportion continued to exist for at least an hour after the 

 hyperpnoea had passed off. We thought that in Geppert and 

 Zuntz's experiments owing to defective circulation in the artifi- 

 cially stimulated muscles of the animal some lactic acid had been 

 produced and thrown into the blood, thus greatly reducing its 

 power of combining with CO 2 . Thus, although the pressure of 

 CO 2 was perhaps actually higher in the arterial blood and caused 



