The specific oxygen capacity of blood 9 



doubt of course as to the theoretical accuracy of the calculation, but 

 it is evidently not impossible that when the greatest accuracy is 

 concerned there may be weak points about its application. We are 

 using fluids for instance which leave films on the glass and exert 

 vapour pressures and so forth, and it may be that the trifling- 

 constant errors involved might in the aggregate amount to one or 

 two per cent. 



Burn therefore went back to quite another method of relating q 

 to x, which was tried, though without complete success, by Roberts 

 and myself. He first defined the conditions under which it might be 

 relied upon, after which he obtained with it results of remarkable 

 consistency. He liberated a known quantity of oxygen in the 

 apparatus and read off the difference in level; thus knowing both 

 q and x he found the constant K by which x must be multiplied to 



(V 

 give q, and which should of course be equal to ( \-a\. 



The known quantity of oxygen was liberated from a known volume 

 of standardised hydrogen peroxide by potassium permanganate accord- 

 ing to the following equation : 



K 2 Mn 2 8 + 3H 2 SO 4 = K 2 S0 4 + 2MnS0 4 + 3H 2 + 50, 

 5O + 5H 2 O 2 = 5H 2 + 50 2 . 



So far as manipulative details are concerned the oxygen is libe- 

 rated in just the same way as it is from a haemoglobin solution by 

 ferricyanide. As the sources of manipulative error are the same in 

 each case, the estimation of K by the H 2 2 method should give a 

 correct value of K for the ferricyanide method. The validity of the 

 method depends upon the assumption that the calculated quantity 

 of oxygen is released. 



The following is the result of one series of experiments in which 

 Burn determined K for a certain apparatus, No. 19, by the two 

 methods. 



K determined by H 2 2 



A" determined by measurement 

 of V, p, and a 



3-72 



Mean 3-83 



The extreme results of the H 2 O 3 method differ from one another 

 by 3 "/.. 



