February 17, 1905.] 



SCIENCE. 



243 



bustion. It had been observed by Lavoisier 

 that the heat of the animal body was de- 

 pendent upon respiration; the heat of the 

 plant body was shown by DeSaussure to be 

 related to a disappearance of oxygen ; com- 

 bustion consmnes oxygen and produces 

 heat; therefore, respiration is a sort of 

 combustion. So the argument ran. 



It is quite impossible to overestimate the 

 influence that this conception has had on 

 the study of respiration. The mischief it 

 has wrought depends chiefly, perhaps 

 wholly, iipon a misconception of the actual 

 mechanism of combustion, a process that 

 has ever been the hete noire of chemistry, 

 as the history of the 'phlogiston' theory 

 well shows. To our changed conceptions 

 of combustion I shall return later. 



The idea of combustion, however, which 

 dominated the argument I have cited, was 

 that oxygen combined with carbon to form 

 COo and with hydrogen to form H2O. It 

 was most natural, therefore, to conceive 

 that the food taken up by the organism 

 stood to it in the same relation as does the 

 fuel to the engine, and that what happens 

 is an actual oxidation of the food innne- 

 diately and directly ; in fact, a process pre- 

 cisely parallel to the burning of the same 

 food outside the body. 



One evident outcome of that idea is the 

 current classification of foods into plastic 

 and dynamogenous, those which are iiseful 

 in building up the body and those that are 

 useful in producing heat within the body; 

 into 'fattening foods' and 'heat-producing' 

 foods. You are doubtless familiar with 

 these phrases. 



But if foods are 'burned' in the body it 

 must be important to know how much oxy- 

 gen enters it, and how much carbon dioxide 

 and water leave it, so as to discern the 

 ratio which exists between them. Plainly 

 a basis for this must be a comparison of 

 the differences between the combustion of 

 foods outside the body and their 'combus- 



tion' within the body. Yet, strangely, this 

 has not been made until recently. Without 

 giving the full tables let me show the re- 

 sults arrived at by two observers, regard- 

 ing two of the most common plant foods, 

 glucose and tartaric acid. These observers 

 assume, you will notice, that the processes 

 are comparable. The results are stated as 

 ratios of CO.,/0.. 



Fool. 



Glucose 

 Tart, acid 



By com- 

 bustion. 



By respiration. 



Dialionow. Puriewicz. 



Diakonow's whole series shows that in 

 combustion the carbon dioxid was always 

 less than in respiration ; Purjewicz found, 

 with the exception of tartaric acid, and 

 even there the difference between his re- 

 sults and Diakonow's is in the same direc- 

 tion, that it was always greater, his results 

 being absolutely different in significance 

 from Diakonow' 's. And this is a good type 

 of the results to be found in examining the 

 literature ! I am not now concerned in 

 determining which set of results is correct, 

 inasmuch as I believe both are valueless, 

 since on the assumption upon which they 

 are based neither can be interpreted. 



RESPIRATORY RATIO. 



Long before this sort of comparison was 

 made, however, a voluminous literature 

 arose which was concerned only with the 

 ratio between the carbon dioxid given off 

 and the oxygen consumed, and how this 

 ratio was influenced by temperature, by 

 light, by this kind of food or that, by mere 

 hunger, or by starvation. This ratio, the 

 so-called respiratory ratio or respiratory 

 quotient, the plant physiologists really in- 

 herited frpm the animal physiologists, by 

 whom it was devised with reference to the 

 gaseous exchange that occurs in the lungs. 

 This respiratory ratio has proved a ver'- 



