16 FOOD INGESTION AND ENERGY TRANSFORMATIONS. 



meter, and the expired air passing first through vessels containing 

 sulphuric acid and finally through a strong solution of caustic potash 

 to absorb the carbon dioxide. The amount of carbon dioxide exhaled 

 was found by weighing, the total amount of ventilation being deter- 

 mined from the volume of air passing through the dry gas-meter. In 

 practically all of the experiments the subject was in the sitting position. 

 A large number of tests were made, both without and with food. In 

 most of the food experiments a substantial mixed diet was used. In 

 giving his results, Smith unfortunately expressed the excretion of car- 

 bon dioxide in English grains per minute, 1 but a large proportion of 

 the original data has been recomputed by Sonde'n and Tigerstedt 

 to grams per minute, 2 and presented in their excellent collection of the 

 literature of early metabolism experiments. The average of the experi- 

 ments made on himself and with three other subjects showed approxi- 

 mately 8.78 grains of carbon dioxide per minute for an 18-hour day 

 with 3 to 4 meals. As the data obtained in the experiments without 

 food gave an average value of 6.64 grains per minute, the increment 

 after food would be 2.14 grains of carbon dioxide per minute, or 32 

 per cent over the fasting condition. In one observation Smith took 

 500 grains of arrowroot boiled in water, and found a slight increment 

 over the fasting value. Generally the maximum quantity of carbon 

 dioxide was observed in from 1 to 2 hours after the meal. 



Since Smith found, in his first paper, that the processes of digestion 

 with a mixed diet increased the metabolism by approximately 33 per 

 cent, he planned the experiments reported in his second article 3 for 

 the especial purpose of studying pure food materials. A large number 

 of food materials of all classes were studied. Certain of Smith's 

 conclusions are recorded herewith: 



"It is evident that foods may be fitly divided into two classes, viz., those 

 which excite certain respiratory changes (excito-respiratory) , and those which 

 do not. The excito-respiratory are nitrogenous foods, milk and its compo- 

 nents, sugars, rum, beer, stout, the cereals, and potato. The non-exciters 

 are starch, fat, certain alcoholic compounds, the volatile elements of wines 

 and spirits, and coffee leaves. 



"Respiratory excitants have a temporary action; but the action of most of 

 them commences very quickly and attains its maximum within one hour. 



"The most powerful respiratory excitants are tea and sugar; then coffee, 

 rum, milk, cocoa, ales, and chicory; then casein and gluten, and lastly, gelatin 

 and albumen. The amount of action was not in uniform proportion to their 

 quantity. Compound aliments, as the cereals containing several of these 

 substances, have an action greater than that of any of their elements." 4 



We can not conclude the discussion of this interesting memoir of 

 Smith's without noting that he recognized at this early stage of research 



1 15.432 grains equal 1 gram. 



2 Sonden and Tigerstedt, Skand. Arch. f. Physiol., 1895, 6, pp. 101 and 143. 



'Smith, Phil. Trans., 1859, 149, p. 715. 



*Ibid., pp. 738-7.39. 



