88 CALORIMETERS FOR STUDYING RESPIRATORY EXCHANGE, ETC. 



ABBREVIATED METHOD OF COMPUTATION OF OXYGEN ADMITTED TO THE 

 CHAMBER FOR USE DURING SHORT EXPERIMENTS. 



Desiring to make the apparatus as practicable and the calculations as 

 simple as possible, a scheme of calculation has been devised whereby the 

 computations may be very much abbreviated and at the same time there 

 is not too great a sacrifice in accuracy. The loss in weight of the oxygen 

 cylinder has, in the more complicated method of computation, been con- 

 sidered as due to oxygen and about 3 per cent of nitrogen. The amount 

 of nitrogen thus admitted has been carefully computed and its volume 

 taken into consideration in calculating the residual oxygen. If it is con- 

 sidered for a moment that the admission of gas out of the steel cylinder is 

 made at just such a rate as to compensate for the decrease in volume of the 

 air in the system due to the absorption of oxygen by the subject, it can be 

 seen that if the exact volume of the gas leaving the cylinder were known 

 it would be immaterial whether this gas were pure oxygen, oxygen with 

 some nitrogen, or oxygen with any other inert gas not dangerous to respira- 

 tion or not absorbed by sulphuric acid or potash-lime. If 10 liters of 

 oxygen had been absorbed by the man in the course of an hour, to bring the 

 system back to constant apparent volume it would be necessary to admit 

 10 liters of such a gas or mixture of gases, assuming that during the hour 

 there had been no change in the temperature, the barometric pressure, or 

 the residual amounts of carbon dioxide or water-vapor. 



Under these assumed conditions, then, it would only be necessary to 

 measure the amount of gas admitted in order to have a true measure of 

 the amount of oxygen absorbed. The measure of the volume of the gas 

 admitted may be used for a measure of the oxygen absorbed, even when it is 

 necessary to make allowances for the variations in the amount of carbon 

 dioxide or water-vapor in the chamber, the temperature, and barometric 

 pressure. From the loss in weight of the oxygen cylinder, if the cylinder 

 contained pure oxygen, it would be known that 10 liters would be admitted 

 for every 14.3 grams loss in weight. 



From the difference in weight of 1 liter of oxygen and 1 liter of nitrogen, 

 a loss in weight of a gas containing a mixture of oxygen with a small per 

 cent of nitrogen would actually represent a somewhat larger volume of gas 

 than if pure oxygen were admitted. The differences in weight of the two 

 gases, however, and the amount of nitrogen present are so small that one 

 might almost wholly neglect the error thus arising from this admixture of 

 nitrogen and compute the volume of oxygen directly from the loss in weight 

 of the cylinder. 



As a matter of fact, it has been found that by increasing the loss in 

 weight of the cylinder of oxygen containing 3 per cent nitrogen by 0.4 per 

 cent and then converting this weight to volume by multiplying by 0.7, the 

 volume of gas admitted is known with great accuracy. This method of 



