294 



Dr. W. Marcet on 



[Apr. 11, 



My experiments amount to one hundred and eleven in number, they 

 relate to the quantity of carbonic acid expired, the volume of the air 

 exhaled, and the number of expirations observed within a given time. 



With respect to the estimation of carbonic acid, the principle of 

 Pettenkofer's method was adopted ; but the mode of applying it was 

 somewhat altered in order to suit my requirements. The air to be 

 analysed being first expired into a bag made of strong india-rubber 

 material, was afterwards drawn into a long glass tube of a known 

 capacity. The tubes used in these experiments held about one and a 

 half litre. The tube, fastened upright to a tripod and closed with 

 india-rubber stoppers, was first filled with a nearly saturated solution of 

 salt water ;* the bag being then connected with the tube through the 

 stopper at the upper end, the salt water was let out at the lower part, 

 thus aspiring the air from the bag into the tube. The capacity of the 

 tube being known, it became filled with a measured bulk of air to be 

 analysed. The air in the bag had been allowed to cool till its tem- 

 perature had become the same as that of the salt water in the tube, so 

 that the temperature of the air in the tube and in the bag was alike. 

 A small pear-shaped vulcanised india-rubber bag, of a capacity of about 

 100 cub. centims. and emptied of its air, was now substituted for the 

 large india-rubber bag, and a pipette holding 100 cub. centims. of the 

 normal solution of barium, and which had been run through the upper 

 india-rubber stopper, was now opened by turning a stopcock. The 

 fluid then fell into the tube, the air thereby displaced being collected 

 in the small india-rubber bag which acted as a diverticulum. A stop- 

 cock in the tube connected with this diverticulum being next closed, 

 the large analysis-tube or cylinder was removed from the tripod and its 

 contents driven from one end to the other repeatedly in order to 

 effect the combination of the carbonic acid ; the tube was then returned 

 to the tripod, and the air in the diverticulum forced into it by 

 pressure with the fingers, when the stopcock was again closed to be 

 followed by fresh agitation. The milky fluid was now drawn into a 

 small bottle holding about 100 cub. centims., which was well corked, 

 and the solution was subsequently analysed with one of oxalic acid 

 of known strength. With but few exceptions the clear fluid was 

 drawn out of the bottle for analysis with a pipette, as I was led, in the 

 course of this inquiry, to object to filtration. The above description is 

 necessarily much condensed and shortened, but the drawing which 

 accompanies the present summary will explain at one glance the form 

 of apparatus 1 employed. 



The air was breathed into the bag through a well-fitting ori-nazal 

 mask from which two tubes projected, each tube being supplied with 

 a valve carefully made by Mr. Coxeter, of Grafton Street. The valves 



* The object of salt water was to avoid the absorption of carbonic acid. I found, 

 however, in the course of the inquiry, that distilled water could safely b% used. 



