1878.] the Function of Respiration at Various Altitudes. 295 



were so constructed as to let in the external air for the inspiration, and 

 transmit the inspired air to the bag. 



There was nmch more difficulty than I had anticipated in determin- 

 ing the capacity of the bags used for collecting the air expired. This 

 was done by filling the bag with a bellows under a certain pressure, 

 and then measuring the bulk of air by means of a water aspirator, the 

 amount of water necessary for aspiring out the whole of the air being 

 carefully measured ; the capacity of each bag was tbus ascertained a 

 number of times, the probable approximate error being finally esti- 

 mated at half a litre. Two of the bags mostly used in last summer's 

 experiments had been found to contain 39 "3 litres of air each under a 

 pressure of one inch of water, another held 46' 5 litres of air under 

 1*5 inch of water. Other bags about the same size were employed, 

 and one still larger was used on a few occasions. 



The use of india-rubber bags, although made of strong material, 

 appeared to me at first open to a great objection from the known 

 diifusibility of carbonic acid through caoutchouc, as shown by Thomas 

 Graham ;* but in the course of the present inquiry I satisfied myself 

 that no appreciable loss, at all events, no loss serious enough to impair 

 my results, occurred through the bag in the course of an hour, and as 

 much as three-quarters of an hour seldom elapsed between the filling 

 of the bag and the introduction of the air into the analysis- tube. 

 The experiments were made either sitting, or walking on level ground, 

 or walking up hill; most of them were made sitting. They were 

 always conducted in the open air, and no smoking was allowed near 

 the spot while the bag was being filled. The posture selected was 

 assumed a few moments before the experiment was commenced, 

 so as to bring the body under the proper physiological condition. 

 The experiments in the sitting posture were conducted in the 

 following way : — I first placed the empty bag in a woollen jacket to 

 preserve the temperature of the air expired and allow of its being 

 taken, then the bag was connected through a neck and india-rubber 

 tube with a water gauge fastened to a post, and a thermometer was 

 slipped into the bag through another neck. An assistant, and my 

 guide while in the mountains acted as such in a most commendable 

 and intelligent way, stood ready by me with a stop-watch registering 

 the minutes and seconds, and at a preconcerted signal set the watch 

 in motion. t At the same time I began expiring into the bag, holding 

 the mask to the face with my hand. When the bag was fall, and the 

 water in the gauge up to a certain mark, the timepiece was stopped. 

 Thus, the time to expire a certain volume of air was determined. The 

 temperature of the air inside the bag was then read off, and the jacket 

 removed from the bag, the latter being next agitated in the air or 



* " Phil. Trans.," 1866, 



f I frequently attended myself to the watch. 



