10H Dr. W. Marcet. On the Chemical [Jan. 1.% 



re-breathed ; Srdly, air expired immediately after re-breathing ami 

 witb the inspiration of fresh air; 4thly, air breathed while no longer 

 under the direct influence of re-breathing. 



The above is a general sketch of the investigation. I must nmv 

 beg leave to go into the details of the work. The diagram on the 

 next page illustrates the disposition of the instrument. 



The person under experiment, in the recumbent position in a deck- 

 chair, held in his right hand an india-rubber tube connected with the 

 bell-jar through a double-way cock, as I have explained in my last 

 paper to the Royal Society. The cock was turned in such a 

 position that the air inspired through the nose was expired into 

 the open air, a little flag showing the movement of the expired air 

 through, the tube. The experiment begins with the operator expiring 

 through this tnbe into the external air. When respiration has 

 become perfectly quiet and regular, the double-way cock is turned 

 during an inspiration, and the air of the next expiration is collected 

 in the bell- jar, which begins its ascending course. At the same time 

 the hands of a chronograph are set in motion. After about 36 litres 

 of air have been expired, the tube leading into the bell-jar is closed 

 at the end of an expiration, and the chronograph is stopped. 



Some trouble was experienced in obtaining similar volumes of air' 

 expired in a given time, say, about every seven minutes. I have 

 come to the conclusion that most people do not breathe, even when 

 perfectly still, exactly the same volume of air in a given time, and 

 after an experience of many years, it was found that the best plan 

 was to repeat the breathing two or three times, or more, in succes- 

 sion, and to take, according to circumstances, either the mean of the 

 different experiments, or the result of the last made. Any experiment 

 differing widely from the others was rejected. The air collected 

 finally was read off on the scale attached to the bell-jar, its tempera- 

 ture was taken, the barometer read, and the air was analysed for the 

 determination of its carbonic acid, by the same method as that which 

 has been described in the paper in the ' Philosophical Transactions ' 

 already referred to. The air left in the bell-jar was then driven out, 

 and the jar made ready for further use. 



The next part of the work is the re-breathing. Perhaps half an 

 hour has elapsed since air was first collected for the determination of 

 CO 2 in normal breathing ; during that time the person under experi- 

 ment has remained perfectly still in the deck-chair. The bell-jar, 

 which has not yet been used, is now thoroughly rinsed with common 

 air and filled with air to the extent of 35 litres. It carries an indi:> 

 rnbber tube, connected with the dome of the jar and supplied at thn 

 end with a fork-shaped nose-piece (see diagram). This nose-piece 

 has been ascertained to fit the nostrils of the operator, air-tight ; a 

 second india-rubber tub?, with a mouth-piece at one end, is connected 



