62 i>r. Marcet. Absorption of Oxygen and [June 4, 



Before the hydrogen was nsed it was tested in nearly every case 

 with atmospheric air. 



The determinations of oxygen were made as follows : The \J -tube 

 was first of all filled to overflowing with mercury through its open 

 limb, the three-way cock being turned so as to let out the mercury 

 after filling the tube ; by this means every trace of gas was driven out 

 of the tube. 



Then the stop-cock was turned so as to close the eudiometer and 

 let the hydrogen gas through it, and the instrument having been 

 brought into connexion with the gas-holder by india-rubber tubing, 

 the hydrogen, under a pressure of an inch or more of water, was 

 driven rapidly through the stop-cock, being, moreover, aspired by 

 the dilatation of an india-rubber syringe. The cock was now turned 

 so as to admit the hydrogen into the eudiometer, and the mercury, 

 being let out at the bend of the \J -tube, aspired the required volume 

 of hydrogen into the instrument ; this amounted to from 18 to 20 c.c. 

 The bell-jar was then placed under atmospheric pressure, the gas 

 turned off, and the height of the mercury if not exactly the same in 

 both limbs was adjusted by adding or withdrawing mercury until the 

 readings were alike. 



The air to be analysed for the determination of oxygen had been 

 collected by displacement with water in a cylinder holding from 1 to 

 1*5 litre, and shaken with a solution of barium hydrate, to rid it 

 entirely of its carbonic acid. The cylinder was now placed on a stand, 

 over which was disposed a glass receiver full of water, communicating 

 by india-rubber tubing with the lower end of the cylinder. The india- 

 rubber tube was carefully filled with water, so as to let no air into 

 the cylinder, and then the cylinder was placed in communication 

 with the eudiometer. The next stage was to wash out the passage 

 in the stop-cock with the air to be analysed ; this was done by con- 

 necting by india-rubber tubing the three-way cock with the cylinder, 

 and some 200 or 300 c.c. of air were driven through it from the cylinder 

 by the pressure of the water in the receiver; the iron tnbe was 

 now stoppered on the opposite side by a short rubber-tube and 

 pinch -cock. The mercury in the eudiometer was next let out at the 

 bottom until a sufficient diminution of pressure had been obtained to 

 aspire the requisite volume of air from the cylinder. This air was 

 admitted from the cylinder while under pressure, and taken into the 

 eudiometer by aspiration, so that the effect produced was that of a 

 piston driving the hydrogen before it, and giving it no time to diffuse 

 out of the instrument. The height of the mercury was now adjusted 

 in both limbs by pouring in mercury, or letting it out at the bottom, 

 and finally the reading was taken and recorded. The air and hydro- 

 gen were now thoroughly mixed by fitting an india-rubber syringe 

 to the open end of the eudiometer and pressing it with the hand ; a 



