28 



air condenses as dew on the black bulb, and at that instant the 

 readings of the two thermometers must be observed. 



118. Eudiometer to estimate oxygen in air. 

 E. 63. 1874. 



This instrument is first filled with water. The air to be examined 

 is then introduced, and the level of the water adjusted to the zero 

 of the scale marked on the tube. A few drops of water being now 

 removed from the open end of the eudiometer by a tube, their 

 place is supplied by an equal quantity of solution of potassic 

 pyrogallate, or, what is the same thing, solutions of caustic potash 

 and pyrogallic acid are separately used to refill the open end. 

 The instrument is now closed with the thumb and violently 

 shaken to promote absorption, and then opened under water so 

 that the liquid may rise and its level be ascertained. All 

 measurements must be taken after the eudiometer has been im- 

 mersed in a glass cylinder containing water, so as to counteract 

 the expansion caused by the warmth of the hand, and while the 

 water is standing at the same level both within and without the 

 tube. 



119. Diffusion tube. 

 E. 64. 1874. 



This is a tube closed at one end by a porous plug of plaster 

 of Paris, and open at the other. In using it the plug must on no 

 account be wetted. Hydrogen is the gas usually employed in 

 diffusion experiments, owing to its greater rapidity of motion. 

 The tube may be filled with hydrogen by displacement, first 

 closing the top of the tube by a cork so that no gas may escape 

 through the plug. On then placing the open end of the tube in 

 coloured water, the rise of the liquid on removing the cork 

 shows that the hydrogen escapes through the porous plug more 

 rapidly than the air enters the tube in the opposite direction. 

 Or the open end of the tube may be dipped into water, and a 

 cylinder of hydrogen lowered over the plugged end. The diffu- 

 sion of h} 7 drogen into the tube is so much in excess : of the 

 diffusion of air out of it, that bubbles are forced through the 

 water as if they were violently blown out of the tube. 



120. Oxyhydrogen blowpipe and gas bags. 

 E. 65. 1874. 



The use of these is sufficiently understood. The chief pre- 

 caution to be observed is, that the hydrogen must always be 

 ignited before the oxygen is turned on, and an excess of oxygen 

 is to be carefully avoided. Twice as much hydrogen as oxygen 

 (by measure) should be burned. 



121. Cavendish's eudiometer (so called). 

 E. 66. 1874. 



This is used for demonstrating the production of water by 

 the explosion of a mixture of oxygen and hydrogen. It is filled 

 by first exhausting it of air at the air-pump, turning off the 



