X INTRODUCTION. 



duction of carbonic acid to an object under the microscope 

 renders it requisite that we should be able to exchange it at 

 will for atmospheric air. I introduce, therefore, between the 

 wash bottle and the slide a T-shaped tube (a, fig. n.). The 

 horizontal portion of this tube lies in the axis of communica- 

 tion between the wash bottle and the slide ; whilst the cross- 

 piece is directed towards the observer. A long caoutchouc 

 tube is attached to the latter, the end of which is seized by the 

 observer between his teeth. 



Fig. ii. 



Between the T'tube and the wash flask W, a clip is intro- 

 duced. When I open the clip,* and by means of the treadle F 

 raise the flask containing acid, and thus cause carbonic acid to 

 flow into the wash flask, and at the same time compress the 

 caoutchouc tube between my teeth, the gas must pass over the 

 slide ; but if I apply the clip, and inspire through the tube in 

 the mouth, I then draw in free air from the opposite end of the 

 chamber. By this arrangement common air can be exchanged 



* The use of the clip may be dispensed with if the column of water in 

 the wash flask is high. 



