288 LECTURES TO SCIENCE TEACHERS. 



by changing the position of my eye, read the mercury in 

 this tube as high as twenty-four or as low as twenty-two. 

 There are two ways by which this difficulty can be 

 avoided ; one is by the mirror method of observing. You 

 take a piece of looking-glass and hold it against the tube, 

 the tube being practically straight and the looking glass 

 being practically plane, the two surfaces will come in 

 contact, and by placing the eye in such a position that the 

 pupil which is visible on the looking-glass is divided into 

 two parts by the surface of the mercury, you are quite 

 certain that the eye is at the proper level. The only 

 objection to this method is that you have to come so close 

 to the instrument that the hand may raise its temperature ; 

 it is, therefore, preferable to use such an instrument as the 

 cathetometer, which enables you to read from the other 

 end of the room. This method of mirror-reading is intro- 

 duced into one of the eudiometers I have here, in which 

 the mirror is graduated instead of the tube, and you read 

 off the level of the mercury by observing at the same time 

 your eye and the surface of the mercury, and being quite 

 certain that it is at the proper level. 



The absorption of gases by Bunsen is carried out by 

 means of solid reagents. He takes the reagent and fuses 

 it in a small tube or crucible, and then pours it into a 

 bullet mould. This mould is conveniently arranged with 

 a small notch filed at the lower part, through which a 

 platinum wire is passed, upon which the reagent is cast so 

 that you obtain a wire with a bullet at the end of it. 

 These wires here have on the ends bullets of caustic 

 potash, and are used for the absorption of such gases as 

 carbonic anhydride. I will show this by way of compari- 

 son, because afterwards I will show you the mode of 

 absorbing the same gas by means of a liquid reagent. 

 This is only a test tube in proper gas analysis one would 

 use an absorption tube, which is longer, and which would 

 rest against this Y-shaped support. This is a trough such 

 as was used by Bunsen, made by Desaga, but it does not 

 at all fulfil Bunsen's requirements. There must be a place 

 on which the eudiometer is supported, so that when the 

 gas is introduced there is no fear of the tubo falling to 

 the bottom. This is effected by a small projection with a 

 notch in it, on which the tube will rest so that there is no 



