cuiy, and allowing all but 5 or lo per cent, of this to be replaced 

 by air, and the remaining space by carbon dioxide. After expos- 

 ing the plants to sunlight for a given length of time, the air in 

 burette is forced over into the potash bulb, and after a short 

 period returned. This is accomplished by the pressure of mer- 

 cury or water, whichever happens to be used. The difference in 

 volume is then noted and from this is calculated the percentage 

 of carbon dioxide absorbed. Either water or mercury may be 

 employed, and when the former is used we usually take the 

 water from a reservoir suspended five or six feet above the appar- 

 atus, in which case we regulate the output of water by the stop- 

 cock shown at Cm Fig. i. We seldom allow the contents ol 

 the burette to go below the 83 or 84 c.c. mark. In using water, 

 a small portion of the carbon dioxide is likely to become absorbed. 

 The absorption of carbon dioxide, however, can be largely 

 prevented by a drop of oil on the surface of the water. In case 

 mercury is used, no such precaution is necessary. It has been 

 our practice to allow students to make a few analyses of the car- 

 bon dioxide, previous to placing the plants in the bulb, in order 

 that they may become familiar with the method and test the ac- 

 curacy of the same. We prefer very small apertures in the two- 

 way stop-cock ; this makes the apparatus much easier to work, 

 and there is less opportunity for leakage. The special bulb 

 burette is made by E. Greiner, of New York. 



When it becomes necessary to make experiments with potted 

 plants, we have used for some years the apparatus represented in 

 Fig. 2. This consists of a bell glass set in a paraffined wooden 

 trough filled with mercury. The potted plant to be experi- 

 mented with is covered tightly with thin rubber sheeting, which 

 permits only the leaves and upper portions of the stems to be 

 exposed. There arc two wide-mouthed tubes, one inside the 

 bell glass, //, and one outside, /, which contain water. These 

 are connected with a U-shaped tube below, with clamps at a, />, 

 and c. 



In supplying the apparatus with carbon dioxide, the generator 

 is attached to one of the inlet tubes at the top of the bell glass, 

 and the inner tube, //, which is completely filled with water, is 



