398 



LECTURE XXV. 



a very heavy gas which does not immediately pass out of the opened cylinder : hence 

 the result of the experiment. In exactly the same manner we may convince ourselves 

 by the same means of the respiration of large developing Fungi, as well as of mould 

 growing on bread or on a liquid. The enormous energy with which growing plants 

 absorb the oxygen in their neighbourhood, and give it back as carbon dioxide, is 

 particularly conspicuous in that the whole of -the oxygen in an absorption-tube is 

 made use of for respiration, as follows at once from the fact that the carbon dioxide 

 absorbed by potash after the experiment corresponds exactly to the volume of the 

 oxygen originally present, unless, indeed, it happens that by means of intra-molecular 

 respiration, to be described later, an excess of carbon dioxide is found. 



For the purpose of more exact studies as to the carbon dioxide formed by 

 respiration, the apparatus here figured may be employed. The two bottles/" and g 



serve as an aspirator, since the 

 ^ water flows out from y down ta 



g, whence of course the air which 

 enters at (on the right) must 

 pass through the various vessels 

 of the apparatus. It is first freed 

 from any small quantities of car-f 

 bori dioxide in the vessel a, which 

 contains pumice-stone saturated 

 with potash : if the lime-water in 

 the iiask i remains clear, it proves 

 that this is accomplished. The 

 air thus comes into the receiver 

 c quite free from carbon dioxide. 

 In this receiver is a trough h 

 covered with wide-meshed gauze, 

 which touches the surface of the 

 water contained in the trough. 

 On the gauze lie 20^30 germinat-i 

 ing seeds of Peas, Wheat, &c., or 

 unfolding buds or flowers or suitr 

 able vitally active parts of plants generally, from which carbon dioxide is evolved 

 by respiration. The receiver c is fixed air-tight on to the glass-plate k. The 

 air, now laden exclusively with respired carbon dioxide, streams through the two 

 flasks d and e containing lime-water. The carbon dioxide is nearly all absorbed 

 in d^\. e. a white precipitate of calcium carbonate is formed — and usually, when 

 the air only streams slowly through the apparatus, scarcely any further precipii 

 tate of calcium carbonate results in the second flask e. Should this be the case, 

 however, a third flask must be interposed. The calcium carbonate is now collected 

 on a filter, and from its weight the quantity of carbon dioxide developed in the 

 plants is calculated. Another form may also be given to the apparatus by replacing 

 the two vessels a 3 as well as the flasks d e with Liebig's bulbs filled with potash 

 solution. The chief point is that in this apparatus new air containing oxygen. 15 

 continually supplied to the plants, and the carbon dioxide formed is removed, so that 



FIG. 252. 



