154 Mr. E. H. Acton. The Assimilation of Carbon 



were obtained the trials were repeated, using the modified cylinder 

 described below and figured on p. 155 (diagram No. 2). 



In testing the tissues for starch Sachs' well-known method was 

 used ; in cases where the results were negative the " potash method " 

 recommended for small quantities of the substance was employed. I 

 generally also made a micro-chemical examination of portions of the 

 tissues in addition to the direct tests. 



The bell-jar (see diagram No. 1) is accurately ground to fit the 

 glass plate, the surfaces in contact are covered with a mixture of 

 vaseline, resin, and beeswax, which is extremely tenacious, and 

 entirely prevents any access of air in this direction.* 



The india-rubber stopper is perforated with two holes, through 

 which glass tubes are inserted connected with soda-lime U -tubes ; 

 this arrangement allows a free circulation between air in the bell-jar 

 and external atmosphere, but entirely deprives any air entering the 

 apparatus of C0 2 . Any C0 2 derived from respiration of the plants 

 is at once absorbed by the KOH or soda-lime, so that the air in the 

 bell-jar is entirely destitute of C0 2 during the whole course of experi- 

 ment. 



Two vessels of water (not shown in diagram ]STo. 1) are also 

 placed under the bell-jar to prevent any chance of the air being 

 rendered too dry by the soda-lime. The water in these vessels is 

 deprived of any C0 2 which it may contain in solution by the addition 

 of baryta- water (Ba(OH) 2 ). 



Since the experiments showing a positive result are open to the 

 possible objection that C0 2 might be evolved by decomposition in the 

 solution, and be absorbed by the leaves before it was taken up by the 

 soda-lime or potash, I repeated these with the modified apparatus 

 shown on the opposite page (diagram No. 2). 



In this case any C0 2 evolved from decompositions in the culture 

 solution could not find its way to the leaves ; but, at the same time, a 

 free circulation of air is allowed between the space at top of the 

 cylinder and that in the bell-jar. Except for this modification in the 

 cylinder containing the culture solution, the apparatus is the same as 

 described above ; instead of the plant stem being simply passed 

 through a hole in cork of cylinder, the insertion is made gas-tight, as 



* Sachs, Oodlewski, &c, in similar experiments close the bottom of the bell-jar 

 by placing it in a dish containing strong KOH solution, through which they intro- 

 duce tubes (curved) to allow a free circulation of air, &c. The arrangement 

 described in the text is equally efficacious and more convenient to employ, as the 

 cylinder, dishes, &c, stand on a glass plate instead of in KOH solution. In tbe 

 first experiment with each apparatus used, a portion of the air was withdrawn from 

 the bell-jar, collected over mercury, and tested for 0O 2 by tbe ordinary metbods of 

 gas analysis at various intervals ; in all cases the air in the apparatus was found to 

 be completely free from C0 2 . 



