PHOTOSYNTHESIS 99 



but, owing to the low solubility of the calcium hydroxide in water (only about 

 .2%), the solution is too weak to form a rapid absorbent of the gas. It can 

 be kept in tightly stoppered bottles, having an excess of lime on the bottom, 

 from which it may with caution be poured off as a clear liquid (or filtered 

 if accidently shaken). 



(4) Baryta water is a saturated solution of barium hydroxide in water, 

 prepared and used precisely as is lime-water; it absorbs carbon dioxide 

 after a similar equation, Ba(OH) 2 + CO 2 =BaCO 3 + H 2 O. It has the ad- 

 vantage over lime-water in that the barium hydroxide is more soluble (form- 

 ing about 3% solution), and hence gives a more copious precipitate of the 

 white carbonate. 



OTHER DEMONSTRATION METHODS. The above-described method is 

 logical, for both sets of tissue are from the same leaf, in natural attachment 

 to the plant, and under conditions precisely alike except as to the carbon- 

 dioxide supply; and it has also the practical advantage that by stopping 

 tightly the necks of the bottles, they may be kept indefinitely always ready 

 for immediate use, without any change of the chemicals. The method 

 depends of course upon the principle that a bottle of this size contains enough 

 atmospheric carbon dioxide to permit the making of a visible amount of 

 starch. I have also used with success another arrangement giving free access 

 to the atmosphere. It consists simply of two vials cut across the middle 

 and bent inward somewhat at the cut rims so as to hold sticks of, respectively, 

 caustic potash and chalk placed crosswise, the vials being attached to the 

 leaf as shown by Fig. 22; it is, however, less convenient, especially as there 

 is drip from the potash, and less effective for demonstration, than the former 

 method. The method most commonly described in the text-books, in 

 which separate plants or shoots are placed under bell jars which communicate 

 with the atmosphere through tubes containing, respectively, soda-lime and 

 a mechanically equivalent substance, is fallacious in theory; in fact the access 

 of the atmosphere is quite blocked mechanically, and such success as the 

 experiment exhibits is due to the amount of carbon dioxide present in the 

 non-absorbing jar. As a matter of fact this experiment is very effective if 

 only single leaves, with petioles in water, are placed in large sealed jars or 

 bottles, one of which contains soda-lime. Another method is described by 

 DETMER, 54, and another, somewhat elaborate, is given by MACDOUGAL, 

 229. Another, upon a different principle, was introduced independently 

 by STAHL (Botanische Zeitung, 52, 1894, 129) and by BLACKMAN, Science 

 Progress, 4, 1895, 30); it consists in sealing the stomata of part of a leaf 

 by a thin coating of cocoa-butter and wax, or else by vaseline; but the method 

 is logically defective in that other conditions (transpiration, respiration) are 

 also radically changed. 



The use of the carbon dioxide of the air in photosynthesis may also be 

 proven by testing the disappearance of the gas from a closed chamber in 

 which photosynthesis is taking place. This is shown very perfectly by the 

 use of the photosynthometer described a few pages later. But there is another 

 method, used somewhat widely in elementary teaching, making use of the 

 fact that if a flame be burned in a closed space, the carbon dioxide generated, 



