168 Dr. Draper on the Decomposition of Carbonic Acid 



stantly the horizontally of the film is disturbed; it swells 

 upwards, and is spontaneously expanded by the passage of 

 the gas through it into a bubble. The play of colour which 

 attends this experiment, and the excessive thinness which the 

 film finally assumes, render this one of the most beautiful ex- 

 periments that chemistry can furnish ; for when the bubble is 

 almost invisible by reason of its incapacity to reflect light, and 

 can only be seen in particular positions, it still discharges its 

 percolating function. 



This percolation of gases through liquids cannot be hin- 

 dered by employing oil or such other liquids as botanical 

 writers seem to imagine. Through common lamp oil, through 

 copaiva balsam, &c, hydrogen gas will escape with rapidity, 

 and protoxide of nitrogen and carbonic acid still faster. The 

 law that regulates these phaenomena is a very simple one, — 

 the gas escapes through the confining medium with a rapidity 

 proportional to its solubility therein. 



These things being understood, it is obvious that when 

 carbonic acid is decomposed in the experiments we have been 

 detailing, a variable proportion of that gas will intermingle 

 with the oxygen collected. The proportions must be variable, 

 for it depends on the amount of carbonic acid remaining be- 

 hind in the water, on the speed with which the experiment is 

 conducted, and other variable conditions. As before stated, 

 therefore, I shall leave out of consideration this carbonic acid, 

 in discussing the analysis of the collected gases, because it 

 is present by accident and is not essentially connected with 

 the phaenomena, except in one instance, where dark heat is to 

 be employed, as will be described presently. 



Analysis of Air evolved from Carbonated Water by the Sun. 



I may remark that this table contains a few out of a great 

 number of experiments, all of which might have been quoted 

 as examples of the observations which I wish to deduce from 

 it. 1st. They all coincide in this respect, that the oxygen is 

 never evolved without the simultaneous appearance of nitro- 

 gen. 2nd. That when certain leaves are employed, as those 

 of the Pinus tceda, there seems to be a very simple relation 

 between the volumes of oxygen and nitrogen. In the first 



