108 DECOMPOSITION OF CARBONIC ACID IN LIGHT OF DIFFERENT COLOURS. 



exist nnder, and can be separated by, the force of capillary attraction. An example 

 will here illustrate what is meant. Oxygen and hydrogen gases may be mingled with 

 each other in the proportion of one to two, the result existing in a compressed state, 

 and forming a capillary compound ; the contact of flame, or the passage of an electric 

 spark, changes it into aqueous gas, a chemical compound ; in the former state, de- 

 composition is rendily effected by capillary attraction ; in the latter it cannot produce 

 such a result. The general law of these decompositions by tissues without pores of 

 sensible size is given in Chapter IV. ; it is a very simple one, showing that a capillary 

 equilibrium is gained only when the composition of gaseous media on each side of a 

 barrier is chemically the same. This was proved by exposing extremely thin soap bub- 

 bles, filled with different gases, to different gaseous atmospheres, and then measuring 

 and analyzing the media within the bubble and without. This law is applicable not 

 only where a barrier separates a gas from a gas, but also when one of the gases is held 

 in solution by water; and in the energy with which the media endeavour to attain an 

 equilibrium, is to be found not only one of the causes of the decomposition of carbonic 

 acid by the light of the sun, but also a very fruitful source of erroneous experimenting. 

 Having made reference to this matter, I proceed to detail the steps which have been 

 taken to illustrate this decomposition. 



419. Take four globular vessels, such as a {fig. 56), three or four inches in diame- 

 ter, with necks a couple of inches long; fill them with spring water, and put a bunch 

 of pine leaves in it; immerse the end of the neck beneath the surface of the mercury 

 contained in a cup, b. Let one of these vessels, designated A, be exposed to the sun's 

 direct ray ; a second, B, to the light which has passed through a solution of bichro- 

 mate of potassa; a third, C, to the light which has passed through a solution of sul- 

 phate of copper and ammonia ; and a fourth, D, in a dark place. It will be found that 

 in the course of a few hours, A has eliminated most gas, B somewhat less, and C and D 

 none at all ; this is a very instructive experiment ; we find from A and D that the sun's 

 rays have the power of eliminating gas from solutions ; from B we learn that the ab- 

 sence of the chemical rays does not affect the apparent result, but that if the calorific 

 rays are obstructed, it ceases to go on. 



420. A variety of experiments having thus convinced me that the mere evolution of 

 gas is neither due to the rays of light, nor to the chemical rays, I have attempted to 

 produce a like effect with the calorific rays emitted from a common fire ; rays in which 

 the light was altogether disproportioned to the heat, and the chemical power totally 

 wanting. The arrangement is as follows: in the focus of a concave speculum of brass 

 eighteen inches in diameter, I placed one of the glass globes of the preceding section, 

 so that it might receive the rays emitted from a common wood fire, converged on it by 

 the mirror. The fire was burning without flame, being what is technically called a 

 dead fire, and the distance of the mirror eight feet. In a few moments gas was copi- 

 ously liberated, more copiously than if it had even been exposed to the solar ray. In 

 fig. 57, this arrangement is depicted : a is the concave mirror, b the glass matrass filled 

 with spring water, and containing a bunch of pine leaves, c a cup of mercury into which 

 its neck dips. 



