CHEMICAL RAYS OF DIFFERENT COLOURS. 



Allowing for unavoidable errors of manipulation, the resulting gas was, therefore, one 

 third nitrogen and two thirds oxygen, united without sensible condensation. 



432. If any farther proof was required that the evolution of nitrogen by the plant is 

 an essential part of this decomposition, it is furnished by the results of an experiment 

 in which spun glass was used to replace the pine leaves. This arrangement, though 

 exposed to the sun under the most favourable circumstances, will not evolve any gas, 

 but on passing into it a leaf, no matter how small, decomposition at once commences, 

 because the requisite quantity of nitrogen is given off. 



433. A box, a, b, e, e, of a cubical shape (Jig. 59), and nearly 12 inches in each of its 

 dimensions, had one of its sides taken out and replaced by a trough, k k, of suitable size, 

 consisting of two glass plates cemented at a distance of one fourth of an inch from each 

 other. This trough was filled with a solution of bi-chromate of potassa ; one of the 

 sides of the box was hung on hinges, e e, as a door, for the sake of obtaining access to 

 the interior. Within this little chamber, a matrass filled with carbonated water, and 

 enclosing a bunch of pine leaves, its neck dipping beneath the surface of some water 

 in a cup, was shut up and exposed to the sun's rays, which, passing through the trough, 

 impinged upon it. In a short time air-bubbles were copiously given off, and when a 

 sufficient quantity was obtained for analysis, its constitution was determined. The fol- 

 lowing is selected from a number of analyses, being probably the most correct, and very 

 nearly the mean : 



Carbonic acid .... 16-00 



Oxygen 8-16 



Nitrogen 4-84 



29-00 



We here remark the existence of a far larger proportion of carbonic acid, but the rela- 

 tive proportion of the oxygen and nitrogen is still observed with tolerable accuracy ; the 

 deviation may be satisfactorily referred to disturbing causes. The greater amount of 

 carbonic acid, as compared with sections (427) and (431), may likewise be due to the 

 higher temperature of the arrangement when shut up in a close box, where currents of 

 air, or other cooling agents, could not have free access to it. Or it may hereafter be 

 found that there are chemical rays of different colours, as it were ; or, more strictly, of 

 different refrangibility and absorbability, and that those which find a passage through bi- 

 chromate of potassa can cause the decomposition of carbonic acid, though they cannot 

 blacken chloride of silver. The doctrine that chemical rays are nothing more than un- 

 dulations of an elastic medium, the waves of which vary in breadth, I shall endeavour 

 to support ; each of these kind of waves is competent to bring about changes peculiar to 

 itself. Not in this place, however, to anticipate what I have to offer on these matters, 

 I shall continue to use the term chemical rays as expressing those which blacken chlo- 

 ride of silver ; and these, I say, are not engaged in the decomposition of carbonic acid. 



434. From the first observations made on the decomposition of carbonic acid by 

 PRIESTLEY, this subject has afforded much scope for chemical speculation. Count RUM- 

 FORD examined it successfully, but wanting means of accurate gaseous analysis, and, 

 above all, not understanding the doctrine and laws of interchange through tissues, his 



