GAS FROM LEAVES CONTAINS NITROGEN. 109 



421. I shall have occasion to show, hereafter, that when a beam of Tight falls upon 

 any surface in contact with a medium, it causes that surface to exert an apparent pres- 

 sure on the medium, capable, at times, of producing singular effects ; it is, therefore, 

 probable that to this action we are to attribute the evolution of gas by vegetable leaves, 

 spun glass, raw silk, &c. The percolation of liquids and gases through tissues, in obe- 

 dience to the laws of capillary attraction, should also, on these principles, be controlled 

 bv the action of a solar beam. If we arrange two Champagne glasses (Jig. 58), with 

 their footstalks cut off, each capped with a thin lamina of India-rubber, their narrow 

 apertures dipping into cups of water, so that they may be in all respects as like each 

 other as possible, and fill them with protoxide of nitrogen, we shall find that one of 

 them exposed to the sunbeam will throw off its gas much quicker than the other, shut 

 up in the dark. Or, if one of them be exposed to an atmosphere much warmer than 

 the other, the liquid confining the gas rises more rapidly. It has been remarked that 

 the experiment of which I gave an account (52), of the passage of hydrogen gas through 

 a thin film without pores of sensible size, is not uniformly attended with success. In 

 examining the causes of failure, I have been able to trace them entirely to this source ; 

 at a certain temperature the effect is scarcely perceptible, but as the thermometer rises 

 it becomes more and more marked. The same observations may be made of ammonia- 

 cal vapour. There are temperatures at which these permeations are imperceptible, but 

 at 75 Fah. they take place with great rapidity. 



422. Radiant heat, whether of the sun or of terrestrial fire, impinging on the surface of 

 an obstacle, causes it to exert an increased action, resembling a force of attraction or 

 pressure, on any medium with which it may be in contact A few fibres of unspun 

 silk being immersed in water containing the elements of atmospheric air in solution, and 

 exposed to the sunshine, became speedily covered with bubbles of gas. The exact 

 chemical constitution of these bubbles is determined by a variety of circumstances: by 

 the velocity of evolution, by the solvent action of the water, which is greater for one 

 gas than another, and by the presence or absence of the chemical rays. I shall here 

 be excused for remarking a circumstance which appears to me indicative of a proneness, 

 even in capillary compounds, to exhibit tendencies of combination by multiple volumes. 

 Atmospheric air contains oxygen and nitrogen in the proportion of 1 to 4 ; the gas ex- 

 pelled from spring water contains the same elements in the proportion of 1 to 2 ; and 

 the gas given off by pine leaves from water, holding carbonic acid in solution, con- 

 tains the same elements in the proportion of 2 to 1. 



423. The chemical rays emitted from the sun are not, therefore, the cause of the 

 evolution of gas from liquids by fibres, or by vegetable leaves, for it takes place in their 

 absence ; the blue, the indigo, and the violet rays have nothing to do with it for the 

 same reason ; and the green, yellow, orange, and red are not the cause of it, for though 

 they are present, it refuses to go on. To the calorific ray we are, therefore, to impute 

 it It happens not by the action of any kind of light operating as a mere stimulus 

 on plants, for when the light is nearly absent, it goes on with undiminished energy. 



424. The evolution of gas depending, therefore, on the rays of heat, we are next led 

 to inquire whether the chemical rays affect the operation in any manner. To determine 



