AND THE ACTINIC CONSTITUTION OF THE ATMOSPHERE. 19 



nomeiion increases witli intensity of light np to a certain niaxinuun, difl'ering in 

 different plants ; beyond this it decreases. Starting from this point, it is natnrally 

 suggested that those actinic clouds which we have just discovered, cannot be witli- 

 out influence on vegetation, since they modify so largely, although often invisibly, 

 the strength of the chemical radiations, which is nearly, if not al)solutely the only 

 active element in the complex whole, which until now has Iteen studied under the 

 name of Luminous Iiiteusltij. And if again the plants themselves disperse into 

 the air the materials which absorb the solar, chemical radiations, how can we 

 avoid thinking that possibly the production of these odoriferous and oxidizable 

 effluvia may be for the plant a means of protection? 



To elucidate this subject fully, would require experiments which I have not 

 the time to make. 1 have been satisfie<l with examining it under various aspects. 

 Odorous and essential oils are not alone able to arrest the passage of chemical radi- 

 ations. The surfaces of plants are, as is well known, fr('(|uently covered with a fatty 

 or waxy layer, which is highly oxidizable. Thei-e are, besides, at all times fatty 

 substances in the aii', as is proved by the gi-easy feel of old dust deposited upon 

 our furniture. What effect can these fatty substances exert upon the combustion 

 of oxalic acid in the sunlight? If our ideas are correct, a slight layer of fatty 

 matter should protect that acid against solar light. 



INFLUENCE OF FATPY SUBSTANCES. 



The presence of fatty matter on the surface of our test solution bi'ings u[) a 

 slight experimental difiiculty. It is this, that solar oxidation of a fatty substance 

 is always accompanied by a production of acid which raises the titre of the oxalic 

 solution at the same time that the solai' combustion lessens it. We must, therefore, 

 either use a very small quantity of fatty matter so that it may barely form an im- 

 perceptible veil to cover the liquid, oi-, what is better still, we must spread it out 

 in a transparent layer ovei- a surface of glass interposed in the path of the luminous 

 rays. Here are some experiments made in connection with this subject: 



JExp. — On June 27, 1885, I exposed to the sun during six hours seven vessels 

 of the same dimensions, containing each 10 c. c. of oxalic acid in half-deci- 

 norraal solution. Two of these vessels, Nos. 1 and 2, had their walls 

 clean. Vessel No. 3 had been rubbed with a weak solution of butter in 

 sulphide of cai-bon, which left upon the sides, hardly tarnishing them, a 

 greasy layer. Moreover, by vii'tue of a well-known phenomenon of super- 

 ficial tension, the walls of the vessel have allowed an invisible layer of 

 fatty matter to spread on the surface of the li(piitl. In order to separate 



