C. GENERAL PHYSICS. 165 



ill the experiment with gruel, and, in fact, in most chemical 

 analyses by this process, parchment paper is found to be a 

 most effective substance. Barthelemy maintains that the 

 cuticle of most vegetable substances, and especially of leaves, 

 is particularly energetic in facilitating the transmission of 

 carbonic gas. From a number of experiments made by him, 

 lie has shown that a given quantity of this gas is diffused 

 through a film formed from a portion of the leaf of a Bego- 

 nia in one hour's time, while the same quantity of nitrogen 

 required fourteen hours, and oxygen required six hours' time; 

 that is to say that the natural colloidal vegetable surfaces 

 liave for carbonic acid a diffusable power thirteen or fourteen 

 times greater than that which they have for nitrogen, and 

 six times greater than they have for oxygen. 6 B^ 1873, 428. 



THE KELATIOX BETWEEX THE PHENOMENA OF ELECTFJCITY 



AND CAPILLARITY. 



When a globule of mercury is placed in a vase of glass, 

 and surrounded by sulphuric acid, it contracts itself and be- 

 comes more convex. When we put it in metallic communi- 

 cation with an iron or copper point, which also touches the 

 acid, there is produced an electric current which polarizes 

 the surface of the mercury. Experiments show that the con- 

 traction of the surface is due to a change in its capillary con- 

 stant.' This constant is the electro motive force of the polar- 

 ization, they being functions of each other. 



Reciprocally, an extension of the surface produces, by pure- 

 ly mechanical action, a polarit}^, the same as that made by 

 the electric current. 



Lippman, to whom we owe the preceding novel remarks, 

 bases on these phenomena the idea of a machine which shall 

 transform the current of a galvanic battery into mechanical 

 work. Two masses of mercury, covered by the acidulated 

 water, served alternately as the negative electrode to the cur- 

 rent from one element of a Daniell's battery. The motions of 

 the expanding and contracting mercury act upon small tubes 

 to which their motion is communicated. 6 B, 18V3, 1408. 



THE DYNAMICAL THEORY OF GASES. 



Maxwell writes that he has recently been revising the 

 theory of gases, founded on the assumption of the collisions 



