266 M. Becquerel on the Development 



pose all the others constant. We shall therefore act upon 

 bodies of which the polish, temperature, and hygrometrical 

 state are sensibly the same: we shall only vary the pressure. 

 It would otherwise be impossible, m the actual state of know- 

 ledge on the subject of the development of electricity, to di- 

 stinguish, in the general effect, the part which each of the in- 

 fluential causes would have had m the production of electri- 

 city, &c. 



The following apparatus fulfils the necessary conditions: we 

 take an electric balance which is placed on a horizontal shelf, 

 gg, fig. 1, covered by a glass ; this shelf is suspended by means 

 of two vertical pieces h h to the cross-piece A A of a strong 

 wooden frame A ABB. The glass cover oo of the balance 

 is pierced at its upper end by an aperture i i, through which 

 is passed a copper tube which descends within it, and which is 

 made fast to the horizontal cross-piece by strong screws. At 

 the extremity of this tube is fixed an apparatus c c, composed 

 of two small circular plates of copper, which may be brought 

 together by means of three screws, and the lower of which 

 has an aperture of two centimeti'es in diameter; between these 

 two plates is placed the body which is to be subjected to pres- 

 sure. This little apparatus communicates with the common 

 reservoir by means of a metallic chain for canning off the 

 electricity. When it is desirable to raise the temperature of 

 the body, a liquid heated to the requisite degree is poured into 

 the copper tube bb. Two apertures are also made in the 

 shelf 0-0-; one, of about a decimetre in diameter, for the inter- 

 nal use of the balance, may be closed at will by a glass capsule, 

 upon the edge of which is a bayonet joint [douille a vis); this 

 capsule is also intended to receive the substances for ab- 

 sorbing the humidity of the inside of the balance ; the other 

 aperture u u serves as a passage for a small glass tube, co- 

 vered with lac varnish, and carrymg at its upper extremity 

 the body which is to press that placed between the two circu- 

 lar plates : when it is intended to apply pressure, this tube is 

 placed, by means of a little foot, upon one of the extremities of 

 the beam of the balance, the other being suitably disposed for 

 the reception of the weights necessary for the production of the 

 pressure. The aperture made in the shelf is sufficiently large 

 not to impede its movements ; and in order to prevent the 

 communication of the external air with that inclosed in the 

 case of the electric balance, a cylinder of gold-beater's skin is 

 fixed, by means of a ring, around the aperture, in the middle 

 of which the little tube passes, and the dimensions of which 

 are such that the light skin of which it is formed does not in 

 any degree impede the movements of the little tube. The 



beam 



