1871.] 



Constant Form of Darnell's Battery. 



257 



The accompanying drawing represents a design suitable for the electric 

 light, or other purposes, for which an interior resistance not exceeding T ? 

 of an Ohm is desired. The zinc is in the form of a grating, to prevent 



the lodgment of bubbles of hydrogen gas, which I find constantly, but very 

 slowly, gathering upon the zincs of the cells I have tried, although the 



solutions used have no free acid, unless such as may come from the ordl 

 nary commercial sulphate of copper and commercial sulphate-of-zinc 

 crystals which were used. 



Postscript. 



Received February 2, 1871. 



The principle which I have adopted for keeping the sulphate of copper 

 from the zinc is to allow it no access to the zinc except by true diffusion. 

 This principle would be violated if the whole mass of the liquid contiguous 

 to the zinc is moved toward the zinc. Such a motion actually takes place 

 in the second form of element (that which is represented in the drawing, 

 and which is undoubtedly the better form of the two) every time crystals 

 of sulphate of copper are dropped into the charging-tube. As the crystals 

 dissolve, the liquid again sinks, but not through the whole range through 

 which it rose when the crystals were immersed. It sinks further as the 

 sulphate of copper is electrically precipitated on the copper plate below in 

 course of working the battery. Neglecting the volume of the metallic 

 copper, we may say, with little error, that the whole residual rise is that 

 corresponding to the volume of water of crystallization of the crystals which 



VOL. xix. x 



