2 
NOTE ON THE lEON-TUENINGS CELL. 
But the electromotive force of such a cell as measured by 
Clarke’s method was by no means constant. Thus, the E. M. F. of 
a copper-turnings cell, when first set up, is very nearly equal to that 
of a Daniell element, but after running for a few minutes through 
a very short external circuit it is reduced by nearly one-half. 
An increase in the external resistance is accompanied by an 
increase in the electromotive force, and this is also the case with 
the iron cell, and with both the E. M. F. is increased by shaking 
the cell, which causes the disengagement of bubbles, which can 
be seen and heard as they come to the surface ; so that it would 
appear that, with a given current passing through such a cell, a 
polarizing layer is formed whose efficiency, and probably whose 
thickness, depends on the rate at which the hydrogen can be 
liberated in the form of bubbles, in such a manner that an 
increase of current by causing the production of hydrogen to 
exceed the rapidity of escape increases the thickness and polar- 
izing efficiency of the layer, and therefore diminishes the electro- 
motive force of the cell. 
Owing to this changeability of the E. M. F., it is difficult to 
obtain satisfactory measures of the resistance of the cell at any 
given moment. 
It is customary to attribute the liberation of bubbles to the 
action of the j)omts of the fragments of metal. The author is of 
ojnnion that this view' is untenable, and that the bubbles have 
their origin in the crevices and hollows of the surface. 
It is not possible in the case of the cell to see the growth of 
the bubbles ; and the reason for assigning the liberation of the 
bubbles to the influence of points is that bubbles of steam in boil- 
ing water, and bubbles of gas in a gaseous solution, such as soda 
•water, appear to be evolved at the points and edges of fragments 
of any foreign solid matter that may be present in the liquid. 
Now with respect to the origin of bubbles of vapour in 
boiling, it is well established that the bubbles are not formed in 
