113 
from 6 to 18 inches long, and consequently be used with 
much longer porous cells. 
The substance has but little porosity, but I prefer having 
the plates or cylinders about two inches above the top of 
the porous cell, as they will then remain in action for 
hours without affecting the connexions. The only pre- 
cautions necessary are, after use, to immerse the plates in 
boiling water for a few minutes to abstract the acid, also 
to dry them well before a fire or in a stove, immediately 
previous to use ; if these precautions are neglected the 
acid will absorb moisture from the air, and be always damp 
and affect the connexions. Often when in haste I place 
them on the fire, and sometimes heat them red, which 
in nowise deteriorates, but rather improves their action. 
Having used the same plates and cylinders for a long 
time, no deterioration of their conducting power, nor any 
decomposition or alteration, has been observed. The con- 
nexion is made by soldering a strip of copper to the zinc, 
and pressing this very strongly against the carbon with a 
clamp. The more firmly the copper is pressed against 
the carbon the better will be the connexion. The copper 
should always be made bright where the contact with the 
carbon is made, and the surface of the carbon rubbed very 
smooth. The other parts of the carbon plate or prism 
may be left as taken from the retort or saw, its roughness 
increasing its acting surface. 
Comparing these plates with plates of platinum of equal 
size, I can detect no difference, and from their cheapness 
and durability they will make a valuable addition to our 
voltaic apparatus. With a battery of 90 plates the voltaic 
light was of the most splendid description, l^ inch long, 
and very voluminous. My garden and a large field were 
brilliantly illuminated. The trees presented a very singular 
appearance ; the leaves looked white instead of green. 
