194 THE PRESERVATION OF 
IT have been guided in my researches by an important consideration, which T 
will here mention: it has been discovered, that in order to decompose one mil- 
ligramme of water, it was necessary to employ in the form of a current a quan- 
tity of free electricity equal to that of 20,000 batteries, each of one square metre 
of surface, and charged in such a way as to give out sparks flying more than 
one centimetre in length. This is the quantity supposed to be combined with 
the substance and which becomes free on the decomposition of one milligramme 
of water, or which experiences some kind of transformation, either by becom- 
ing heat or by being changed into a vis-viva, of which I have tried to deter- 
mine the effects in the cases under consideration. This much is certain, that but 
an exceedingly small portion of the enormous quantity of the electricity capable 
of producing the effects of thunder is collected ; but before arriving at this de- 
termination I have investigated with the utmost minuteness, by means of the 
compass of sines and suitably arranged apparatus, the electro-motive force of 
zine as well as that of iron, of copper, of lead, and their alloy, plunged in sea- 
water, which forces beara relation to the developed affinities and naturally serve 
as starting points for finding the protecting metal or alloy; this latter acting 
only with etliciency when the negative condition of the protected metal, which 
is derived, is superior to that which it assumes when it is attacked by salt 
water. I subsequently determined the electrical condition of all the compo- 
nents of a protected metal in order to see what became of the vis-viva, of which 
I have just spoken, and to discover the laws upon which it will be necessary 
to rely to secure protection. My mode of proceeding was as follows: 
When a copper plate 5 metres in length and 6 centimetres in width, con- 
taining a surface of 3,000 square centimetres, and armed at one of its extremi- 
ties with a very small band of zinc one centimetre square, and furnished at short 
distances with vertical rods of the same metal and other details, is plunged into 
sea water, it will be found that from the zinc to the other extremity of the plate 
the electrical condition of each point of the latter gradually diminishes, and 
if the curve of the intensities be traced, by taking for the axis of the ab- 
scissas a line which represents the electro-motive force of copper, for the abscis- 
sas themselves the distances to the zinc, and for ordinates the corresponding 
electrical conditions, this curve will appear to have the axis of the abscissas 
for its asymptote, showing that it cannot be determined how far the protection 
extends. ‘I'he whole surface of the copper retains its brightness, with the ex- 
ception of the part on the side of the zinc to a distance of about one or one and 
a half metres, which becomes covered with earthy metallic deposits when the salt 
water is not pure. In another experiment made at sea, the law has been veri- 
fied to about fourteen metres. It is evident, therefore, that in the oxidation of 
the zinc the electricity which ceases to be united with the substance, and which 
is enormous in amount, acts as a living force, when it is transmitted to the eop- 
per at distances the precise limits of which are not known. We must not for- 
get here to observe that there are in circulation over the whole metallic surface, 
by means of the liquid which moistens it, derived currents which produce elec- 
tro-chemical decompositions, and which are created at the expense of the elec- 
tricity disengaged in the oxidation of the zine. 
‘Hence we pereeive, that if we wish to protect a copper surface in sucha 
manner as to avoid electro-chemical deposits, we must arm the surface with a 
metallic protector having an electro-motive force equal to that of the point where 
these deposits begin to be insensible; this is an important condition to fulfil in 
order to prevent deposits of shells and other marine bodies, which seem to be 
formed on parts already covered with limestone, magnesia, and other substances. 
Copper plates armed with iron, and iron plates protected by zinc, present 
similar effects, with the slight difference that the sphere of electric action is less, 
it being understood that its extent depends on the difference between the elec- 
tro-motive forces of the protecting and the protected metal. 
