530 Progress in Sctence. [October, 
mains, or be buried several feet deep in a bed of coke, or be attached to a 
mass of metal in moist earth, or be carried down a well; (3) Each conductor, 
if there be more than one, should make a separate earth if possible, and 
they should all be connected together below the surface. The lead roofing 
and all external masses of metal should be connected with them; (4) All 
joints and connections should be soldered. It is better that each chimney 
stack should have its own conductor, and they should be periodically 
examined to see that their points remain inta&, and that their metallic con- 
tinuity is perfect. The custom is to fix them, and then to leave them to chance. 
The precautions that are unnecessary are these :—(z) It need not be copper; 
(2) It need not be insulated; (3) It need not be carried externally to their 
disfigurement in the cases of church spires, columns, and chimneys. I never 
pass Trafalgar Square without regretting the disfigurement of Nelson’s statue. 
It is, however, better to carry the wire externally in the case of dwelling- 
houses, lest it pass too near the lead gas-pipes, which, being good conductors 
and soft metal, might be fused. The wire can go round a corner as well as 
through it, but acute angles are best avoided. The more direct the course to 
the earth the better. The area protected by a conductor is said to be that whose 
radius is equal to twice the height of the condudtor from the ground; but it 
is safer to take the radius as equal to the height of the conductor. Thus, for 
small houses one conduétor is enough, but it is safer to attach one to each 
stack. If it project a yard above the stack it is sufficient, and it is within reach 
for inspection. The stack pipes down the sides of a house are convenient 
conduits for the wire, and there is no reason why it should not be carried 
down to the ground inside them, so as to be out of sight. If there be no 
convenient stack pipe the wire can be run up and stapled to the brickwork or 
stone. With thirty shillings for materials and ten shillings for labour any 
intelligent man can, with these directions and precautions, safely protect his 
house from the destructive effets of thunderstorms. 
Mr. J. Baynes Thompson has shown that common eleétro-plating is not 
applicable to steel or iron, as these metals cannot be got perfectly clean, that 
is, chemically clean; therefore no adhering coating can be obtained. In faét, for 
all manner of plating or soldering, the first requisite is, that the two metals that 
are to be applied to each other must be chemically clean, or no proper adhesion 
can be obtained. This cleanness is obtained in various ways: in soldering 
by various fluxes; in ele@ro-plating, to such metals as that method is applicable, 
by dipping the article in an acid which will readily dissolve the metal of which 
it is made, and not only so, but the salt formed by this solution of the metal in 
the acid used must be readily soluble in water, or no clean surface can be 
obtained. There is still another condition to be considered ; that is, when the 
surface of the metal has been made thoroughly clean, it must be protected 
from conta with the air in its transit from the cleansing-baths to the solution 
wherein it is to be coated. This condition, not being recognised in the first 
attempts at electro-plating, caused many failures and much trouble, until it 
was discovered that a film of mercury prevented the contact of the air with the 
cleaned metal. Moreover, mercury has the advantage that it amalgamates 
with the metal tu be coated, and with the coating, though this amalgamation 
is not absolutely necessary ; yet it facilitates the coating of metals with other 
metals by electro-deposition, when the two metals will readily amalgamate. 
There are cases where amalgamation is not possible ; for example, where one 
of the metals will not amalgamate, as with steel or iron coated with aluminium 
or nickel; not that it is impossible to form an amalgam with these metals, for 
even steel can be amalgamated with the intervention of sodium, but it is not 
possible for plating purposes, as a diluted solution of a mercuric salt must be 
used. For all such cases as these, where the amalgamation process cannot 
be used, pyro-plating is employed, this name being given to distinguish the 
process from the eleéro-plating process, and because the coating is driven 
into the surface of the metal on which it is put by means of heat and pneu- 
matic pressure. It is not confined to coating with silver, as its name might 
indicate, but it is at present applied to coating with gold, platinum, silver, 
nickel, aluminium, copper, brass, or bronze and aluminium-bronze. ‘The 
