606 
silver is not in all respects in accordance with the account given 
on page 598 of the formation of metallic films by evaporation; it 
might be possible that silver is actually deposited on the wall as 
molecules, but that subseqnently changes in the structure of the film 
occur, consisting in the molecules combining to complex groups, 
and that this would cause the behaviour of silver to be different in 
various Ways. 
At much thinner films than that for which the conduction of silver 
begins the glass wall showed a coloured deposit; the colour changed 
with increasing thickness in the order: yellowish brown, pink, 
violet, blue *). 
Simultaneously with the beginning of conduction the film began 
to show metallic reflection and the retlected light ceased to show 
any colours; the transmitted light continued a blue colour. 
Discussion of the results. 
In comparing the results obtained by us with PoGany’s measure- 
ments (le) on films of platinum and silver, formed by cathode- 
deposit a tolerable agreement is found to exist as regards the thickness 
at which the layers first begin to conduct. Assuming as we do 
— at least for platinum — that conduction in a film formed by 
vapour-deposit first begins, when a sufficient number of molecules 
have been deposited for a continuous layer to be formed on the 
wall, the agreement in question would show, that with cathode- 
deposition films consisting of molecules and not of complexes may also 
be formed. 
If we assume the dimensions of metallic molecules to be of the 
same order of magnitude as the value given for gaseous molecules, 
that is about 5 > 10 em., the results show, that for conduction 
a film is needed two or three molecules thick with platinum, one 
molecule with tungsten and twelve molecules with silver. 
For the thicker films the specifie resistance may be satisfactorily 
represented as a function of the thickness by a hyperbola, as already 
observed by Pocáxy (Le.). This relation may be interpreted on the 
electronic theory in the following simple manner: we shall consider 
a thin film of the metal 1 em. large and of a thickness d and we shall 
assume, that the electrons are impeded in their freedom of movement 
by the walls of the film. Let N be the number of electrons per 
cem.; the mean free path of the electrons inside the bulk of the 
metal may be called 4 and the mean velocity of the electrons 2. 
The number of collisions per second with the metallic molecules 
2 Comp. L. Hourrevieve Ann. chim. phys. S. 8. T. 20. p. 143. 1910 and L. 
Hawpurcer, Chemisch Weekblad. 1916. p. 551. 
