Alain schrieb:
> Back then, the outer wall of any brick or granite house more than 5 
> years old where mostly black.
> Now, those same walls have recovered ther original colours, mostly due 
> to the rain.

This is how the spires of the Cologne Cathedral look like today:

   http://en.wikipedia.org/wiki/File:Koelner-dom-spire.jpg

And this is a photograph from ca. 1880:

   http://en.wikipedia.org/wiki/File:Detail,_Cologne_Cathedral_Tower.jpg

Note that the cathedral is - at least in parts - 750 years old. The dark 
color seen today is almost exclusively from soot, probably most of it 
from the nearby central railway station (the cathedral is just some 100 
meters from the platforms).

The soot has "eaten" its way so deep into the stone that any attempt to 
wash it off is futile. Only in the process of restoration of other 
damage does the cathedral regain its original bright color again in some 
sections.

Man, that must have been a beautiful building!

> And, now, old peoples that lived during that time are complaining about 
> the polution levels we expeiencing now... And say that in ther time 
> there was no polution.

Well, the pollution was entirely different. I'm not sure whether it was 
any better or worse back then: I'm sure it was more obviously bad, but 
given that we're consuming far more energy these days, I guess the grand 
total of today's emissions is just as bad.

For instance, there appears to be a correlation between particulates 
(which weren't that much of a problem in historic times, most of the 
emitted sold matter having been bound in larger, visible particles) and 
pollen allergies (i.e. people seem to more readily develop hayfever in 
regions with strong particulate pollution).

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