By now you've probably run into one of the curbside nitrogen tanks in the city. Not too long ago we got an email inquiring about them, a reader wanted to know about "the random nitrogen tanks everywhere at the corner of city blocks. They are always bubbling and oozing out. Are they dangerous?" There are many guesses as to why they are there (to asphyxiate rats, to reduce odors, to scare off tourists), but most......