Briefly describe what happens when ammonia evaporates from a beaker that is sitting in a fume hood?

Why does the author attempt to convince the reader that what he has described of the middle colonies is factual?

The valve seat performs two functions: it provides a seal for the valve in the combustion chamber, and it provides a cool surface to carry heat away from the valve. most valves use a seating angle.

Assume that the banner needs to be 18 feet long instead of 15 feet