Closed Manometer Question | Get the Answer

[chemistry1]

Hi, Just have a basic question on a closed manometer :

http://postimg.org/image/6tq089eoz/

So I do understand the basic idea behind manometers. I see that in this case the pressure of gas is lower than that of the liquid, which is why the liquid is higher on the left side. Now, what I don't understand is why do we say that the gas as a negative pressure ? Wouldn't that mean there's no gas at all ? Also, how do we know that it's negative ? Usually, I only take the difference between the highest point and the lowest ...
Can somebody help ? Thank yoU!

 

[NascentOxygen]

If it's an open-tube manometer, and the gas pressure is less than atmospheric, you could say the gas has negative pressure [relative to atmospheric pressure].

I can't see the term "negative pressure" being directly applicable to the closed-tube manometer, where the pressure is measured relative to a vacuum*.

There is plenty of interesting material on the web, e.g., http://www.kentchemistry.com/links/GasLaws/manBar.htm