Really basic question on Pressure 
*

*I'm reading this book.It says: Doubling the gauge pressure does not double the amount of air in the tank. But doubling the absolute pressure does.

I don't understand why...when : $\text{Absolute}\ P= \text{Gauge pressure}+ \text{atmospheric}\ P$ ?


*Also why is pressure a scalar when it always perpendicular to the surface.
It's acting perpendicular like a local thing and would change if the body moves.Is that the reason?

 A: Let's just use the ideal gas law.  Let $P_{ab}$ = absolute pressure, $P_g$ = gauge pressure and $P_a$ = atmospheric pressure.  $$P_{ab} V = NkT$$ From this is it obvious if I double Pab and keep the volume the same, then N must double.  Now write Pab as $$(P_g + P_a)V = NkT$$ If I only double $P_g$, then N does not double.
Your question about pressure being a scalar has been answered by @Ashwin Balaji.
A: The reason that pressure acts normal to all surfaces is that, in reality, it is not actually a scalar.  At static equilibrium, the (2nd order) stress tensor is isotropic, and the parameter we call pressure is the magnitude of this stress tensor:  $$\boldsymbol{\sigma}=-p\mathbf{I}$$where I is the so-called (isotropic) identity tensor or metric tensor.  The identity tensor when contracted with a unit normal $\mathbf{n}$ to a surface is equal to the unit normal itself.  So, according to the Cauchy stress relationship, the stress vector acting on a surface of orientation $\mathbf{n}$ is equal to $-p\mathbf{n}$.
A: The gas does not 'know' what its gauge pressure is, it only knows what its absolute pressure is.
Suppose you are at sea level, and opened a valve on a tank so that it equilibrated at a pressure 1 atmosphere absolute (= 0 gauge).
You then closed the valve to seal the tank (still 1 atmosphere absolute, 0 gauge).  You then carry the tank to the top of Everest, which has only 30% of sea level atmospheric pressure.  (Assume the temperature stays the same.)
The absolute pressure in the tank is still 1 atmosphere.  The gauge pressure has now increased from 0 to 0.7 atmosphere.  But despite the increase in gauge pressure there is no more air in the tank than there was before.
The gas, sealed in the tank, has no idea that anything has changed, and happily occupies the same volume as before.
