The density of (1) mercury is 13.6 times that of water, so for the same pressure difference, the height difference when using water is 13.6 times that when using mercury, so the height difference when using water is large and the phenomenon is obvious;

(2) When the glass bottle is placed on the platform 1.2m above the ground, the atmospheric pressure drops by 0. 1mmHg, and the water level in the capillary rises by 0.1mm×13.6 =1.36mm;

(3) The atmospheric pressure is 750mmHg, and the gas pressure in the bottle is: p1= 750 mmHg+20413.6 mmHg = 765 mmHg;

According to the ideal gas state equation, there is p1v1t1= p2v2t2, which is obtained by substituting the data: 765×V300=P2V30 1+0, P2 = 767.55mmhg; ;

So the atmospheric pressure at point B is Pb = P2? Ph=767.55mmhg？ (204+40.8)mmhg 13.6 = 749.55 mmhg；

So the height difference between A and B is △ H = (750-749.55) ×12 = 5.4m;

So the answer is: (1) The density of water is smaller than that of mercury, and the height difference is obvious when the pressure changes. (2) It rises,1.36; (3)5.4.