The bubble is larger in winter since the thermal coefficient of expansion is greater for the liquid than the glass. The volume of the liquid is reduced more by cooling than the volume of the glass container and the space occupied by vapour increases. In summer, on the other hand, the liquid expands when the temperature rises, and since the vapour is easily compressed under pressure, the volume occupied by the vapour decreases in size.
This can be observed by experiment. Take a flask, fill it with water and bring the water to boiling-point. Part of the water will be driven out of the flask during this. Cool the flask slightly so that it can be corked and so that the water fills it entirely. Lay the flask horizontally. As the flask cools further a bubble will appear above the water and if the cooling process be continued, the dimensions of the bubble will increase.
154. The internal pressure on the cork equals the weight of the column of mercury above the level of the opening and this is less than the pressure of the atmosphere which balances the whole column of mercury. Consequently, if the cork be removed, air will enter the tube and the mercury will fall till its level in the tube is the same as its level in the vessel.