I have read about the experiment of Torricelli, he filled a tube with mercury and placed it standing in a basin filled with mercury, then mercury poured out into the basin from the tube. He concluded that a vacuum was created.

The problem is I don't think that there is enough evidence to reach this conclusion. Maybe some air came through the mercury and filled the vacuum. Or maybe there is a space between the glass tube and mercury so air could come through that space.

How can one know that air can not move through the matter or there is no space between mercury and glass tube?

I would be glad if you answer this question without the assumptions of atomic theory because I don't know much about it. Thanks.


1 Answer 1


It was known that air was a substance which could exert wind pressure and be compressed. Torricelli reasoned that it would have a natural pressure of its own.

He then reasoned that his mercury tube was long enough that the weight of the mercury was greater than the atmospheric pressure, so when he upended it the mercury could no longer be held in and would fall, even if there was no air to replace it.

He tried it, sticking the bottom end in a bowl of mercury to stop air bubbling up round. It fell a bit but then stopped at around 75 cm tall.

If air had been able to get in, then the tube would have emptied steadily. The experimental observation that the level stays permanently around 75 cm is proof in itself that air cannot diffuse through mercury. This answers part of your question.

Having established that the gap could not be filled with air, Torricelli reasoned that there was nothing else it could be filled with either. There was genuinely nothing there, a vacuum. (In practice there is a small amount of mercury vapour, but it is so small as to be insignificant here, and would not be measured for centuries yet. Torricelli's conclusion was correct by the science of the day). This answers the rest of it.

Thus, he could use the height of his dense mercury column to calculate atmospheric pressure. He had invented the mercury barometer.

You can do the same thing with water. But you need a tube of 10 m or more length to get the same weight, because it is so much less dense. And over the following days and months the level will very slowly fall, because air can dissolve in water and diffuse through it.

  • $\begingroup$ Thanks for your answer. But the statement'' İf air had been able to get in, then the tube would have emptied steadily'' might not be true if you don't assume that the air in the gap would have the same pressure as the atmospheric air. Maybe the air in the gap exerts less pressure than atmospheric air because the glass tube might prevent the air above the gap from exerting pressure to the mercury or maybe there isn't enough air in the gap to make a noticeable pressure effect. $\endgroup$
    – Umut
    Oct 31, 2020 at 10:28
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    $\begingroup$ I agree that a stable height would indicate that air was not leaking in, however we know now that there would be a small amount of mercury vapor in the top of the tube. $\endgroup$
    – R.W. Bird
    Oct 31, 2020 at 14:08
  • $\begingroup$ @Umut I think you have your reasoning backwards. Torricelli left no way for air to get in, so it could not have filled the gap when the gap first opened. That was the whole point of the mercury bath. Moreover we "don't assume that the air in the gap would have the same pressure as the atmospheric air", quite the opposite, we deduce a difference of one atmosphere of pressure. $\endgroup$ Oct 31, 2020 at 15:12
  • $\begingroup$ I'd guess that experiments with water and air preceded Torricelli's experiment with mercury. Immerse a glass bottle under water then raise the bottle above water while keeping the lip below water. Then the experimenter can watch the water level drop as he bubbles air into the bottle. $\endgroup$
    – MaxW
    Oct 31, 2020 at 17:56
  • $\begingroup$ Hand piston pumps date at least back to the 1400's so the fact you can't pump water from a depth below 7-8m was probably known long before vacuum was discovered... $\endgroup$
    – bobflux
    Oct 31, 2020 at 23:23

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