Experiences

From the idea of horror vacui to the concept of atmospheric pressure

Paola Falsini

Italy

High Secondary School

Physics

Teaching in class, Laboratory

Direct

Below you can find some negative aspects that the current didactic course aims to remove:
- students are used to memorise scientific notions, but they lack in knowledge about “the way to make science”;
- among this list of notions students do not manage to distinguish between facts, hypothesis and interpretation;
- students are unable to abandon some common sense supposition (in the current case the theory of horror vacui).
For the students of the first two years I propose some easy experiments to be implemented at home and in the Physics lab: for instance, to separate the bellows arms plugging the hole, to drink something with the straw with the hole plugged, to pour through a syphon a liquid between two containers. Then I highlight that, up to 17th century, these events were explained by the idea that the nature horrified of the empty spaces and thus it does everything to fill them.
However, already at that time, it was well known an event contradicting this theory: it was possible to take the water up to 10.3 meters high. Watching some videos on the phenomenon, it can be discussed the reason of its incompatibility with the horror vacui.
At this point, I describe Torricelli’s experiment, I show a video and I read Torricelli’s letter of the 1644 to M. Ricci with which he affirms that the air sustains the mercury: so the cause is external to the pipe.
By comparing the results of the two experiments (one with water and the other with mercury) we discover that the product between density and height is constant and is about 1kg/cm2. What does it mean this product? By answering to and by the discussion following this question we reach the definition of pressure and the identity between the pressure column of the liquid and the atmospheric pressure.
By using whether the website of the Museum of Galileo of Florence or directly in the Lab, finally, we analyse the various confirmations to the Torricelli’s experimental hypothesis together with the functioning of Boyle’s pneumatical engine.
The experimental side of whatever method is always appealing and in the current case is very surprising (for instance thinking about the Magdeburg hemispheres). However the key element of my methodology is the analysis of how students challenge themselves interpreting through Torricelli’s point of view the events they observed.