SPAC demonstration

dh321 at dh321 at
Wed Dec 12 18:42:49 EST 2001

How high were you able to raise the column of mercury with the hemlock
The demonstration setup with either a porous clay cup or a living branch at
the top is not equivalent to a mercury barometer, which is made by inverting
a glass tube of mercury into an open dish of mercury. The porous clay cup
and hemlock branch are not sealed to air at the top as is the mercury
barometer. Thus, there is no need to subtract off 76 cm of mercury. 

Most of the porous ceramic cups from Soil Moisture Corporation are rated at
1 or 2 bars (0.1 or 0.2 MPA) for air entry. If the mercury column did not
break before that, the porous clay cup rated at 2 bars could theoretically
support a mercury column about 152 cm high, which would be equivalent to a
water column of 20.5 meters because the density of mercury is 13.5 times
greater than water. That would be a quite impressive demonstration
especially for a glass tube with a 1 mm inside diameter, more than 6 times
the diameter of an average vessel (0.15 mm according to Taiz and Zeiger) and
more than 12 times the diameter of an average tracheid (0.08 mm
according to
Salisbury and Ross).

It would be very nice to videotape the demonstration, especially to see the
maximum height obtained and where the column snapped. 

David R. Hershey

Virginia Berg <Virginia.Berg at> wrote in message
news:9v3826$glq$1 at> I have somewhere a copy of
instructions for a transpiration = tension
> demonstration that was absolutely convincing. It involved a .3m branch
> of a conifer (hemlock, I think), a short length of tygon tubing, then a
> 1 m long 1 mm inner diameter glass tube (this is a standard type). The
> trick was to get all the air and grease out of everything, so it
> involved boiling the water and the bottom of the debarked stem to degas
> them, and boiling the tygon, as well as freshly cleaning the glass tube
> with chromic acid. Assembling it with the hot water wasn't fun, but we
> didn't want gas to redissolve in the water. The bottom end of the glass
> tube was in a beaker of water, but then after things were working well,
> mercury was poured in. Only when the mercury is above 760 mm have you
> shown that the water is being pulled up, not pushed up by atmospheric
> pressure as in a barometer.
> Practically, you need to set up a bunch of these, because inevitably
> some of the water columns will break as the tension builds up from all
> that weight of mercury. You need a very clean mercury
> surface...everything needs to be absolutely clean. Building vibrations,
> people walking by, and bumping the system can all cause the column to
> break, often right where the stem starts, where you may not be able to
> see it.
> It was really a pain to set up. But it worked, and is absolutely
> convinving. I want to do this once more some day, and videotape it.
> Retirement in a few years seems like a very active proposition.
> --Gini Berg
> --
> Dr. Virginia Berg
> Professor, Plant Physiology
> Biology Department
> University of Northern Iowa
> Cedar Falls, IA USA 50614-0421
> bergv at
> office: 319 273-2770
> fax: 319 273-2893

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