\subsection{Experiment by Wood}
%
Although the warming phenomenon in a glass house
is due to the suppression of convection,
say air cooling%
%
\footnote{A problem familiar to those who are 
          involved in PC hardware problems.},
%           
it remains true that most glasses 
absorb infrared light at wavelength 
$1\,\mu{\rm m}$ and higher almost completely.

An \textit{experimentum crucis} therefore
is to build a glass house with panes consisting
of ${\rm NaCl}$ or ${\rm KCl}$, which are 
transparent to visible light as well as
infrared light. For rock salt (${\rm NaCl}$) 
such an experiment was realized as early 
as 1909 by Wood\TschSpace%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\cite{Wood1909,Jones1990,Schloerer2007,Connolley2007}:
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

\begin{quote}
\lq\lq
There appears to be a widespread belief 
that the comparatively high temperature 
produced within a closed space covered 
with glass, and exposed to solar radiation, 
results from a transformation of wave-length, 
that is, that the heat waves from the Sun, 
which are able to penetrate the glass, 
fall upon the walls of the enclosure 
and raise its temperature: the heat energy 
is re-emitted by the walls in the form 
of much longer waves, which are unable 
to penetrate the glass, the greenhouse 
acting as a radiation trap. 
\\[2mm]
I have always felt some doubt as to 
whether this action played any very 
large part in the elevation of temperature. 
It appeared much more probable that the 
part played by the glass was the prevention 
of the escape of the warm air heated by the 
ground within the enclosure. If we open 
the doors of a greenhouse on a cold and 
windy day, the trapping of radiation 
appears to lose much of its efficacy. 
As a matter of fact I am of the opinion 
that a greenhouse made of a glass transparent 
to waves of every possible length would show 
a temperature nearly, if not quite, as high 
as that observed in a glass house. The 
transparent screen allows the solar 
radiation to warm the ground, and the 
ground in turn warms the air, but only 
the limited amount within the enclosure. 
In the \lq\lq open\rq\rq, the ground is 
continually brought into contact with 
cold air by convection currents. 
\\[2mm]
To test the matter I constructed two enclosures 
of dead black cardboard, one covered with a 
glass plate, the other with a plate of rock-salt 
of equal thickness. The bulb of a thermometer 
was inserted in each enclosure and the whole 
packed in cotton, with the exception of the 
transparent plates which were exposed. When 
exposed to sunlight the temperature rose 
gradually to 65~$^\circ{\rm C}$, the enclosure 
covered with the salt plate keeping a little 
ahead of the other, owing to the fact that 
it transmitted the longer waves from the Sun, 
which were stopped by the glass. In order 
to eliminate this action the sunlight 
was first passed through a glass plate. 
\\[2mm]
There was now scarcely a difference of one 
degree between the temperatures of the two 
enclosures. The maximum temperature reached 
was about 55~$^\circ{\rm C}$. From what we know 
about the distribution of energy in the spectrum 
of the radiation emitted by a body at 55~$^\circ{\rm C}$, 
it is clear that the rock-salt plate is capable 
of transmitting practically all of it, while the 
glass plate stops it entirely. This shows us that 
the loss of temperature of the ground by radiation 
is very small in comparison to the loss by convection, 
in other words that we gain very little from the 
circumstance that the radiation is trapped. 
\\[2mm]
Is it therefore necessary to pay attention 
to trapped radiation in deducing the temperature 
of a planet as affected by its atmosphere? 
The solar rays penetrate the atmosphere, 
warm the ground which in turn warms the 
atmosphere by contact and by convection 
currents. The heat received is thus stored 
up in the atmosphere, remaining there on 
account of the very low radiating power 
of a gas. It seems to me very doubtful 
if the atmosphere is warmed to any great 
extent by absorbing the radiation from 
the ground, even under the most favourable 
conditions. 
\\[2mm]
I do not pretend to have gone very deeply 
into the matter, and publish this note merely 
to draw attention to the fact that trapped 
radiation appears to play but a very small 
part in the actual cases with which we are familiar.\rq\rq 
\end{quote}

This text is a recommended reading 
for all global climatologists referring 
to the greenhouse effect.  
