vertical spacing is witchcraft, as far as I can tell.  I have put in everything that I could think of that could possibly force the explain environment contents to be tightly vertically spaced.  It works in the main text.  The macros themselves inside the explain environment are working, too.  alas, the environment definition itself does not when inside the table.  huh?

\documentclass{article}

\usepackage{setspace}

\newenvironment{explain}{%
  \medskip\par%
  \renewcommand{\baselinestretch}{0.1}
  \setstretch{0.1}
  \large\mbox{X}\footnotesize
  }{%
}

\setstretch{0.1}


\begin{document}

\begin{table}

\begin{explain}
  This fails. This is the time. This is the time. This is the time. This is the time. This is the time. This is the time. This is the time. This is the time. This is the time. This is the time.
\end{explain}

\end{table}

\begin{table}

  \renewcommand{\baselinestretch}{0.1}
  \setstretch{0.1}
  \large\mbox{Y}\footnotesize
  This works.  This is the time. This is the time. This is the time. This is the time. This is the time. This is the time. This is the time. This is the time. This is the time. This is the time.


\end{table}



\begin{explain}
  This works.  This is the time.  This is the time. This is the time. This is the time. This is the time. This is the time. This is the time. This is the time. This is the time. This is the time.
\end{explain}

\end{document}


what did I do wrong (this time)??


Question: Conductivity of Aqueous Ionic Solutions as a Function of Time
Answer: This is really physical chemistry, but since the question is here:

When you electrolyse solutions you need to consider secondary reactions that might take place at the electrodes. For example if you produced sodium at the cathode it would immediately react with water to produce Na$^+$ and hydrogen. Therefore the cathode produces H$_2$ not sodium metal.

Likewise, if the salt solution is dilute the anode produces oxygen. So the net result is that you are splitting the water into hydrogen and oxygen, and as the water is removed over time the concentration of the sodium chloride will get higher and the conductivity of the solution will increase.

However as the concentration of NaCl increases, the reaction at the anode changes and starts producing chlorine instead of oxygen. This produces sodium hydroxide in the cell, which means the concentration of NaCl decreases and the concentration of sodium hydroxide increases. The molar conductivity of NaOH is greater than the molar conductivity of NaCl, so as before the overall conductivity carries on increasing with time.

The solution in the cell will eventually become saturated in sodium hydroxide, but the solubility of NaOH in water is extremely high: from memory it's about 50% by weight. At some point the mobility of the ions is going to start decreasing and the conductivity will fall. However I'm not sure at what point that will happen and whether it happens before or after saturation is reached. Eventually of course the cell will dry out completely and solid NaOH is an insulator.