The study of solidification has a long and 
diverse history, and is of key importance in every 
branch of materials science.  The main champions of 
the field have been the metallurgists, and before 
them, the ceramists.  
With the development of new technologies, new fields 
have arisen, in which solidification once again plays 
a primary role, such as semiconductor/electronic 
materials and polymer science, as well as the 
pharmaceutical and food industries.  

In all of these fields, the use of model systems is 
necessary to facilitate the fundamental study of 
solidification processes.  One rather popular model 
system for such studies is that of Ammonium Choloride 
($NH_4Cl$) in water\footnote{The crystallization 
in the $NH_4Cl-H_2O$ system is quite similar to that 
of austenitic steel \cite{Saratovkin:59} and is therefore 
of use to the metals industry.  In addition to this 
system, there are a number of other very good model 
systems (including several organic systems such as the 
succinonitrile-acetone system), which are the systems 
of choice for studying metals.}, in which the solid 
$NH_4Cl$ precipitates out of its aqueous solution.  
This system has been popular because the materials 
are readily available and the experiments are more 
easily performed and observed than in most other systems.
The crystallization, in 
addition to being beautiful to watch, is quite 
fascinating.  The spectrum of morphologies/patterns and the 
dynamics giving rise to them are so diverse that one 
can quite easily believe oneself to be a pioneer exploring 
an undiscovered wonderland.  This has resulted in such 
unfortunate recent titles in the literature as 
``New Periodic Morphologies...'' \cite{Raz:91} 
and ``New Experimental Findings...'' \cite{Honjo:85} 
for studies which had been more carefully 
performed in works published decades earlier.  

The three most exacting studies found on the $NH_4Cl-H_2O$ 
system are\footnote{The 
listed studies work with an approximately two-dimensional 
chamber geometry, which is explained in 
section Experimental Apparatus.  Some excellent studies have been 
performed with fully three-dimensional geometries 
\cite{Jackson:66, Chen:91, Copley:70}.  However, with such geometries, 
much of the behaviour of interest in this dissertation is not observed.}:  

\begin{itemize}

\item	P. P. Ewald's and A. Papapetrou's work 
	culminating in Papapetrou's doctoral dissertation 
	in ``general science'' in 1935 \cite{Papapetrou:35}; 
\item	D. D. Saratovkin's beautiful exposition, translated 
	into English in 1959\footnote{The translation is of the 
	second edition, which was presumable written in 1954, 
	judging by the dates of the cited publications.}, 
	also serving as a full and 
	critical bibliographic record of Russian work 
	in metallurgy and solidification \cite{Saratovkin:59}; 
\item	Manfred Kahlweit's and Hans-Hermann Reimer's 
	(as well as others) work culminating in Reimer's 
	Doctoral dissertation in physical chemistry in 
	1974 \cite{Reimer:74}.  

\end{itemize}
As evidence of the general applicability of such research, 
the above three works alone span three different fields.  
Mechanical engineers, among others, are also working on this system in 
order to study the gravity-driven convection occuring in 
three-dimensional systems.  Yet more recently, the 
development of powerful theories to understand 
pattern formation from a unified perspective has 
attracted a large number of physicists 
and applied mathematicians to the $NH_4Cl-H_2O$ 
system.  

Among these different fields, there is no uniformity 
in language or terminology.  As a result, 
any special terms used in this work will be explicitly 
defined.  More detailed treatments are available in 
any of the good introductory texts.  
One noteworthy example is the text 
of Wilfred Kurz and D. Fisher \cite{Kurz:92} 
which is accessible on many levels.  In addition to this, 
\cite{Kingery:76} and \cite{Jones:84} are quite encompassing, 
while \cite{Rogers:51} and \cite{Smith:51} serve as very clear 
introductions written in layman's terms.