1. INTRODUCTION

2. DATA AND MODELING  
2.1. HST Data  
2.2. FUSE Data  
2.3. McDonald Data  
2.4. ESO Data  
2.5. Ismod.f Spectrum Synthesis  

3. OBSERVED RELATIONSHIPS  

4. DERIVED PHYSICAL CONDITIONS  

5. ANALYTICAL CHEMISTRY  
5.1. CN Chemistry  
5.2. $CH^{+}$ Chemistry  
5.3. The Role of $C_{2}$ Data  

6. NUMERICAL MODELS WITH CLOUDY  
6.1. Computational Details  
6.2. Forming and Modeling $CH^{+}$  

7. DISCUSSION  

8. CONCLUSIONS