We discuss interstellar temperature determinations using the excitation equilibrium of the ^ { 2 } P levels of Si II and C II .
We show how observations of the ^ { 2 } P _ { 3 / 2 } fine structure levels of Si II and C II ( which have significantly different excitation energies , corresponding to \sim 413 and 92 K , respectively ) can be used to limit gas kinetic temperatures .
We apply this method to the z = 4.224 damped Lyman- \alpha system toward the quasar PSS 1443+27 .
The lack of significant absorption out of the Si II ^ { 2 } P _ { 3 / 2 } level and the presence of very strong C II ^ { 2 } P _ { 3 / 2 } provides an upper limit to the temperature of the C II ^ { * } -bearing gas in this system .
Assuming a solar Si/C ratio , the observations imply a 2 \sigma 
limit T < 954 K for this absorber ; a super-solar Si/C ratio gives stricter limits , T < 524 K. The observations suggest the presence of a cold neutral medium ; such cold gas may serve as the fuel for star formation in this young galaxy .