testing for significantly different slopes

Hi,

How would one go about determining if the slope terms from an analysis of 
covariance model are different from eachother?

Based on the example from MASS:

library(MASS)

# parallel slope model
l.para <- lm(Temp ~ Gas + Insul, data=whiteside)

# multiple slope model
l.mult <- lm(Temp ~ Insul/Gas -1, data=whiteside)

# compare nested models:
anova(l.para, l.mult)
Analysis of Variance Table

Model 1: Temp ~ Gas + Insul
Model 2: Temp ~ Insul/Gas - 1
  Res.Df    RSS Df Sum of Sq      F  Pr(>F)  
1     53 52.045                              
2     52 48.704  1     3.341 3.5673 0.06451 .

It seems like this approach would yield insight into whether or not the two 
slope terms (InsulBefore:Gas  and InsulAfter:Gas) were different. However, is 
there a formal test for this sort of question, and can it be generalized to 
differences between more than 2 slope terms?

Thanks,

Dylan

Dylan Beaudette
Soil Resource Laboratory
http://casoilresource.lawr.ucdavis.edu/
University of California at Davis
530.754.7341