[R] Help with Bartlett's test on linear model

[Stephanie Bernard]

Hi all,
I would like to test the homoegeneity of variances between several 
linear model for some analysis of covariance. It seems that the 
Bartlett's test is a good test to use but I am having problem using with 
linear model and I cannot find any examples on the internet. There are 
some examples for comparisons of variances but not linear models.
If I take the hellung data set, which is the example in Dalgaard's book. 
I know var.test works fine but I want to learn how to use the Bartlett's 
test.
 > hellung$glucose <- factor(hellung$glucose, labels=c("Yes","No"))
 > attach(hellung)
 > tethym.gluc <- hellung[glucose=="Yes",]
 > tethym.nogluc <- hellung[glucose=="No",]
 > lm.nogluc <- lm(log10(diameter)~log10(conc), data=tethym.nogluc)
 > lm.gluc <- lm(log10(diameter)~log10(conc), data=tethym.gluc)
I guess I have two questions. 1) How to use bartlett.test with linear 
model (using the model above) and 2) how to test for homogeneity of 
variances of linear models when there are more than two groups.
Thanks,
Stephanie