bug in qplot (library ggplot2)

Hello,

this following code give a nice png:

/library(ggplot2)
i <- 1
png(file=paste('test ',i,'.png',sep=''))
qplot(carat, data=diamonds,
fill=color,geom='histogram')+scale_y_continuous(i)
dev.off()
/
I would like to get more files, but the following code doesn't make  
any
file:

/library(ggplot2)
for (i in 1:2) {
    png(file=paste('test ',i,'.png',sep=''))
    qplot(carat, data=diamonds,
fill=color,geom='histogram')+scale_y_continuous(i)
    dev.off()

}
/
Have you got any idea?
Have I made something false?

Thanks,

Pierre-Olivier