problem in install RCurl in R (Ubuntu Linux)

I have some problem in running R-cran's Demography package.
The hmd.mx function need Rcurl. I tried to install RCurl, but meet the following error:
*********************************************************************
...
* installing *source* package ?RCurl? ...
checking for curl-config... no
Cannot find curl-config
ERROR: configuration failed for package ?RCurl?
* removing ?/home/z/R/i686-pc-linux-gnu-library/2.12/RCurl?
...
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How to fix the problem?
Ethan Chang