One hundred years ago this month, the June 1926 issue of Science and Invention showed this simple science project, that’s probably good for a blue ribbon today at the science fair. Just as a lens made out of glass can refract light waves, a lens made of gas can refract sound waves. If this project looks familiar, it’s because we previously showed another version.

The 1926 version calls for the balloon to be filled with “carbonic acid gas.”  But since carbonic acid is unstable, it immediately turns into CO2 and water, so the balloon is just carbon dioxide.  You should be able to fill the balloon using vinegar and baking soda.  For those who want to experience more danger, you can look at the 1927 version.  If you need step-by-step instructions, you can find them at this site.