If gas is produced after inoculation and incubation, a visible gas bubble will be trapped inside the small tube.

On 14 May 1796 he was selected by Jenner, who took a healthy boy, about eight years old for the purpose of inoculation for the Cow Pox .

It was simply to encourage the inoculation of children by, not only making it a social event I think good use of behavioral economics in that, if you turn up with several other mothers to have your child inoculated, your sense of confidence is much greater than if you turn up alone.