""Mr. Rapp [a medical student], what is the difference between an element and a compound body?" Mr. Rapp is again obliged to confess his ignorance. "A compound body is composed of two or more elements," says the grinder, "in various proportions. Give me an example, Mr. Jones." [A "grinder" is an outdated term for a person who coaches students for an upcoming examination.] "Half-and-half is a compound body, composed of the two elements, ale and porter, the proportion of the porter increasing in an inverse ratio to the respectability of the public-house you get it from," replies Mr. Jones."