March 15th is Josef Loschmidt's birthday. Loschmidt was an Austrian chemist who is best known for his close estimation of the size of air molecules. His value was nearly twice the actual size, but the correct order of magnitude. He also is known for the Loschmidt constant which sets the number of particles in a given volume of an idea gas as a constant. This value of 2.687 x 1025 particles per cubic meter at STP is occasionally confused with Avogadro's number in textbooks.