JOHN TYNDALL (1820–93)
Extract from ‘Atoms, Molecules and Ether Waves’, Longman’s Magazine (1882)

We must, however, refresh ourselves by occasional contact with the solid ground of experiment, and an interesting problem now lies before us awaiting experimental solution. Suppose 200 men to be scattered equally throughout the length of Pall Mall. By timely swerving now and then a runner from St James’s Palace to the Athenaeum Club might be able to get through such a crowd without much hindrance. But supposing the men to close up so as to form a dense file crossing Pall Mall from north to south: such a barrier might seriously impede, or entirely stop, the runner. Instead of a crowd of men, let us imagine a column of molecules under small pressure, thus resembling the sparsely-distributed crowd. Let us suppose the column to shorten, without change in the quantity of matter, until the molecules are so squeezed together as to resemble the closed file across Pall Mall. During these changes of density, would the action of the molecules upon a beam of heat passing among them resemble the action of the crowd upon the runner?