Nor did Faraday require elaborate apparatus to illustrate his meaning. Steaming up the Thames one July day in a penny boat, he was struck with the offensiveness of the water. He tore some white cards into pieces, wetted them so as to make them sink easily, and dropped them into the river at each pier they came to. Their sudden disappearance from sight, though the sun was shining brightly, was proof enough of the impurity of the stream; and he wrote a letter to the _Times_ describing his observations, and calling public attention to the dangerous state of the river.[20] At a meeting of the British Association he wished to explain the manner in which certain crystallized bodies place themselves between the poles of an electro-magnet: two or three raw potatoes furnished the material out of which he cut admirable models of the crystals. Wishing to show the electrical nature of gun-cotton, he has been known to lay his watch upon the table, balance on it a slender piece of wood, and, charging a morsel of the gun-cotton by drawing it along his coat sleeve, cause the wood to revolve towards the electric fibres. "An artist was once maintaining that in natural appearances and in pictures, up and down, and high and low, were fixed indubitable realities; but Faraday told him that they were merely conventional acceptations, based on standards often arbitrary. The disputant could not be convinced that ideas which he had hitherto never doubted had such shifting foundations. 'Well,' said Faraday, 'hold a walking-stick between your chin and your great toe; look along it and say which is the upper end.' The experiment was tried, and the artist found his idea of perspective at complete variance with his sense of reality; either end of the stick might be called 'upper,'--pictorially it was one, physically it was the other." Faraday's manner of experimenting may be further illustrated by the recollections of other friends who have had the opportunity of watching him at work.