Fig. 136.
a. Flask containing iodine heated by spirit lamp. b. Cold flask above to receive the vapour. c C. Sheet of cardboard to cut off the heat from the spirit lamp.
Second Experiment.
Upon a thin slice of phosphorus place a few small particles of iodine; the heat produced by the combination of the two elements soon causes the phosphorus to take fire.
Third Experiment.
Heat a brick, and then throw upon it a few grains of iodine; by holding a sheet of white paper behind, the splendid violet colour of the vapour is seen to great advantage. It was by the discovery of iodine in the ashes of sponge—which had long been used as a remedy for goitre, a remarkable glandular swelling—that this element began to be used for medical purposes, and the important salt called iodide of potassium is now used in large quantities, not only in medicine, but likewise for that most fascinating art, which has made its way steadily, and is now practised so extensively, under the name of photography.
THE ART OF PHOTOGRAPHY.
It was the great George Stephenson who asked the late Dean Buckland the posing question, "Can you tell me what is the power that is driving that train?" alluding to a train which happened to be passing at the moment. The learned dean answered, "I suppose it is one of your big engines." "But what drives the engine?" "Oh, very likely a canny Newcastle driver." "What do you say to the light of the sun?" "How can that be?" asked Buckland. "It is nothing else," said Stephenson. "It is light bottled up in the earth for tens of thousands of years; light, absorbed by plants and vegetables, being necessary for the condensation of carbon during the process of their growth, if it be not carbon in another form; and now, after being buried in the earth for long ages in fields of coal, that latent light is again brought forth and liberated, made to work—as in that locomotive—for great human purposes."