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Page:Popular Science Monthly Volume 22.djvu/411

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cate plant that is dried and bottled. With the true spirit of an original investigator, who sees that there is more to be learned than has already been discovered, and with a modesty that well becomes one who has accomplished so much, Dr. Lawes (now Sir John Bennet Lawes) writes me: "We are getting greatly in arrears of what may be called published work, and both Dr. Gilbert and myself are much more interested in searching after the unknown than in making public what little we do know. I think, however, it is not right to keep back so much valuable matter, and I shall try and publish next year the whole series of our ash analysis, without comment of any sort, merely giving the history of the experiments in regard to manures, etc., so that the reader may be able to trace the remarkable changes which take place from time to time. You will see that we have got to a point in our experiments in which the mere growth of the crop is one item, and a very small one, in the scope of our inquiry; the relations of the crop to the manure and to the soil and atmosphere bring us face to face with problems of great difficulty, which require several life-times to elucidate."


By Captain ABNEY, R. E., F. R. S.

TAKING the case of a daguerreotype plate which has been exposed, and which we are about to develop by the action of mercury, I should like you to understand exactly what takes place in the plate when it is exposed and developed. On the surface of the plate we have a mixture of silver iodide and bromide; but, for simplicity's sake, I will suppose that it is simply silver iodide. When light acts on such a compound, the result is the liberation of iodine and the formation of a new salt, which we call silver subiodide, Ag2I2 Ag2I I. The iodine is taken up by the silver plate at the back of the sensitive film. To develop the picture, mercury-vapor is caused to condense on the subiodide, and leave the iodide intact. In the Talbotype process, the picture, which has been taken on a paper that has been washed with nitrate of silver, iodide of potassium, and nitrate of silver again, is developed by washing with gallic acid and silver nitrate. The picture begins to appear on washing after a very short exposure to the light, and becomes gradually more visible as the washing goes on. A paper process is a most fascinating process, because you can dabble about, and do exactly what you like; it is not like the gelatine plates of the present day, which you have to leave to come out mechanically. With paper, if you want to bring out a little better detail in one place,

  1. Abstract of four Cantor Lectures delivered before the Society of Arts.