Page:Popular Science Monthly Volume 59.djvu/118

108 absent. In forest air traces of hydrogen were present, and about half as much methane as in the air of Paris. At a mountain station in the Pyrenees at an elevation of 2,785 meters only two volumes of methane per 100,000 were found, but seventeen volumes of hydrogen. At a sea station, 40 kilometers from the coast of Brittany, only traces of methane were found, but nearly two volumes of hydrogen in 10,000, an amount two-thirds as great as that of carbon dioxid. The source and fate of atmospheric hydrogen is a problem which now awaits solution. Liveing and Dewar seem of the opinion that there is a continual accession of hydrogen to the atmosphere from interplanetary space, and Stoney holds that the earth's gravitational attraction is insufficient to retain hydrogen in the atmosphere. Experiments of Gautier show that when certain crystalline rocks are heated with water a considerable quantity of hydrogen is evolved, which might cause a constant accession of the gas to the atmosphere. The problem must be considered for the present unsolved.

of Washington University, St. Louis, has discovered that the most sensitive photographic plates may be manipulated in open daylight, and perfect pictures may be developed upon them in sunlight instead of in the dark room. The pictures are separately wrapped in black paper in the dark room, and boxed. They may then be separately unwrapped, in the open fields if necessary, and placed in the plate holders. The camera exposure must be very much greater than in the dark room methods. After the exposure, the plate is taken out into the light and placed in the developing solution. Even if direct sunlight falls upon the plate for a moment during these changes, fine pictures may be developed. There is, however, no advantage in unnecessarily exposing the plate. The developing bath may always be in shadow, but beautiful pictures have been developed in direct sunlight. The pictures produced in this way are positives, while those produced in the dark room by ordinary methods are negatives. The positive is the picture ordinarily obtained by printing off from the negative. The shadows show light on the negative and dark on the positive. The positives produced in this way are greatly superior to those produced in the dark room on over-exposed plates, and the exposure time is very much less, but may be very great. Such pictures of a crowded street show the street with perfect clearness, every moving thing being eliminated. In one exposure lasting for several hours, a team which had stood in one position for half an hour showed no trace upon the plate when developed.

one who has had experience in photography has lost valuable plates by over-exposure. But Professor Nipher shows that all exposures may be successfully developed. Exposures ranging from a snapshot to an overexposure of about 2.000 may be developed in the dark room as negatives. The fogging in over-exposed plates is an approach to a zero condition, where the plate is blank. For such exposures bromide is freely used, and a few drops of saturated hypo are added. In ordinary dark room work hypo is carefully avoided. But as the zero condition is approached, it is very useful in keeping the plate clear. As soon as the exposure is so great that the plate cannot be controlled in the dark room, it may be developed in the light. Plates a million times over-exposed can be thus developed. The amount of illumination of the plate while being developed depends upon the amount of exposure in the camera. Instead of using the camera, the plate can be exposed in a printing frame, where it takes the place of the sensitive paper. An exposure of two or three minutes, just out of direct sunlight at a south window, may be developed in the same light. The best results are obtained with a hydrochinone developer. Some photographic