126. CHEMICAL DECOMPOSITION OF THE GLASS BULB. During the generation of the X-rays the sodium line of the spectrum appeared in the spectroscope, thereby indicating decomposition of the glass. With combustion tubes the glass gave the weakest soda line, while lime soda glass gave the strongest, and was most permeable to the X-rays. “The continuous decomposition of the glass makes it almost impossible to maintain a vacuum except when connected to the pump and even then the effect of the current is greater in producing gas than the capacity of the pump to exhaust, but the ray is very powerful.” It is supposed that for this reason, as well as for others easily apparent that Edison as well as other experimenters have always carried on their investigations with the discharge tube permanently connected to the pump. The next best thing is to let the tube contain a stick of caustic potash for maintaining an exceedingly high vacuum. By gradually heating this, the desired degree of vacuum can be obtained. § 54. 127. SCIAGRAPHS. DURATION OF EXPOSURE DEPENDENT UPON DISTANCES. With the given discharge tube, he obtained sciagraphs at a distance of 3/8 inch from the phosphorescent spot in one second, a vulcanized cover being between; at two ft. distant the time was 150 sec.; at three ft., 450 sec.; the opaque plate being interposed each time. Consequently “Roughly, the duration of exposure may be reckoned as proportional to the square of the distance.” 128. DIFFERENCE BETWEEN X-RAYS AND LIGHT ILLUSTRATED BY DIFFERENT PHOTOGRAPHIC PLATES. TIME OF EXPOSURE. The rapid plate for light gave not the deepest images by X-rays. Several different kinds of small sensitive plates were laid side by side. A sciagraph of a metal bar was taken upon them all simultaneously. In this way, he obtained the result, whereby it would appear preferable to employ the mean rapid plate for the purpose of obtaining sciagraphs. On account of the opacity of platinum, it occured to E. B. Frost, (_Sci._, N.Y., Mar. 27, ’96,) to try platinum photographic paper of the kind used for portraits, but such paper (intended for long exposures in printing in sunlight) was far too lacking in sensitiveness to produce any effect.