CHEMISTRY 247 



demonstration, and easily carried out by anyone practically ex- 

 perienced in photography. A developing glass tank must be 

 provided of proper size, and the plate cut small enough to dip 

 easily into it. This plate should be a chloride plate and not 

 a bromide, which is too opaque, while the chloride plate is also 

 so much less sensitive as to be more manageable. A suitable 

 negative being provided, and superposed in a proper frame upon 

 the chloride plate, may be exposed in the parallel beam from the 

 condensers, time 5 to 15 seconds, according to the power of 

 the jet. The developing tank is then placed in the stage, with 

 a plate of ruby glass between it and the condensers, and a 

 plate of glass with some black diagram being placed in it, in 

 the position the photographic plate will occupy, is focussed. 

 This being withdrawn, the plate is taken from the frame and 

 placed in the tank, upside down of course. The developing 

 fluid, previously arranged for instant use, is then poured in, 

 rather a weak preparation being used. Ferrous oxalate is 

 best, as its own red colour will allow the ruby plate to be then 

 withdrawn and so allow more light to pass. The image will 

 then gradually appear, and when of the desired strength, 

 the plate can be washed and replaced in another tank of fixing 

 solution, which will show the film gradually getting clear. If 

 a tank be provided with waste-pipe and tap, the same may be 

 employed throughout the whole operation. 



CHAPTER XVII 



SOUND 



A LABGE number of experiments in acoustics are of necessity 

 incapable of projection : those for instance which depend 

 entirely upon the hearing, cannot be demonstrated by another 

 sense. But so far as it is desired to show that the phenomena 



