began and we were unable to check it until to-day when we happened to think of using muriatic acid. We immediately applied a dilute solution of the acid to the tray using a cloth to take off the black coating. After rinsing we applied a solution of soda and other tests without any action of the mercury. We raphers to never use an aluminium tray for in tensification but if they have already spoiled a tray by it to try the acid which we think We suppose it is not possible that every one who has to do with chemicals should first study their chemical actions sufficiently to avoid the mistake of our correspondent of putting a chemical into his tray which would dissolve it. He knows the fact regarding aluminium Experience is a good schoolmaster though her instruction comes high it has been said. the humming in electric street railway motors? They are noiseless when new but after about six months or a year they begin to hum. A. If this statement is true we are not able to give a reason for it. There is no electrical develop a humming noise nor any mechanical cause for such a universal effect. We suggest a broader investigation to see if all motors hum at the end of six months. 2. In cast welding rail joints do they allow for any ex pansion or contraction? If s how? A. No. If the joint is made stronger than the force of contraction the rail will not break. If the rail Is held down more rigidly than the its fastenings. Hence it will stay in its place both in winter and in summer. This is the theory. 3. Is there any direct incorporation of the metal in the rail and in the cast? I have heard some claim there is not while others claim that the rail is fused at one or two points generally about the size of a half two metals. A. There is firm adhesion. We do not know whether there is incorporation or not of the two metals. 4. We have made a box-kite with 2 cells 16 inches long and 15 tween them. When we try to set it up it will dive down after going up about 25 or 30 feet sometim
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