282 LECTURES TO SCIENCE TEACHERS. 



hydrogen in a properly constructed vessel. Here is a 

 modern example of a Volta tube. It is made of very 

 thick glass with a stopcock at the lower portion, and is 

 graduated into cubic inches. At the top are placed two 

 platinum wires almost touching. The air and the hydrogen 

 are collected in this tube over water or mercury ; they 

 are well mixed ; the stopcock is then closed, and an 

 electric spark passed between the wires, which produces an 

 explosion. You have the hydrogen combining with the 

 oxygen, the watery vapour is instantly condensed, and on 

 opening the stopcock under water or mercury there will 

 be a certain rise in the tube : the diminution in the 

 volume will consist of two volumes of hydrogen and one 

 of oxygen, and, therefore, one-third of the diminution will 

 represent the oxygen present in the air. It is obviously 

 necessary to employ an excess of hydrogen. 



This instrument of Volta's was afterwards improved by 

 Ure, and is known as lire's Syphon Eudiometer, of which 

 this is a modern example. It consists of a bent tube 

 closed at one end with platinum wires sealed through it, 

 graduated on the closed limb. When an explosion has to 

 be made, the gas is introduced by filling the whole tube 

 with mercury, and inverting it over mercury in the trough 

 and allowing the gas to pass up in what are supposed to 

 be the right proportions. You first put in the air, turn it 

 over and measure the volume, and after having carefully 

 levelled the mercury in both tubes, once more invert it, 

 introduce the hydrogen, and measure again. The mixture 

 is exploded by holding the tube in the hand, leaving a 

 column of air in the open side, and closing the open end 

 firmly with the thumb. The knuckle of the third finger 

 is brought in contact with the wire, and by means of an 

 electrophorus a spark is sent through the mixture. I 

 never performed this experiment, because one imagines 

 there might possibly be a little nervousness at feeling the 

 electric shock passing through one's knuckle, especially 

 when holding a tube containing gases to be exploded. I 

 am told, however, that there is no danger whatever, that 

 it only wants a little strength of mind, and that the 

 concussion is entirely taken up by the cushion of air 

 between the mercury and the thumb, and that the results 

 are very concordant indeed. 



