96 



Prof. J. J. Thomson. 



[Mar. 9, 



discharge tube is decomposed by the spark, so that the volume 

 of the mixed gases bears a very small proportion to that of the 

 steam — certainly nothing like 1 : 100 in my experiments — it is 

 evident that if the steam contains anything like 1/10 per cent, of 

 air, the oxygen in this air will be comparable with that produced by 

 the sparking, and its presence will prevent any reliable results being 

 obtained. 



This air may come from two sources — (1) it may be present in the 

 tube originally ; and (2) it may have been absorbed by the water. 



To get rid of the air from, the first source, the tube was so con- 

 structed that there were no blind alleys ; every part of it was a 

 thoroughfare for the steam. In addition to this, the vessel H (fig. 2) 

 was, at the beginning of the experiment, filled so full of distilled water, 

 by dipping one of the delivery tubes under the water and connecting 

 the other to a water pump, that when the water was heated its ex- 

 pansion was sufficient to cause it to fill the whole of the tube and 

 overflow. 



To get rid of the air dissolved in the water, I found no plan so 

 efficacious as prolonged boiling. In the earlier experiments, in addi- 

 tion to the boiling, I tried to absorb the oxygen by mixing oxidising 

 agents with the water; finally, however, I dispensed with these and 

 trusted entirely to the boiling to remove the air. 



The distilled water was boiled vigorously for six or seven hours with 

 the ends of the tubes F, G open to the atmosphere. The eudiometer 

 tubes filled with mercury were then placed over the ends of the de- 

 livery tubes F, G, so that, if any air were mixed with the steam, it 

 would be collected in these tubes. The steam was then allowed to 

 run into the eudiometer tubes for about an hour, when the tubes were 

 examined to see if they contained air. If any air was observed, the 

 eudiometer tubes were removed and vigorous boiling maintained 

 until, on repeating the experiment, the air was found to have dis- 

 appeared. 



As the excess of hydrogen obtained in the hour by sparking through 

 the steam would have been at least 1 c.c, and in many experiments 

 much more, while the air did not form a bubble large enough to be 

 visible, we may, I think, conclude that there was not enough air 

 present to affect the result appreciably. 



Method of Producing the Sparks. 



The sparks were produced by means of a large induction coil r 

 which would give sparks about 5 cm. long when the current from 

 five large storage cells, which was the usual battery power employed, 

 was sent through it. The break used was, generally, the ordinary 

 electro-magnetic break supplied with these coils, but in some experi- 

 ments a slow meroury break was employed. 



