On the Viscosity of Hydrogen as affected by Moisture. 113 



a lower value for hydrogen, and have consequently diminished the 

 number expressing the ratio of the viscosity of hydrogen to that of 

 air. In 1876 I found the ratio to be O508. In 1877 I reduced this 

 ratio to 0462. Lasc year, with improved apparatus, I obtained the 

 ratio 0*458, and 1 have now got it as low as 0'4439 " (p. 425). The 

 difficulty was attributed to moisture. Thus (p. 422) : "After work- 

 ing at the subject for more than a year, it was discovered that the 

 discrepancy arose from a trace of water obstinately held by the 

 hydrogen — an impurity which behaved as I explain farther on in 

 the case of air and water vapour." 



When occupied in 1888 with the density of hydrogen, I thought 

 that viscosity might serve as a useful test of purity, and I set up an 

 apparatus somew T hat on the lines of Sir W. Crookes. A light mirror, 

 18 mm. in diameter, was hung by a fine fibre (of quartz I believe) 

 about 60 cm. long. A small attached magnet gave the means of 

 starting the vibrations whose subsidence was to be observed. The 

 viscosity chamber was of glass, and carried tubes sealed to it above 

 and below. The window, through which the light passed to and fro, 

 was of thick plate glass cemented to a ground face. This arrange- 

 ment has great optical advantages, and though unsuitable for experi- 

 ments involving high exhaustions, appeared to be satisfactory for the 

 purpose in hand, viz., the comparison of various samples of hydrogen 

 at atmospheric pressure. The Toppler pump, as well as the gas 

 generating apparatus and purifying tubes, were connected by seal- 

 ing. But I was not able to establish any sensible differences 

 among the various samples of hydrogen experimented upon at that 

 time. 



In view of the importance of the question, I have lately resumed 

 these experiments. If hydrogen, carefully prepared and desiccated 

 in the ordinary way, is liable to possess a viscosity of 10 per cent, in 

 excess, a similar uncertainty in less degree may affect the density. 

 I must confess that I was sceptical as to the large effect attributed 

 to water vapour in gas which had passed over phosphoric anhydride. 

 Sir W. Crookes himself described an experiment (p. 42b) f rom which 

 it appeared that a residue of water vapour in his apparatus indicated 

 the viscosity due to hydrogen, and, without deciding between them, 

 he offered two alternative explanations. Either the viscosity of water 

 vapour is really the same as that of hydrogen, or under the action of 

 the falling mercury in the Sprengel pump decomposition occurred with 

 absorption of oxygen, so that the residual gas was actually hydror 

 gen. It does not appear that the latter explanation can be accepted, 

 at any rate as regards the earlier stages of the exhaustion, when a 

 rapid current of aqueous vapour must set in the direction of the 

 pump; but if we adopt the former, how comes it that small traces of 

 water vapour have so much effect upon the viscosity of hydrogen ? 



