OP GASES AT HIGH EXHAUSTIONS. 
425 
695. The diminution of logarithmic decrement which MM. Kcjndt and Warburg 
obtained at moderate exhaustions, and their high ratio between the viscosity of 
hydrogen and air, is probably due to the presence of a trace of foreign gas, most likely 
water. They remark that: “ On proceeding further to investigate the laws of gas- 
friction below the before-mentioned limit of rarefaction,— 
s>a- 
we could not succeed, even with the most careful drying, in removing with sufficient 
completeness the last traces of aqueous vapour, which, insensible in the above 
experiments, with the low pressures here employed distorted the results. The 
presence of aqueous vapour was shown, inter alia, by this—that the damping moment 
for a vacuum (so we name a space filled with a gas of x<mth of a millim. pressure mixed 
with vapour) rose considerably when the apparatus was left to itself. This arose from 
water separating from the solid parts and evaporating into the vacuum. In con¬ 
sequence of this the theory cannot be quantitatively tested on the results obtained.” 
It is not unlikely that a trace of foreign gas or aqueous vapour was present in 
Graham’s hydrogen, for he himself remarks that: “ The small addition of 5 per cent, of 
air to hydrogen has a surprising effect in retarding the transpiration of that gas. 
The effect of 5 per cent, of air in retarding the rate of hydrogen is nearly four times 
greater than it should be by calculation. The experiment shows the effect which a 
small amount of impurity must have in deranging the transpiration rate of that gas.”'" 
The same effect is noticed by Maxwell, who sayst that a small proportion of air 
mixed with hydrogen was found to produce a large increase of viscosity. 
696. Graham states that in his experiments the hydrogen “was prepared from zinc 
which contained no arsenic, and was passed through a wash-bottle containing oxide 
of lead dissolved in caustic soda, and dried by passing over asbestos moistened with 
oil of vitriol.” t 
T 
In another paper he speaks of drying the gases by means of chloride of calcium 
tubes, and pumice moistened with oil of vitriol. Guided by the light of recent 
experience I do not hesitate to affirm that by this means it is not possible to remove 
the last traces of aqueous vapour from hydrogen gas. It is not even enough to pass 
the gas through a tube containing phosphoric anhydride lying loose in it. The 
anhydride must be tightly packed in the tube, so as to offer considerable obstruction 
to the passage of the gas, which should pass through several feet of such desiccating 
tube (641, 688). Not until I adopted all these precautions to dry the gas was I able 
to get perfectly concordant results with different lots of hydrogen gas. 
697. With each improvement in purification and drying I have obtained a lower 
value for hydrogen, and have consequently diminished the number expressing the 
'* ‘ Chemical and Physical Researches,’ by Thomas Graham, p. 123. 
f “ On the Viscosity or Internal Friction of Air and other Gases.” Phil. Trans., 1866, Part I., p. 257. 
+ ! Chemical and Physical Researches,’ by Thomas Graham, p. 176. 
