On the Viscosity of Argon as affected hy Temperature. 73 



AVhen the low temperature observations were finished, the gas was 

 lighted under a small boiler placed upon a shelf above the apparatus, 

 and steam was passed through the jacket. It was necessary to see 

 that there was enough heat to maintain a steady issue of steam, yet 

 not so much as to risk a sensible back pressure in the jacket. The 

 time of transpiration for air was now about 139 seconds. Care was 

 always taken to maintain the temperature of the bulbs at the same 

 point as in the first observations. 



There are one or two matters as to which an apparatus on these 

 lines is necessarily somewhat imperfect. In the high temperature 

 measiu'ements the whole of the gas in the capillary is assumed to be at 

 the temperature of boiling water, and all that is not in the capil- 

 lary to be at the temperature of the room, assumptions not strictly 

 compatible. The compromise adopted was to enclose in the jacket the 

 whole of the capillary and about 2 inches at each end of the approaches, 

 and seems sufficient to exclude sensible error when we remember the 

 rapidity with which heat is conducted in small spaces. A second 

 weak point is the assumption that the instantaneous pressures are 

 represented by the heights of the moving mercury columns. If the 

 connecting U-tubes are too narrow, the resistance to the ^o^Y of mer- 

 cury enters into the question in much the same way as the flow of gas 

 in the capillary. In order to obtain a check upon this source of error 

 the apparatus has been varied. In an earlier form the connecting 

 U-tubes were comparatively narrow ; but the result for the ratio of 

 viscosities of hot and cold air was substantially the same as that sub- 

 sequently obtained with the improved apparatus, in which these tubes 

 were much widened. Even if there be a sensible residual error arising 

 from this cause, it can hardly affect the comparison of temperature- 

 coefficients of gases whose Adscosity is nearly the same. 



I will now give an example in detail from the observations of 

 December 21 ^Y\\h purified argon. The times of transpiration at the 

 temperature of the room (15° C.) were in seconds 



104f, 1041, i04f. Mean, 104-67. 



When the capillaries were bathed in steam, the corresponding times 

 were 



1671, 1671, 167f. Mean, 167-58. 



The barometer reading (corrected) being 767-4 mm., we deduce as 

 the temperature of the jacket 100-27° C. Thus = 287-5, 0' = 372-8. 

 The reduction was effected by assuming 



(6). 



