434 Mr. F. Guthrie on the Influence of Temperature 



end of the funnel-tube passes air-tight through the cork of a 

 bottle, V, and has hanging from its end a little thimble 

 of glass, D. By this means, whatever amount of water has 

 entered V, the pressure in V is that due to the difference of 

 height of the water in b and D. The air from Y is dried by 

 two chloride-of-calcium tubes, F and Gr,and then passes through 

 the experimental capillary tube H, which can be heated. 

 Thence it passes down the tube I and is collected over water 

 in the tube J, which has a marked stricture. The vessel in 

 which J stands is always overflowing. 



It was found that the utmost attention was required to keep 

 the air perfectly dry. A joint of caoutchouc in water is found 

 to be porous to water near its boiling-point, the water probably 

 penetrating as a vapour. As the slightest intrusion of water 

 would vitiate the experiment, paraffin was used in such cases. 



The first point to be decided on was, whether heating a 

 current affected its rate of motion independently of its friction. 

 At first sight this seemed not to be impossible ; the expansion 

 of the air while its temperature is being raised might, it was 

 thought, react on the air behind it and thus check its outflow. 

 Experiments, however, showed that this was not the case, and 

 that the effect of heat in checking the current is due solely to 

 its influence on gaseous friction. 



It was found that the amount of air passing down a given 

 capillary tube varies approximately in the inverse ratio of the 

 square of the absolute temperature, and directly as the differ- 

 ence of pressure at the two ends of the tube. 



Neither of these relationships, however, is quite exact ; the 

 following formula more nearly expresses actual results. Call- 

 ing t the time required to fill with air a vessel of given capa- 

 city, and T being the absolute temperature reckoned from 

 — 273° C, and denoting by p 1 and p 2 the pressures at the ends 

 of the tube, 



Pi-f»\ T(Pi-A)V' 



where a is a small fraction depending probably on p^. 



The facts that the time varies approximately as the square 

 of the absolute temperature, and is not exactly in inverse pro- 

 portion to the difference of pressure, are worthy of notice ; and 

 their theoretical investigation will throw some light on the 

 molecular theory of gases. The fact that the temperature 

 affects the time according to the square of the absolute tem- 

 perature is consistent with known molecular laws. It has 

 been shown that the viscosity of air and its consequent shear- 

 ing friction is in proportion to T. And it is obvious from the 



