256 Mr. R. Gill on an Air-pump without Clearance. 



absorption to ordinary coal-gas as to moist air, and the same 

 ratio of the radiating powers of these two gases is very nearly 

 given by the above-mentioned results of the observations of 

 Professor Magnus. In the last place, it must be observed that 

 such experiments on radiation are not of themselves suited to 

 decide definitively the present disputed point ; for whilst the 

 dry and the moist air, in the experiments of Professor Magnus 

 on radiation, had a temperature of about 200°, the experi- 

 ments on their absorption were instituted at the ordinary tem- 

 perature of 15° C. The principle, however, according to which 

 the ratio between the powers of absorption and radiation is the 

 same for all bodies, is only true when the latter absorb and ra- 

 diate at the same temperature. It is conceivable, therefore, that 

 the absorption or radiation of the non-luminous rays by dry and 

 by moist air may be very different at ordinary temperatures, 

 although at a temperature of 200° they may be very nearly 

 equal. 



Berne, June 14, 1866. 



XXXII. Oil an Air-pump without Clearance. 

 By Robert Gill*. 



{With a Plate.] 



WITH air-pumps as ordinarily constructed, it is impossible 

 to obtain a perfect vacuum, for two reasons. First, 

 because the pump at each stroke can extract only a certain por- 

 tion of the air contained in the receiver, and it follows evidently 

 that another portion must always remain ; but this consideration 

 is not of practical importance, as, if we had a pump capable of 

 extracting air from the receiver at each stroke without ceasing, 

 we should soon reduce the pressure so much as to make it im- 

 possible for our instruments to indicate it. A second and more 

 important reason is the following : — The air which the pump ex- 

 tracts from the receiver is of a tension lower than that of the 

 atmosphere, and it loses more and more tension in proportion as 

 the rarefaction becomes more perfect; this air enclosed in the 

 pump-barrel must be discharged at each stroke either into the 

 atmosphere or into a space in which the pressure is kept lower 

 than that of the atmosphere : the valves which close the dis- 

 charge orifices must have a certain weight, as, if they were in 

 equilibrium, they would have no tendency to close the apertures ; 

 the rarefied air must be compressed so much as to acquire a pres- 

 sure capable of raising the weight of the valves, in order to escape ; 

 and in ordinary air-pumps, not only must it overcome the weight 



* Communicated by the Author. 



