178 



ATTRACTION. 



Method of ascertaining the specific gravities of the gases. Dr. 

 Hare. 



" Suppose the globe, A, 

 to be removed from the 

 receiver, R, and exhausted 

 during a temporary at- 

 tachment to an air pump, 

 by means of a screw with 

 which the globe is fur- 

 nished, and which serves 

 also to fasten it to the re- 

 ceiver, as represented in 

 the figure. Being pre- 

 served in this state of ex- 

 haustion, by closing the 

 cock, let it be suspended 

 from a scale beam, and 

 accurately counterpoised ; 

 air being then admitted, 

 will cause it to preponde- 

 rate decidedly. If in lieu 

 of admitting air, the globe 

 be restored to the situation 

 in which it appears in this 

 figure, so as to be filled 

 with hydrogen from the 

 receiver, R, anjl after- 

 wards once more sus- 

 pended from the beam, in- 

 stead of preponderating de- 

 cidedly, as when air was 

 allowed to enter ; unless 

 the balance be very deli- 

 cate, the additional weight, arising from the admission of the hydro- 

 gen, will scarcely be perceptible. Supposing, however, that the ad- 

 ditional weight thus acquired, were detected ; and also the weight 

 gained by the admission of exactly the same bulk of atmospheric air, 

 after a similar exhaustion of the globe, the weights of equal volumes 

 of hydrogen and air, would be represented by the weights thus ascer- 

 tained. The specific gravity of atmospheric air is the unit, in mul- 

 tiples, or fractions of which, the specific gravities of the gases are ex- 

 pressed. Hence the weight of any given bulk of hydrogen, divided 

 by the weight of an equal bulk of air, gives the specific gravity of 



