OF AIR AND OTHEE GASES. 3 



disks instead of one, so that there are six surfaces exposed to friction, which 

 may be reduced to two by placing the three disks in contact, without altering 

 the weight of the whole or the time of vibration. The apparatus was con- 

 structed by Mr Becker, of Messrs Elliott Brothers, Strand. 



Description of the Apparatus. 



Plate XXI. p. 30, fig. 1 represents the vacuum apparatus one-eighth of the 

 actual size. MQRS is a strong three-legged stool supporting the whole. The 

 top (MM] is in the form of a ring. EE is a brass plate supported by the ring 

 MM. The under surface is ground truly plane, the upper surface is strengthened 

 by ribs cast in the same piece with it. The suspension-tube A C is screwed 

 into the plate EE, and is 4 feet in height. The glass receiver N rests on a 

 wooden ring PP with three projecting pieces which rest on the three brackets 

 QQ, of which two only are seen. The upper surfaces of the brackets and the 

 under surfaces of the projections are so bevilled off, that by slightly turning the 

 wooden ring in its own plane the receiver can be pressed up against the plate 

 EE. 



F, G, H, K are circular plates of glass of the form represented in fig. 2. 

 Each has a hole in the centre 2 inches in diameter, and three holes near the 

 circumference, by which it is supported on the screws LL. 



Fig. 6 represents the mode of supporting and adjusting the glass plates. 

 LL is one of the screws fixed under the plate EE. S is a nut, of which the 

 upper part fits easily in the hole in the glass plate F, while the under part 

 is of larger diameter, so as to support the glass plate and afford the means of 

 turning the nut easily by hand. These nuts occupy little space, and enable 

 the glass disks to be brought very accurately to their proper position. 



ACS, fig. 1, is a siphon barometer, closed at A and communicating with 

 the interior of the suspension-tube at B. The scale is divided on both sides, 

 so that the difference of the readings gives the pressure within the apparatus. 

 T is a thermometer, lying on the upper glass plate. F is a vessel containing 

 pumice-stone soaked in sulphuric acid, to dry the air. Another vessel, containing 

 caustic potash, is not shewn. D is a tube with a stopcock, leading to the air- 

 pump or the gas generator. C is a glass window, giving a view of the suspended 

 mirror d. 



For high and low temperatures the tin vessel (fig. 10) was used. When 

 the receiver was exhausted, the ring P was removed, and the tin vessel raised 



12 



