164 



SCIENTIFIC AMUSEMENTS. 



purchased, answering the same purpose ; this consists of a sort of 

 earthenware cylinder or basin with a wide flat base perforated by 

 a hole, and provided with a slit or circular orifice in one side ; 

 this stands in the basin of water or "pneumatic trough" upside 

 down, the delivery tube of the gas generator passing through the 

 slit in the side so that the gas bubbles into the interior of the 

 "bee-hive" or inverted basin-shaped cavity, and thence passes 

 through the orifice into a jar standing on the flat base or " shelf." 

 Fig. 72 represents the mode of use of the bee-hive shelf with a 

 trough constructed of glass plates. 



Any gas not too readily soluble in cold water can be thus collected 



Fig. 72. Collection of Gas at the Pneumatic Trough. 



by means of the pneumatic trough in jars or such like receptacles. 

 When gases easily soluble in cold water are to be examined, hot 

 water may sometimes be employed, as, for example, in the case of 

 nitrous oxide or "laughing gas" (Expt. 187). In other cases a 

 " mercurial trough " is employed (fig. 73), i.e., a basin containing 

 mercury instead of water, but manipulated in much the same way 

 as that above described. Large vessels cannot well be employed 

 in this way on account of their great weight when filled with 

 mercury. When the gas is considerably lighter or heavier than 

 air, it may be collected by displacement ; thus, in Expt. 75, a jar 

 full of ammonia gas was thus collected, and in Expt. 104, one of 

 hydrogen, both gases lighter than air, the jars being held mouth 

 downwards, and the delivery pipe being so turned that the escap- 

 ing gas passes upwards ; on the other hand, carbonic acid gas 



