110 PURIFIERS. 



quantity of caloric passes from a sensible to an insensible state during the 

 formation of vapour, it follows that cold should be generated by evaporation. 

 If the sun shines upon the pipes, they will be colder than when it does not 

 (always supposing the quantity of water supplied is greater than the evapora- 

 tion), owing simply to the quicker generation of vapour. If after conden- 

 sation " dry lime*" is used for purifying, the gas must pass through a wash- 

 vessel, that a portion of sulphuretted hydrogen may be absorbed before the 

 gas comes in contact with the lime in the purifiers; and to effect a final 

 separation of the essential oil, ammoniacal and carbonic acid gases, the essen- 

 tial oil is so intimately mixed, that it may be detected in gas after it has 

 passed through a pipe a mile long. (Its presence may be proved by shaking 

 the gas with a little alcohol contained in a bottle ; the oil will unite with 

 the latter, forming a soapy liniment.) A wash-vessel is shown in Plate XI. 

 A is the inlet pipe for the gas, which displaces a column of water about three 

 inches high, and passes first through the openings Z> b b and at the sides of 

 the wrought-iron box B, then through the continuous opening or slit C C 

 (which must be equal in area to that of the inlet and outlet pipes, viz. in 

 this example, 5O265 square inches, the diameter of the pipes being eight 

 inches), and finally through the water. The use of the opening C C is to 

 divide the gas into small portions and distribute it over a large surface. The 

 more minutely it is divided, the better will the wash-vessel effect its object. 



D is the outlet-pipe f- 



E is a siphon, for maintaining the water at a certain level ; the part which 

 enters the tank is conducted to the bottom, in order that the sediment may 

 run off. The upper end of this dip-pipe is open, or it would otherwise form 

 an actual siphon and drain the tank. 



F is a portion of a condenser called by workmen a " gridiron condenser," 

 which, when separators are used, may be dispensed with. 



After this last process, the gas is in a state to come in contact with the dry 

 or hydrate of lime to be purified, which is much the best for that purpose j, 



* The term " dry-lime" is used in contradistinction to lime-water, the first being simply a 

 hydrate, the latter holding lime in suspension with a large quantity of fluid. 



f A better situation for the outlet-pipe is represented by the dotted lines on the top of the 

 vessel, since all liability of its filling with water is avoided. 



J It has been stated that the use of hydrate of lime, for the purposes of purification, is more 

 expensive than lime-water ; but I do not think it is so, if properly managed. 



