PURIFIERS. 



119 



these machines will therefore do the same work as the three dry-lime purifiers 

 before mentioned, viz. 130,000 cubic feet. 



To reduce the operation of one of these machines to theory, the gas should 

 be so divided that each atom of sulphuretted hydrogen should be brought into 

 close contact with its equivalent atom of lime ; the chemical change would 

 then be effected instantly, a hydrosulphuret of lime being formed, and the 

 depth of water holding the lime in solution need not exceed that of a single 

 atom. It is impossible practically to effect this perfect contact, but we can 

 approach in some considerable degree towards it, by allowing the gas to pass 

 through the liquid only in small bubbles, which is effected by the ring and 

 plate touching each other within half an inch, the gas being made to pass 

 through this annulus. In some machines the gas is allowed to pass through 

 the lime-water in masses, as it would escape from under the plate if no ring 

 confined the space : then the purification would not be effected by double the 

 pressure, for the chemical reason that the atoms of sulphuretted hydrogen 

 would not be brought into contact with their equivalent atoms of lime ; a 

 quantity would therefore escape through the machine unchanged, and remain 

 as an obnoxious impurity. The most essential thing then to be attended to 

 in the construction of both dry and lime-water purifiers is surface. 



Notwithstanding, however, that the quantity of lime required may be well 

 known, it is necessary to test the gas in its progress through the various puri- 

 fiers. In some cases it is advisable to use the test every twelve hours, or 

 oftener, in districts, for instance, where coal is of inferior and various qualities. 

 Every morning, as soon as the superintendent arrives at the works, he ought 

 to test the action of his purifiers, more especially if he has received a fresh 

 supply of coal or lime. A saturated solution of the acetate of lead in distilled 

 water is an excellent test, detecting the presence of the minutest quantity of 

 sulphuretted hydrogen, and more convenient than the carbonate, from its com- 

 plete solubility. Test-papers may be printed in the following form : 



Station and Date. 



Cmde Gas. 1st Purifier. 2nd Purifier. 3rd Purifier. 



Lime-machine having been charged hours with bushels of lime. 



