Where a carbonate of considerable purity is employed, the thorough purification of carbonic acid gas may be obtained by mere filtration of carbonic acid gas. This is done by using such materials as divide the gas bubbles in minute particles, thus presenting a larger surface to the washing liquid in purifier and causing the absorption of all traces of sulphuric acid by the filter medium and retaining all particles of marble dust that are carried over from the generator with the eliminated gas. Chunks of marble we have already mentioned. Other filter materials for this purpose are: Cotton, fragments of well-purified sponges, coarse vegetable or, better, animal charcoal, pumicestone in small pieces.

Hager recommends to adjust at the outlet of the gas pipe in the purifier some linen in the form of a bag to filter or divide the gas bubbles. Either one of these appliances would cause the minute division of the gas, and retain marble dust that might be carried over from generator. Charcoal also would act as an absorbent of contaminated gases. The pieces would be put loose into the purifiers and renewed from time to time.

Even separate cylinders filled with animal charcoal might be connected, through which the gas passes in its course to the fountain or condenser, and this is highly commendable where an exceptional bad carbonate has to be employed or the utmost care and cleanliness is the desire of the carbonator. This coal cylinder would purify the gas from all contamination by bituminous or animalic matter, and, when placed between the pump and condenser with a continuous apparatus of the English plan, would purify the gas from all greasy particles that it may be loaded with in passing greasy valves of the pump. For the latter purpose only, instead of charcoal a washing fluid may be also employed, consisting of 4 parts by weight of soda in 100 parts of water.

The illustrations of these cylinders, called, "Repurgators" the reader will find in the chapter on apparatus, with description.