They are either of glass or wood where not much pressure is exerted, as on the English plan. On the American plan they are of the same material as the generator, lead-lined, and connected by means of a lead pipe with the generator and among themselves. They should contain perforated diaphragms or sieves for the purpose of breaking and subdividing the gas bubbles, which would otherwise pass up through the water in large globules, a form incompatible with thorough purification.

The condenser or compressor on apparatus after the English plan is made of gun-metal or brass; the former is preferable, and should be carefully tin-lined in the interior. The agitator in condenser must be coated in the same way.

The gasometer on the "English plan" of apparatus is generally of galvanized sheet iron, or of copper, carefully tinned inside.

The pumps are best of bronze metal, gun metal, an alloy of copper and tin, inside thickly tinned. The plunger of the forcing pump may be of gun metal, instead of silver or glass, which it always ought to be.

The fountains or cylinders on the apparatus, American plan, are generally of the same material as the generator, iron or copper. They must be tin-lined with care.