The washer is an appliance intended to condense and clean gas, which, on leaving the hydraulic main, holds in suspension a great many properties that are injurious to its illuminating power, and cannot, if retained, be turned to profitable account. This cleaning process is not difficult to carry out effectually; and most of the appliances invented for the purpose would be highly efficacious if they did not in other respects present certain very serious inconveniences. The passage of the gas through a column of cold water is, of course, sufficient to condense it, and clear it of these injurious properties; but this operation has for its immediate effect the presentation of an obstacle to the flow of the gas, and consequently augmentation of pressure in the retorts. In order to obviate this inconvenience (which exists notwithstanding the use of the best washers), exhausters are employed to draw the gas from the retorts and force it into the washers. There is, however, another inconvenience which can only be remedied by the use of a second exhauster, viz., the loss of pressure after the passage of the gas through the washer--a loss resulting from the obstacle presented by this appliance to the steady flow of the gas. Now as, in the course of its passage through the remaining apparatus, on its way to the holder, the gas will have to suffer a considerable loss of pressure, it is of the greatest importance that the washer should deprive it of as little as possible. It will be obvious, therefore, that a washer which fulfills the best conditions as far as regards the cleaning of the gas will be absolutely perfect if it does not present any impediment to its flow. Such an appliance is that which is shown in the illustration on next page. Its object is, while allowing for the washing being as vigorous and as long-continued as may be desired, to draw the gas out of the retorts, and, having cleansed it perfectly from its deleterious properties, to force it onward. The apparatus consequently supplies the place of the exhauster and the scrubber.