The fermenting room has three main requirements for successful commercial distillation. It must allow a uniform temperature to be maintained in the vats; it must have thorough ventilation without any draftiness, and it must be absolutely clean. It should have also plenty of light so that it may be thoroughly inspected. It is true that in the primitive plants all these requisites were violated, but there is no reason for this. The first cost is but little added to by building with these requisites in mind and it is far more profitable in the long run; and it is only by the elimination of the bacteria which are inimical to proper fermentation that the fermenting operation can be performed with any certainty. For the regulation of the temperature reliance may be had on stoves or heaters, or on special mash heaters and coolers by which the temperature of the mash in the tubs may itself be controlled without reference to the temperature of the fermenting room. When, however, no special and adequate-heating means is provided, the walls should be double with an air space between and the doors and windows should either be also double or limited in number. To ensure good ventilation and plenty of space above the vats wherein to work or install suitable vatting machinery, the walls should be at least twelve feet in height. Outlet openings should be formed around the base of the room leading to the outer air and closed by controllable shutters. These are to allow the escape of the carbonic acid gas evolved during fermentation. These should be most carefully constructed, however, to prevent drafts. The walls and floor of the fermenting rooms should be so made that they may be easily washed down and kept clean. Concrete floors are excellent for this purpose and the walls also may be faced with concrete or cement covered with a coating composed of a mixture of asphalt and coal tar. This mixture may be also applied to plaster walls with good results. The fermenting vats, as before stated, are made of wood for small plants, and of iron for larger plants, and are usually from three and a half to four and a half feet in height. After the chief fermenting period, it is necessary that the temperature of the mash be prevented from rising beyond 86∞ F. and to that end movable cooling tubes, coils and stirrers are used. These consist of parallel frames made up of tubes, preferably of copper, through which cold water is passed and which are moved about in the vat, either vertically or rotatively. There must be space above the vats, therefore, for the introduction and removal of these cooling frames, and for the gearing whereby they are driven.