One of the most efficient forms of incubators introduced into the bacteriological laboratory is that known as Hearson’s (Fig. 259). This consists of a chamber surrounded by a water-jacket, with water space below, to afford room for the pipe, L, which conveys the heated products from the flame of the lamp, T, through the water and back again to the lantern. A is the water-jacket surrounding the chamber containing the cultures; O, the pipe through which the water supply is admitted; N, the tap for employing the same; M, the overflow pipe; S, the capsule in a case attached by a tube to the lower plate outside; D, a lever pivoted on the left, carrying at its free end a damper, F, which, when resting on the chimney, V, effectually closes it; P, a screw for adjusting the damper when starting the apparatus; H, a lead weight for bringing more pressure on the capsule; K, a thermometer, the bulb of which is inside and the scale outside the chamber. [Illustration: Fig. 259.--The Baird-Hearson Biological Incubator.] The treated products of combustion move in the direction indicated until the water and chamber are sufficiently heated to distend the capsule. When this point is reached the wire between S and P is pushed up by the capsule, and the lever causes the damper to rise more or less off the chimney, V, and on examining the thermometer the inside of the chamber is at length found to remain steadily at the required temperature. When the thermometer registers the desired temperature, the lead weight must be damped to the lever by means of the milled-head screw which goes through it. After having been once adjusted the heat in the interior will remain constant, notwithstanding the utmost changes of temperature occurring in these latitudes, nor will very great alterations in the size of the lamp-flame seriously interfere with the results. The milled-head screw, P, must be turned, after the first adjustment, during the whole time that the incubator is in use. Observe the temperature before opening the door; observations taken afterwards are worthless. Preparation of Nutrient Media--Separation, and Cultivation of Bacteria. [Illustration: Fig. 260.--Plate Cultivation Showing Colonies.]