Fig. 257.—Crookshank’s Incubator.
Hot-air Incubators and Sterilisers are usually made of sheet-iron, in the form of a cubical chest, with double walls, supported on four legs, as that of Dr. Crookshank’s ([Fig. 257]). They are heated by gas or a lamp from below, while the temperature is indicated by a thermometer inserted through a hole in the top, as in that of the Hearson’s incubator. Test-tubes, flasks, funnels, cotton-wool, &c., must be sterilised by exposure to a temperature of 150°C. for an hour or more.
Wire Cages or crates are used for containing test-tubes, especially when they are to be sterilised in the hot-air steriliser, or for lowering tubes of nutrient jelly into the steam steriliser. All instruments, needles, scalpels, &c., before using must be carefully sterilised.
Fig. 258.—Dr. Koch’s Steam Steriliser.
Steam Sterilisers are made either of iron or tin, jacketed with thick felt, and provided with a conical cap or lid perforated at the apex to receive a thermometer ([Fig. 258]). Inside the vessel is an iron grating or diaphragm about two-thirds of the way down, which divides the interior into two chambers, the upper or steam chamber, and the lower or water chamber. A gauge outside marks the level of the water in the lower chamber; this should be kept about two-thirds full. The apparatus stands upon three legs, and is heated from below with a Bunsen burner or a lamp. It is employed for sterilising nutrient media in tubes or flasks, for cooking potatoes or hastening the filtration of agar-agar. When the thermometer indicates 100° C. the lid is removed, and test-tubes are lowered in a wire-basket by means of a hook and string, and the lid quickly replaced. Potatoes or small flasks are lowered into the cylinder in a tin receiver with a perforated bottom, which rests upon the grating, and admits of the contents being exposed to the steam generated.
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.
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.