156 



LABORATORY EXERCISES IN BACTERIOLOGY. 



side connections. The variations in pressure in the mains leading directly from the 

 gas works are usually gradual and a good thermostat is capable of close regulation 

 from such a source without the need for further intervention ; but to prevent the sudden 

 changes in a house pipe carrying a large number of burners in frequent and intermittent 

 use, a gas-pressure regulator should be interposed between the thermostat and the 

 pipe from which the incubator is supplied. A number of devices are used for this 

 purpose, but the adaptation of the principle of an ordinary gas tank having a constant 

 weight superimposed, as in the Murrell regulator (Fig. 46), is probably the most efficient 

 form. 



The cost of the bacteriologic incubators, as well as of other set apparatus used 

 in well-equipped laboratories, is often prohibitive to private individuals, who in the 

 course of their medical practice might otherwise be anxious and able to conduct in- 

 vestigations in clinical bacteriology to the advantage of themselves and their patients, 

 and to that of science in general. In this connection it may therefore be suggested 

 that the ordinary egg incubator, with a lamp as the source of heat and the thermostat 



used in such an appliance, may easily 

 be arranged to answer every probable 

 demand. There is shown in the ac- 

 companying diagram the plan of a 

 cheap incubator of large size used 

 by the writer for class work, which 

 can be duplicated for a compara- 

 tively small sum (Fig. 47). The 

 arrangement of the hot-water tank 

 on the inside of the incubator cham- 

 ber, instead of as an outside jacket, 

 is not so efficient as the latter in the 

 maintenance of an even temperature 

 through the whole inclosure, but it is 

 a much cheaper mode of application. 

 It has been possible to keep the tem- 

 perature of selected parts of the ap- 

 FIG. 46. MURRELL GAS PRESSURE REGULATOR, paratus uniform within two degrees, 



and if a proper circulation of the 



heated water were arranged by a series of tubes this variation could be reduced. The 

 degree of warmth for each part of the chamber is, however, constant, the warmest 

 part being in the lower tier of wire cages near the tank, and the coolest near the top 

 and front. Every incubator should occupy a place in which as few atmospheric draughts 

 as possible prevail, to escape disturbances of the flame and to lessen the loss of heat by 

 convection. It is well to have a small room, separate from the general laboratory, 

 devoted to the accommodation of the incubators and arranged so as to best protect 

 them. 



As an extemporization in clinical work it may be possible to utilize some nook 

 close to a range or house furnace for the accommodation of culture tubes; but this is 

 at best precarious. One or two tubes of infected medium in a small metal box, wrapped 

 in several layers of cloth or paper, and put in an inner pocket, carried to bed at night, 

 may be successfully incubated by body warmth should necessity place one in extremes. 

 4. Atmosphere. From their relations to the oxygen of air bacteria are usually 

 classified in three groups obligate anaerobic bacteria (obligate anaerobes'), obligate 



