238 



STATE BOARD OF AGRICULTURE 



flagella, stains are essential. A.s in fabrics, substances are employed to fix 

 the stain in the tissue. The flap:ella are first treated with this fixing solu- 

 tion called a mordant, then followed by an application of a strong staining 

 solution. 



INCUBATOR. 



Attention has been called to the sensitiveness of some bacteria to 

 slight variation in temperature. The tubercle bacillus will not develop 

 the temperature varies over two de- 

 grees, and is susceptible to smaller 

 changes. The best temperature at 

 which it will grow is that of the body 

 37^° C. [98° F]. The same is true of 

 many other infectious bacteria; the 

 pathogenic bacteria as a rule require 

 a suitable temperature. For many 

 other purposes of a technical nature 

 in a bacteriological laboratory, a con- 

 stant temperature, a temperature that 

 will not vary over the one-tenth of a 

 degree, is required. To comply with 

 these conditions an apparatus called 

 an incubator has been devised especi- 

 ally for bacteriological work. The 

 principle underlying it is much the 

 same as an egg hatcher, but the egg 

 hatcher bears about the same relation 

 to the laboratory incubators that coal 

 tongs do to watch makers' pincers or 

 tweezers. The incubator has double 

 copper walls between which is kept 

 distilled water. Over the whole is 



& 

 if 



placed a thick covering of felt, 



Fig. 17. Incubator. 



The doors are double and 

 the outer one is also covered with felt. In the top is a 

 place for a sensitive thermometer, a thermo-regulator 

 and an escape for foul air within. 



Thermo- To regulate the amount of gas supplying the 



regulator, burner, a thermo-regulator has been made. 

 It consists of a tube with arms, containing 

 mercury. Through the arms the gas passes into the tube fc j^g w 

 and out to the burner. There is another adjustment arm 

 which regulates grossly the height of the mercury in the 

 tube. When ready for use one arm is connected with 

 the gas supply by a rubber tube and another with the 

 burner. The tube with its mercury is inserted into the 

 incubator. The mercury as in a thermometer, contract- 

 ing with the cold and expanding with the heat, operates 

 upon an opening through which the gas passes. When 

 the mercury in the tube has been adjusted to 37^° C, this 

 regulator will hold it constantly at that point. If the 

 temperature of the incubator should fall the mercury in 

 the tube would contract and increase the size of the hole 

 through which the gas passes and thus increase tho supply 

 of gas to the flame; if the temperature should rise above 



Fig. 18 



regnlator. 



