62 



INCUBATORS 



lost by evaporation. In actual practice it is found that the temperature can 

 be maintained within half-a-degree centigrade in spite of great changes of 

 gas pressure and of air temperature in the room in which the incubator is 

 working. 



[The incubator is rectangular in shape, and consists of a chamber sur- 

 rounded on five sides by a stout copper water jacket enclosed in an outer 

 wooden case with panels of uralite, the space between the case and the water 

 jacket being packed with some non-conducting material. The sixth side is 

 closed by a double door, the inner of glass, the outer of wood also panelled 

 with uralite. 



[The heat is supplied below by an ordinary fish-tail gas burner, the supply 

 of gas necessary to maintain the temperature being controlled by a capsule 

 let in to the roof of the incubator.] 



The regulator is based upon the same principle as that of Rohrbeck (p. 59). 

 An hermetically sealed metal capsule containing a few drops of a liquid 

 boiling at the temperature at which the apparatus is required to work is 

 placed within the incubator. When the liquid in the capsule boils, the 

 expansion of the liquid lifts the upper part of the capsule, and so raises a 

 metal rod which actuates a lever controlling the supply of gas. A small 

 safety tube prevents the gas being extinguished when the main supply is 

 cut off by the lever. 



[Technique. As full instructions are attached to every incubator, it is 

 unnecessary here to describe the details.] 



[2. Hearson's " cool " incubator. The incubating chamber is similar in 

 construction to the " hot " incubator, but on the top is a metal box of the 

 WATER same size as the incubator, surrounded by a 



thick layer of non-conducting material or wood, 

 and with a large hole in the top through which 

 ice can be introduced. The temperature is con- 

 trolled by a capsule similar to that used for the 

 hot incubator, but designed to work at about 

 20 C. 



[The source of heat is a small bath of water 

 placed at the side but near the top of the in- 

 cubator, and heated by a Bunsen burner. The 

 capsule controls a small pipe connected with a 

 supply of water in such a way that when the 

 temperature is that for which the capsule is set, 

 the water runs to waste ; when the temperature 

 has fallen below that required the pipe conveys 

 the water to the little bath, and hot water runs 

 into the incubator jacket, displacing some of the 

 cooler water. When the temperature is too high 

 the cold water runs directly into the water 

 insufficient to reduce the temperature then ice 

 In this country it is very rarely, if ever, that ice 



FIG. 50. Hearson's " cool 

 incubator. 



jacket, and if this be 

 must be put in the box. 

 is required. 



[The only difficulty likely to be experienced with this incubator is in con- 

 nexion with its water supply. If the supply be taken directly from the main, 

 the water may be cut off or the pressure for one reason or another be so 

 reduced from time to time that the automatic regulation breaks down ; so 

 that for satisfactory working it is desirable to have a tank from which a 

 supply of water under constant pressure may be derived.] 



