THE INCUBATOR 39 



type. These regulators depend upon the rise and fall of 

 mercury upon the application of heat and cold ; thus, when 

 the temperature falls the gas-flame increases in size, since 

 by the contraction of the mercury in the thermo-regulator 

 more gas passes; again, when the temperature rises too 

 much of the gas is partially cut off by the expansion of the 

 mercury. In case the main gas opening should become 

 quite closed by the expansion of the mercury, the thermo- 

 regulators are fitted with a small by-pass pipe, which 

 allows so much gas to pass as to maintain a small pilot 

 light to prevent the flame becoming extinct. In addi- 

 tion to the mercury gas-regulators, of which so many 

 types are in use, various other ingenious devices are em- 

 ployed to regulate the temperature of the incubators, 

 among which may be mentioned those depending upon 

 the differential expansion of metals, electric alarms, and 

 the most recent invention, which is known as the Excelsior 

 gas-valve, in which the pressure of ether and other vapours 

 is employed, contained in a flexible envelope ; this, acting 

 upon a lever, controls the gas-supply. 



The best form of incubator for bacteriological purposes 

 is a modification of the Champion egg-incubator devised by 

 Messrs. Hearson and Co., of 235, Eegent Street, W., which 

 is fitted with the Excelsior gas-valve. The following is a 

 description of the Hearson incubator : 



The tank which forms the water-jacket is made of stout 

 copper, the junctions in which are effected by a means 

 which the experience of many years has proved effectual 

 in avoiding the local galvanic action so prejudicial to 

 ordinary solder. The outer case is made of pine, and the 

 space between it and the water-jacket is filled with a non- 

 conductor of heat. The chamber is closed with an inner 

 glass door and an outer wooden one. In the incubator, 

 and immediately below the Excelsior valve, which occupies 



