HKARSON'S INCUBATOR. 83 



pipe P (which is advisable to prevent explosion in all lamp 

 incubators, and is also necessary for the insertion of a ther- 

 mometer into the tank) should be filled up with warm water 

 every other day, and about the same number of times the 

 evaporating pans will need refilling, for which luke-warm 

 water should be used. Otherwise the general management 

 will be much the same as before described. 



While, however, the great desideratum of uniform tem- 

 perature may be secured with many forms of regulators by 

 using a large water tank, it will be obvious that the same 

 result might also be secured by a more perfect regulator. This 

 has been attained by Mr. Hearson in his regulator, which 

 depends for efficiency upon the fixed boiling point of a fluid. 

 Just as water boils at 212, so sulphuric ether boils and 

 expands into vapour at 94. Other liquids boil at higher tem_ 

 peratures, and as a mixture generally boils at a heat inter- 

 mediate between that of its two components, it is easy to 

 prepare a slightly modified ether which shall boil (at ordinary 

 barometrical pressures) at 98 or 99, the lowest admissible 

 incubator temperature. Mr. Hearson's regulator consists of a 

 few drops of such volatile fluid enclosed between two flat brass 

 plates, soldered together all round their edges into a closed 

 flattish capsule. Then, directly the heat of 98 is exceeded, 

 the two plates "bulge" under the ether vapour which is 

 formed ; and hence we have a very powerful force, which acts 

 instantly on a given temperature being attained. The incu- 

 bator is shown in section in Fig. 20. A A is the tank of water, 

 much smaller than in preceding machines, traversed by the 

 flue, L w, from the lamp, T. The flue really returns through the 

 tank, so that the outlet, w, is on the same side as T ; but this 

 cannot be shown with clearness. B is the concave egg-tray of 

 perforated zinc, supported in a drawer floored with open strips 

 of wood, K. The concavity brings the outer eggs rather nearer 

 the heat, and obviates the necessity for moving about the eggs 

 o2 



