412 Mr. F. D. Brown on the Maintenance of 



Physical Society. Here a small closed vessel containing methyl 

 acetate is placed in the water the temperature of which is to 

 be maintained constant ; this vessel communicates with a ma- 

 nometer containing mercury ; and as the vapour-tension of 

 the methyl acetate increases, the mercury in the manometer 

 rises, obstructing the flow of gas to the burner in the usual 

 way. 



I have applied this form of thermostat to a bath of water 

 used for heating a long column of mercury. The water is con- 

 tained in a vertical copper cylinder 42 inches in length by 6 

 in diameter ; the cylinder is packed with felt on the outside, 

 and contains within it a second smaller and shorter one open 

 at both ends, and extending to within about 2 inches of the top 

 and bottom. By means of a suitable mechanical arrangement, 

 the water is made to flow continuously down the space be- 

 tween the two cylinders and up the inner cylinder ; the upper 

 and lower portions of the bath are thus kept at exactly the 

 same temperature. The water is heated by allowing it to cir- 

 culate through a copper coil placed over a gas-burner. Even 

 when the small vessel of the thermostat is filled with ether, 

 the tension of which varies much more for a given variation of 

 temperature than does that of methyl acetate, the temperature 

 is subject to fluctuations of as much as o, l, and this indepen- 

 dently of the change which necessarily occurs when the pres- 

 sure of the gas is altered. 



Gas-regulators are employed perhaps more successfully 

 where the gas-flame can be applied directly underneath the 

 bath ; for the rise of temperature in the water then follows 

 more rapidly upon the increase of the flame, the flow of gas 

 is sooner checked by the mercury, and the tendency to allow 

 too much heat to be communicated to the water is thereby 

 lessened. Still better results are obtained if the thermostat 

 be applied to an outer jacket of water surrounding the bath iii 

 which the observations are made. Both these conditions, how- 

 ever, are generally very difficult to carry out where large 

 quantities of water are of necessity used. Even when all 

 these precautions are taken, a constant temperature, in the true 

 meaning of the words, is not attained. For a further discus- 

 sion of the defects of this form of thermostat see Laspeyres*. 



The only practicable way of attaining the object in view ap- 

 pears to be afforded by the fact that the vapour emitted by a 

 boiling liquid does not vary in temperature, provided that 

 there is no variation either in the composition of the liquid 

 or in the pressure to which it is subjected. Taking their 

 stand on this consideration, Laspeyres (Joe. cit.) and Sprengel 

 * Pogg. Ann. clii. p. 132. 



