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THE POPULAR SCIENCE MONTHLY. 



ings with the cathetometer, and since the thermometers are self-regis- 

 tering, they may be lifted from the water to be read. 



The space between the outer and inner tanks is filled with cotton- 

 wool which has been picked with the fingers until it has as little body 

 as possible. The object of this wool is to prevent currents of air, 

 which would otherwise cause a much greater conduction of heat to or 

 from the body of water in the inner tank. 



The determination of the freezing-points of standards is accom- 

 plished by the apparatus shown in Fig. 2, where a a' is a tinned-iron 

 cylindrical vessel 9x9 inches, inclosing a smaller one 7^X5 inches. A 

 strainer allows the water from the finely crushed ice or snow to escape 

 into the open space b', and the space between the outer and inner 

 vessels is filled with cotton- wool. Close-fitting covers prevent currents 

 of air from the outside, and when in use each thermometer is fitted to 

 a cork which is imbedded partly in the ice. 



One boiling-point appai'atus is constructed after Regnault's plan, 

 and consists essentially of a brass stand (Fig. 3) supporting a water- 

 tank w w', 6 inches in diameter and 3 "5 inches deep, upon which in turn 

 rests a brass section of double tubing having an inside diameter of 5 

 and an outside diameter of 6 inches. This section, which extends up- 

 ward 3"1 inches, has three open tubes each 0*7 inch in diameter [v v') 

 let into its outer wall. At the place which would be occupied by the 

 fourth there is a small manometer-tube m, with a stopcock s, by which 



the difference of the pressure of the steam inside 

 and the air outside may be noted. Any one of a 

 series of four brass double cylinders, ranging in 

 height from three to twelve inches, may be fitted 

 to this first section by a telescope-joint at will. 

 Each of these double cylinders has perforations 

 at its top for the insertion of thermometers. 

 Around the top of the inner cylinder there is a 

 series of ten holes, each three fourths of an inch 

 in diameter, to allow the steam to pass from the 

 inner chamber to the outer, and thus through the 

 vents V v' to make its escape. When in use, the 

 tank 10 w' is filled with pure water, taking the 

 precaution to put several feet of brass ribbon in the bottom to equalize 

 the boiling ; and the heat is communicated by means of the Bunsen 

 burner b'. The thermometers are suspended as at t t', with their 

 bulbs at h. 



Another boiling-point apparatus, to be used for very long ther- 

 mometers, and where it is desirable to take the greatest care in the 

 boiling-point determination, is made entirely of glass. The ther- 

 mometer is completely immersed in steam, and the readings are made 

 with the cathetometer by looking through the glass and steam which 

 surround the thermometer. 



Fig. 2. 



