144 Mr. Carl Barus on the Pressure- Variations of 



and jackets, pppp, are suitably added. The upper end of 

 the tube, a a a a, is in communication with the air-pump, 

 through the lateral tubulure h. Finally, to obviate breakage, 

 the lower end of the tube is protected by a metal jacket, mm. 



The thermocouple to be calibrated, a o /3, is introduced into 

 d d from below, and raised until the junction o is slightly 

 above the plane of ebullition of the liquid, k k. The upper 

 part of the tube is closed with asbestos wicking (not shown) 

 as far down as o. The wires of the couple pass through 

 parallel canals in a rod of fire-clay, x y, and are thus well 

 insulated. The boiling-points of most organic substances, 

 mercury, sulphur, &c, are conveniently obtained in this way, 

 provided the pressure be not too low (§§6, 7). The vacuum 

 boiling-point of cadmium is also easily reached. In general, 

 boiling may be kept up for any length of time — often a great 

 desideratum. If the substance k k be very volatile, like 

 methyl alcohol for instance, a condenser is added at h. By 

 using diphenylamine and a thickly jacketed tube, I found no 

 difficulty in keeping the whole column (30 cm. long) at 

 a practically constant temperature (310°). When normal 

 boiling-points (76 cm.) only are wanted, it saves breakage to 

 leave the tube aaaa open above, and to close it with a 

 cork*. 



4. In the case of boiling-points above 500° it is more conve- 

 nient to use glazed porcelain or fire-clay crucibles, resembling 

 aebbfa, fig. 2. In some forms I run the central tube bcb 

 quite through the crucible, so as to project at the lid ; but 

 this makes the apparatus very much more fragile. Usually 

 the closed tube b c b of the figure is satisfactory. The ebul- 

 lition liquid is shown at k k, and the thermocouple a o fi is 

 again introduced from below, d d is the fire-clay insulator. 

 To vary the pressure under which boiling takes place, the 

 tube a is connected with an air-pump. Finally a tight joint 

 at ef is secured by calking with asbestos fibre, and then filling 

 up the annular space left above the asbestos with fusible 

 metal. 



The crucible aeb bfa is surrounded by a small furnace of 

 Fletcher's composition, F F F F, and heated by one or more 

 blast-burners, AB. Products of combustion escape at D. 

 If the flame impinges directly on the cold crucible the latter 

 is apt to get broken ; but such breakage may be prevented by 

 surrounding the crucible with a conical shell of asbestos. 

 Unfortunately crucibles can be used but once, for they are 



* For details of construction, manipulation, degree of constant tem- 

 perature, burner, insulators, &c, see the ' Bulletin ' (54) cited, pp. 80 to 

 89, 94 to 97. 



