4 Mr. C. Barus on the Fusion 



whereby radiation here is made imperceptible. In this re- 

 entrant pattern symmetry of form has been sacrificed in order 

 to secure greater identity in the exposure of the air-ther- 

 mometer and the thermocouple. Deville and Troost condemn 

 the non-spherical bulb, but I have not in my experiments with 

 the reentrant form found any evidence to agree with them. 

 At all events the expansion error is much less serious than 

 the calibration error in question (§ 14) . 



The amount of gas or moisture forcibly retained by the 

 pores of the non-inglazed porcelain bulb may be greater than 

 the manometer pressure indicates. There is apparent difficulty, 

 moreover, in defining the interval volume, a quantity which 

 in constant-pressure air-thermometry enters essentially into 

 the formulae for computing temperature. Hence Deville and 

 Troost devized bulbs which could be glazed both within and 

 without, as shown in fig. 3. Here the bulb cdp and the stem 

 cf are distinct parts. They are calibrated separately, and 



Fig. 3. — Inglazed non-reentrant porcelain air-thermometer bulb 

 and stem. Scale 1/4. 



/" I - - :■ 



<c > 



40 cm 



prior to using are soldered together with felspar and the 

 oxyhydrogen blowpipe. The bulb, cdp, ends in a short neck, 

 cdrs, just large enough to receive the stem, ef snugly, and 

 the canal, cd, through wdiich the bulb is glazed, eventually 

 becomes the prolongation of the capillary canal of the stem. 



Bulbs combining the features of figs. 2 and 3, being both 

 reentrant and inglazed, were not in hand at the time. 



Porcelain soldering is difficult, and calls for skill and 

 patience. I found that after fastening the bulb to a rapidly 

 rotating whirling-table, with the stem-axis vertical and coin- 

 ciding with the axis of rotation, the operation was much 

 facilitated. Symmetrical heating greatly diminishes the 

 danger of breakage to which the bulb is liable because of the 

 intense heat of the impinging flame. The machine is de- 

 scribed in the Bulletin. Cf § 18. 



At best, however, vacuum-proof joints are sparingly ob- 

 tained, say one in every five solderings, and samples neat in 

 external appearance are frequently worthless. If, therefore, 



