182 Mr. Carl Barus on the Fusion 



§§5, 6, is turned on in full, and the furnace fired as far as 

 1400° or 1500°, when the measurements may be commenced. 

 The length of the fusion-tube is first accurately measured by 

 cathetometer observations at the bottom and the top. The 

 telescope is then left adjusted for the top, and the vertical 

 micrometer (centrally fixed) screwed down until the image of 

 its lowest point is in contact with the cross hairs. After this 

 the micrometer is further screwed down, until the telephone 

 indicates contact between the platinum micrometer-tube and 

 the meniscus of molten rock. The difference of readings 

 gives the depth of the latter below the top plane of the fusion- 

 tube. I usually repeated these measurements three times. 



Hereupon the temperature measurements were made by 

 connecting the terminals of the zero method with the lower, 

 the middle, and the upper thermocouple. 



Finally, the cathetometer and micrometer measurements 

 were again repeated in full. Uniform heating presupposed, 

 it was permissible to regard the two sets of length measure- 

 ments as coincident with the intermediate temperature mea- 

 surement. 



This done, I frequently waited 15 minutes or more, to 

 make another complete measurement, under better conditions 

 of constant temperature. 



The graduated stopcock was then partially closed by a 

 proper fractional amount, and after waiting a sufficiently long- 

 time, the same series of measuring operations was gone over 

 again. Thus I continued until the glass became sticky and 

 solidification imminent, when longer waiting and more finely 

 graded changes of temperature were essential. Fortunately 

 the observer can infer the state of fusion very well, by noting 

 the time necessary for the glass threads drawn out by the 

 micrometer to break, §11. Finally, the enamel on the end 

 of the micrometer-tube becomes solid and ceases to change 

 its form, simultaneously with which the marked contraction 

 of the molten mass in the fusion-tube begins. 



Having waited long euough for the lowest position of the 

 meniscus, temperature may be varied in larger steps again. 



When the furnace is dark, measurement is no longer pos- 

 sible. I then allowed the whole arrangement to cool over 

 night, and next morning determined both the depth of the 

 solid meniscus and the length of the cold tube. The former 

 was computed from bulk measurements (with water) as well 

 as measured micrometrically. These are the normal or 

 fiducial data to which all the other volumes were referred. 





