162 



Prof. R. Bunsen on the Condensation of 



Three experiments, at temperatures varying between 13° # 7 

 and 14°, gave for the specific gravity required, 



2-5063, 2-5059, 2-5057, Mean 2-50596. 



The threads showed under the microscope a perfectly bright 

 and smooth surface, and the section at right angles to the 

 axis was circular ; the diameters of the threads, varying from 

 •00589 to -0295 millim., showed a very gradual decrease, a 

 difference being perceptible only at points far removed from 

 one another. 



To estimate the surface-area of n such threads the diameter 

 must be found. 



If we put d l ,d 2 ,-..d n for the diameters in micrometer- 

 divisions of the microscope, and the value of a division of the 

 micrometer-scale in millimetres is a, then the mean diameter 

 d required is expressed in millimetres by 



d=z -(d 1 + d 2 + . ..d n ). 



(i) 



The means by which the micrometer-measure of 1000 of 

 these could be effected with sufficient accuracy and in a rela- 

 tively short time were the following. On the stage of a Che- 

 valier's microscope a plate of mirror-glass was cemented, and 

 over this another glass plate, admitting of easy removal, w T ith 

 etched ordinate rulings. On this removable glass plate were 

 spread out a hundred glass threads, as nearly parallel as 

 possible, lying loosely, and successively brought into the field 

 of view by gently moving the upper plate. By turning the 

 micrometer-eyepiece each thread to be measured w T as placed 

 parallel to its graduations, and a gentle tap on the lower fixed 

 glass plate with a rod then brought the edge of a thread into 

 coincidence with a division on the scale, and the reading of 

 the other edge was taken. In this way were measured the 

 diameters of 1000 separate threads, which w T ere drawn from 

 eleven different bundles. 



The mean of each ten diameters measured is given in the 

 following Table: — 



Table I. 



631 



504 



5-82 



513 



4-76 



4-86 



4-71 



5-59 



5-03 



4*50 



6-10 



570 



5-35 



6-82 



5-79 



5-05 



511 



5-38 



4-90 



5-35 



6-36 



5-87 



600 



5-42 



520 



4-80 



334 



6*43 



509 



491 



5-93 



5-74 



5-50 



504 



5-33 



5-06 



4-62 



5-69 



432 



4-58 



5-39 



5-58 



622 



6-49 



512 



541 



4-81 



6-25 



4-58 



5-06 



595 



570 



567 



5-83 



Ir'li) 



6 02 



5-09 



5*90 



4-44 



636 



6-30 



5-93 



5-69 



617 



521 



544 



4-72 



601 



5-35 



501 



643 



5-99 



519 



5-32 



5'49 



595 



4-70 



594 



4'58 



4-97 



615 



5-75 



513 



5-04 



5-99 



543 



5-53 



4-52 



4-55 



506 



566 



517 



4-86 



5-83 



610 



4-89 



392 



518 



5-26 



527 



