Study of the New York Obelisk as a Decayed Boulder. 141 



(c). The surface of the stone was rarely subjected to a higher 

 temperature than 85° C. (185° F.), and probably never, when the 

 stove was used. 



(d). Only a very thin outer layer of the granite of the Obelisk 

 was heated beyond the melting-point of paraffin, 60-63° C. (146° 

 F.), probably between 6 and 12 mm. (^ to ^ inch) in thickness. 



Some effort w^as also made, in these experiments, to ascertain 

 whether the surface of the granite of the Obelisk could have suffered 

 damage from the temperatures (60° to 85° C.) and treatment indi- 

 cated. Among the samples of granite imbedded in the surface of 

 the cement were pieces of the original fresh stone of the Obelisk,, 

 each with a polished face set in flush with the general surface. These 

 faces had been previously studied under pocket-lens, and then under 

 a microscope, with magnifying power of 30 diameters. On re- 

 examination, after the conclusion of the treatment, no effect what- 

 ever was detected on the surface subjected to the stove ; on the 

 other, treated by the blast-lamp, two or three very minute checks or 

 crevices, perhaps a millimeter in depth, seemed to have developed. 



On the same question, some information may be derived from 

 the experience of lithologists, in the mode of mounting thin rock- 

 sections for microscopic examination. After having been ground 

 down to transparent pellicles of extreme thinness and delicacy, these 

 are commonly immersed, on a slide, in a drop of partially inspissated 

 and hardened Canada balsam. 



In the first experiments on this subject, a drop of balsam on a 

 glass slide was heated upon a mounting-table, usually from 3 to 5 

 minutes, for the partial evaporation of the excess of turpentine, its 

 natural solvent. At this point, in place of a rock-section, the bulb 

 of a delicate thermometer was inserted into the drop, and a tem- 

 perature of 107° C. (220° F.) was noted. 



Again, a quantity of the balsam, about 200 c. c, was slowly 

 evaporated in a shallow tin-pan, over a low flame. The tempera- 

 ture, 50° C. during the first half hour, then rose to 108-110° C, 

 and so remained for 3 hours ; after 7 hours, when the medium had 

 attained the proper viscidity, the temperature fell to 80°, and, while 

 cooling and still viscid, to 60°. 



Since, therefore, the scrupulous needs of the lithologist, in the 

 investigation of intricate structures of rocks and minerals, are not 

 endangered by subjecting a thin rock-section to a temperature of 

 'ven 107° C. for a minute or more : there seems to be no reason to 



