286 BECKER AND DAY 



tween the weight and the crystal. Here, however, an unex- 

 pected difficulty was encountered. The face of the crystal in 

 contact with the lower surface of the vessel is not plane and 

 does not even distantly approach this configuration. On the con- 

 trary, a terraced cup forms below the crystal so that the bearing 

 surface remains a mere edge throughout its growth. The ac- 

 companying figure may serve to give a fair idea of a section 

 through the crystal at any time during its growth. 



The closer the examination made of these cupped faces the 

 smaller the actual bearing surface was found to be. One 

 method of determining this area is obviouslv to print it off on a 

 piece of paper, but it was found that the edges were often so 

 fine that the printed lines appeared several times broader than 

 the true edge. The process finally adopted was this : A fine 

 micrometer screw was mounted vertically so as to carry the 

 crystal downward in a motion accurately parallel to itself. 

 Chlorophyll made up with fat was selected as printing ink. It 

 gave a good color in extremely thin la3'ers, showed no disturb- 

 ing capillary action while the imprint was being made, and ex- 

 erted no solvent effect upon the crystals. A very thin coating 

 of this mixture upon bristolboard made a good inking pad. No 

 paper was found sufficiently hard and flat to take the impression 

 accurately, and our ingenuity was considerably taxed to find 

 something which would do so ; finally we hit upon the following 

 device : White celluloid was dissolved in ether and alcohol 

 and flowed upon a glass plate somewhat as a photographic plate 

 is coated. When the volatile solvent had evaporated a level 

 surface of opaque celluloid remained behind which was ex- 

 tremely smooth and flat. On this surface prints of the crystals 

 could be taken, portions of the impressions often being so fine 

 that they quite escaped notice unless seen through a reading 

 glass. They would defy reproduction in illustrations. 



The measurement of the minute areas thus recorded is a mat- 

 ter of great difficulty and uncertainty, and the force per unit 

 area which the crystals exert is, therefore, hard to estimate. It 

 was at once evident that it amounted to man}' pounds per square 

 inch, and as observations multiplied, it became reasonably cer- 

 tain that it is actuallv of the same order of maiinitude as the 



