243 



The Tenacity of Gelatine. 



[Publication No. 35.] 



By Arthur L. Foley. 



Some years ago the author called attention to the fact that the co- 

 hesive forces of gelatine must be considerably greater than those of glass 

 in order that a single drop of gelatine in drying and contracting on a glass 

 plate may pull a ring or disk of glass from the plate. 1 The forces here 

 exhibited are apparently greater than shown in the common, though not 

 well known, process of producing chipped glass by flowing a pane of glass 

 with gelatine and allowing the gelatine to dry- Inasmuch as the author 

 could not find in any of the literature at hand any recorded values of the 

 tensile strength of gelatine, he requested one of his students to attempt 

 to determine its value. Several plans were tried, the one giving the best 

 results being as follows : 



Gelatine threads were drawn out between the ends of small wooden 

 sticks (about the size of a match) after dipping one end of each in melted 

 gelatine. The diameter of a thread was varied by varying the size and 

 temperature of the gelatine drop, the thickness of the fluid, the length 

 of the thread and the time spent in drawing it. To the other end of the 

 wooden sticks there had been attached previously small wire hooks for 

 suspending the upper end of the threads and for attaching a small cloth 

 sack to the lower end. Into this sack dust shot were slowly run until the 

 thread broke. The cross-section of the thread was then measured at the 

 point of break. 



When the section of a thread was regular its cross-sction was cal- 

 culated from the diameters measured by a micrometer microscope. Threads 

 of irregular cross-section were placed under a microscope with a camera 

 lucida attachment and a tracing made of the perimeter. The area of the 

 tracing was measured with a planimeter and the area of the section of 

 the thread itself calculated from the known magnifying power of the 

 microscope and attachment. 



When glass threads are drawn they are usually almost cylindrical. 

 Gelatine threads also are probably approximately cylindrical at the time 



1 Note on the Molecular Forces in Gelatine. Science, Vol. 23, p. 790, May 18, 

 1906. 



