506 



IOWA ACADEMY OF SCIENCE Vol. XXVI, 1919 



TABLE I 



*"n. r." means "not readable on MacLeod gaug^." 



fThe dash and numeral in this column indicate the time in minutes from 

 the beginning of the run, when the vacuum reading was taken. 



tThe first of the two numliers in column ab for a given run gives the 

 length of the zone determined by its characteristic lustrous appearance by 

 reflected light, while the second number indicates the opaque length. 



