12 
Research Bulletin Ao. 3 
STUDIES OF THE HYGROSCOPIC COEFFICIENT BY HILGARD AND 
LOUGHRIDGE. 
Hilgard, soon after being appointed State Geologist of Mis- 
sissippi in 1858, began a study of the soils of that state, using 
essentially the methods of chemical analysis employed by Owen 
and Peter in connection with the geological survey of Kentucky 
and Arkansas. The most notable change he made in the method 
was the introduction of the determination of the hygroscopic co- 
efficient, altho he did not use this term at that time, it appearing 
first in 1874. 1 In the case of each soil sample analyzed he de- 
termined this first of all. He states : 2 "The methods I have pur- 
sued in the quantitative analysis of soils, etc., are essentially 
those described by Dr. R. Peter in the Third volume of the Ken- 
tucky Geological Report. * * * I have substituted for the 
determination of moisture contained in the air-dry soil (varying 
greatly within brief periods) that of the moisture absorbed by 
the same at a fixed temperature if possible, in an atmosphere at 
the point of saturation with aqueous vapor; which renders the 
determinations directly comparable and seems to offer a very 
important element in the treatment of soils." He recognized in 
it an index of the degree of heaviness of a soil. 
Details of Hilgard's method were published earliest by Lough- 
ridge, 3 who was a student under Hilgard and who continued the 
study of the soils of Mississippi. The method was described 
later by Hilgard. 4 Loughridge states : "In beginning the analy- 
sis of Mississippi soils in 1859 Dr. Hilgard adopted the following 
method which has also been adhered to by his successors in this 
work, in over two hundred analyses made. The soil is pulver- 
ized with a rubber pestle. * * * The hygroscopic moisture is 
determined, after exposing it in a space saturated with vapor in 
a layer not exceeding 1 mm. in thickness, for 12 hours, by drying 
at 200° C. in a paraffine bath. 7 ' Loughridge expressed this as 
"the hygroscopic moisture from 7° to 21° C." Evidently the 
terms hygroscopic coefficient and hygroscopic moisture had been 
in common use between Hilgard and Loughridge during their 
early association. 
1 Hilgard, E. W. Silt Analyses of Mississippi Soils and Subsoils, 
American Journal of Science, vol. 7, 3d Series, p. 9, 1874. This paper 
had previously been read at the Portland meeting of the A. A. A. S. in 
1874. 
2 Hilgard, E. W. Geology and Agriculture of the State of Mississippi, 
1860, p. x. 
3 Loughridge, R. H. On the Influence of Strength of Acid and Time 
of Digestion in the Extraction of Soils. American Journal of Science, vol. 
7, 3d Series, 1874. pp. 20-23. 
4 Bulletin 38, Bureau of Chemistry, U. S. Dept. of Agr., 1893, and Cir- 
cular No. 6, University of California Experiment Station, 1903. 
