38 BULLETIN 6 2, HAWAII EXPEKIMENT STATION 
The method consists in dispersing 50 or 100 grams of soil by an 
electrical stirring apparatus in a specially constructed metal cup. 
The dispersed soil is made up to a definite volume with water in a 
glass cylinder, and after the required settling time the density of the 
soil suspension is read by means of a hydrometer constructed for the 
purpose. The hydrometer is calibrated to give correct readings 
(grains of solid matter per liter of water) at 67° F. Under or above 
this temperature a necessary correction is made, that is, 0.35 is added 
to or deducted from the reading for each degree above or under 67°. 
The corrected reading divided by the weight of soil used gives the 
percentage of the fraction in question. The results by this method 
are said to check with those of the heat-of- wetting method of deter- 
mining soil colloids. 
In the study of Hawaii soil colloids in the station laboratory, 
this method was used as a routine analysis to classify a larger num- 
ber of soil samples taken from the different parts of the four prin- 
cipal islands. Because of this fact it became desirable to determine 
(1) how closely the rapid method proposed by Bouyoucos (con- 
sisting of two readings only) agrees with the pipette method, and 
(2) how closely a detailed mechanical analysis, as performed with 
the aid of a hydrometer, compares with a pipette method of analysis. 
In checking the rapid method of hydrometer analysis no consid- 
eration was taken of the rather wide variations in specific gravity 
of Hawaii soils. The readings were taken always after settling 
for 1 and 15 minutes, regardless of the specific gravity of the soil 
in question. That this sometimes introduces appreciable error is 
known from the fact that the specific gravity of Hawaii soils was 
found to range from 2.17 to 3.36. In the detailed hydrometer analy- 
sis, however, the correct settling period was calculated from the 
specific gravity of the soil and from the viscosity of water at the 
temperature of the measurement. The depth in the hydrometer 
method was considered as 32.5 centimeters, in accordance with 
Bouyoucos' calculations, the Bouyoucos hydrometer cylinder used 
registering the average density of a soil-suspension column 32.5 
centimeters high. 
The method with which the hydrometer method was compared in 
this investigation had the same principle as the original pipette 
method of Robinson (30, p. 311), that is, sampling a soil suspension 
at a constant depth at calculated intervals and determining the con- 
centration. It differs from it in that sampling was not done by 
means of a pipette, but by a stopcock fitted in the wall of the cylin- 
der. In this method the samples were taken from a 100-millimeter 
depth, corresponding to about an 8-hour settling period for the 
clay fraction. The exact time of settling was calculated from the 
specific gravity of the soil and the temperature. Puri, Amin (28, 
p. 5), and Robinson (30, p. 318) show that closely agreeing figures 
Avere obtained from sampling at different depths at the corre- 
sponding calculated settling time. Temperature conditions were 
observed by placing the soil suspensions in an electrically controlled 
thermostat, regulated at 30° C. with an accuracy of plus or minus 
one-half degree. The cylinders were kept in the thermostat at 
this temperature throughout the experiment, the door being opened 
only to permit taking a reading or for making a sampling. 
