1922] GORTNER & HOFFMAN—MOISTURE CONTENT 309 
to the changed environment by the elaboration of more colloidal 
materials. In some of the earlier papers (2, 6, 7) it was possible to 
secure these data by the rather laborious procedure of drying 
weighed portions of the saps in a water oven at 100° and weighing 
the residue. This method, of course, is wholly unsuited to field 
studies where hundreds of samples are involved, and is likewise 
inaccurate, inasmuch as caramelization of the sugars always takes 
place when plant saps are dried by means of heat. Marked caramel- 
ization can only be prevented by drying at room temperature in 
vacuo over sulphuric acid, or by drying in a vacuum oven at not to 
exceed 50°C. When such methods are employed constant weight 
is not reached until several days have elapsed. 
Another objection to any drying process for field laboratory 
work lies in the fact that there may be suspended cell débris in 
expressed plant sap. With abundant laboratory facilities at hand 
it is comparatively easy to remove such materials by means of a 
high speed centrifuge or rapid filtration, but in a field laboratory 
not equipped with a powerful centrifuge, the removal of such débris 
may be so incomplete as to seriously affect values of dry matter 
determinations obtained by a drying process. 
It recently occurred to one of the writers that it might be possible 
to determine the moisture content by making use of the refractive 
index of the plant sap. This method has been employed by sugar 
manufacturers for many years, and refractometers may be purchased 
which have a special “‘sugar scale” from which the percentage of a 
sugar in a syrup may be read directly. 
Tables of refractive indices were consulted and they confirmed 
this theory, for the refractive indices of solutions of inorganic salts 
and proteins in the concentrations normally present in plant saps 
appeared to be sufficiently near the values for solutions of carbo- 
hydrates so that no excessive errors should result. Accordingly a 
high grade Abbé refractometer was secured, provided with a special 
sugar scale, and carefully standardized by the Bureau of Standards, 
and determinations were made on a series of plant saps with the 
resultseshown in table I. This table does not represent selected 
determinations, but instead every determination which was com- 
pleted is included, with the exception of two or three where accidents 
