132 
Journal of Agricultural Research 
Vd. XXIV. Now • 
dried lodgepole cones, in a calorimetric kiln, had clearly demonstrated 
that the amount of heat required to extract a gram of water from cones 
was not 536 calories but an amount always in excess of that, which 
increased rapidly as the amount of sap in the cones decreased through 
preliminary drying. This apparent increase in the latent heat of vaporiza¬ 
tion, it was thought after a study of the physical chemistry of solutions, 
might be related to the phenomena of rising boiling points and decreased 
Vapor pressure with increases in the concentration. Unfortunately, no 
direct experimental work on this problem had been done, so far as I 
have been able to learn to date, and a number of physicists consulted 
agreed that in their interpretation of the theory of solutions a solute 
could not increase the latent heat of vaporization of the solvent. 
The writer, in the most dependable tests it has been possible to make, 
has found that at the respective boiling points of various concentrations 
of common salt in water, the latent heat of vaporization decreased 
slightly with increased concentration, up to the point of saturation. 
Making allowance for the higher boiling point of the concentrated 
Solution, it would appear that for a given temperature the latent heat of 
vaporization was practically a constant. In these tests an electric 
imme^ion heater was employed for the heat supply, the wattage being 
precisely m^ured; the evaporation was directly measured by weighing 
the Solution; and allowance was made for direct radiation from the solu¬ 
tion and vessel. 
The greatest objection to these tests, or to any that we have so far been 
able to devise, lies in the diflSiculty of maintaining a constant temperature 
with a constant and measurable heat supply, at a relatively low tempera¬ 
ture such as plant tissues may experience, and also at a relatively low tem- 
pa^ttore such that the radiation factor is not a great possible source of 
error. Until these difficulties are overcome we can hardly say that the 
problem of the latent heat of vaporization as it relates to plant evapora- 
tidU has been satisfactorily treated. 
' On the other hand, it is a fairly simple matter, at either high or low 
temperatures, to determine that the rate of evaporation is very materially 
reduced with increase in concentration when the source of heat is outside 
the solution. This would make it appear that there may be a problem in 
conductivity quite as important as, if not more important than, that 
relating to latent heats. The resistance to diying, shown by rather 
concentrated solutions such as sirup, is quite well known. As an illustra¬ 
tion of what we mean by heating from the outside, let us take the case 
of two identical evaporating dishes placed over a steam bath. The steam 
is constantly in contact with the bottoms of the dishes. It may not, 
however, give up its heat unless the surface of the dish is being cooled 
by radiation above or evaporation of the liquid in the dish. Such an 
exposure can not possibly give any indication of the quantity of heat 
utilized in evaporating from the dish. 
Under such circumstances as these the writer found the evaporation 
from a saturated salt solution to be less than one-twentieth as rapid as 
that from pure water. 
Similarly, exposing a number of test tubes mainly to the heat of the 
air in contact with them, the rates of evaporation were found to be 
depressed by somewhat dilute saline and sugar solutions. 
Vessels and bottles in which the contents have been heated primarily 
by the rays of the sun have not shown any consistent depression of the 
evaporation rates due to solutes. 
