HEAT OF EVAPORATION OF WATER. 291 
I. I made a large number of determinations by this method, especially during the 
“preliminary experiments,” of which an account is given in Section IX., where the 
evaporation was promoted by passing a current of dry gas through water. 
As I shall give details of the operations when describing the “ preliminary experi- 
ments,” I will not enter into particulars here, but I may now mention that the results 
by this method were by no means satisfactory, either as regards the determination of 
the stirring heat supply (Q,) or of the values of L. The discrepancies in the value of 
Q, are, however, not alone sufficient to explain the comparative failure of this method 
of finding L, for when both the rate and the temperature were the same, the varia- 
tions in Q, never attained to 1 in 20, and as Q, was but about one-hundredth of the 
total supply, the approximation was nearly sufficient. I omit any detailed account of 
these experiments, as I made no use of them in my final calculations, and it would 
needlessly fill much space. I will, however, give the conclusions they led to, as they 
corroborate the results obtained by the method finally adopted. 
Let 
m; = mass of water evaporated per 1 second by the stirring supply, 
and 
7 = the rate of stirring (7.e., number of revolutions per second). 
When @, diminished, the value of m, increased, and this rate of increase was much 
greater after the aniline was replaced by oil; the change being evidently due to the 
increase of viscosity as the temperature diminished. When the stirring took place 
in aniline m, varied very nearly as 7’, but when oil was the liquid m, varied nearly 
as 7, 
In a former paper* I have proved that when the liquid was water the stirring heat 
varied almost exactly as 7?, and I gave particulars of 108 experiments bearing out 
that conclusion. Again, in Paper A., I have shown that when using aniline with a 
different form of stirrer Q, 0 7? approximately. Now with the same stirrer as that 
used in aniline we get with oil Q,~«7*; at 50° the power of 7 should be slightly 
higher, at lower temperatures rather smaller. These differences, although surprising, 
are none the less real. 
In the case of water the conditions were so different (e.g., 7 was about 30 instead 
of about 5) that no comparisons could be drawn, but, in the case of the aniline and 
the oil, the conditions were almost identical. My incredulity with respect to the 
relations between Q, and 7 led to considerable delay, as I repeated these experiments 
unnecessarily, and it was not until a different method of observation led to the same 
conclusion that I felt any confidence in the results. 
II. I now pass to the method indicated in II. (supra), in which no weighings were 
involved, and where I was also able to dispense with the passage of air through the 
apparatus, 
* Paper J. 
2P2 
