CERTAIN SOLUTIONS AND SUSPENSIONS 197

fold, and finally vanished gradually upon further dilu-
tion. (As a matter of course a Nicol prism was added
as an analyzer, whenever necessary, in order to observe
the fluorescent cone alone without interference with the
colloids in the water.) Sometimes a kind of cloud for-
mation could be seen, but so indistinct that no con-
clusion can be drawn from this phenomenon. Several
repetitions of the experiment led to the same result, even
when objectives made entirely of quartz and fluorspar
were used for illumination. April 10, 19, and 23, 1902.
The very strongly fluorescent aescorcein, which had
been kindly placed at our disposal by Prof. G. Lieber-
mann ^ of Berlin, for which we here express to him our
sincere thanks, showed under the same conditions a
distinct light-cone with a content of 2.5.10"^ mg. per
cubic centimeter. Upon further dilution the light-cone
disappeared.
Tetraiodofluorescein behaved in the same manner;
several other non-fluorescent dyestuffs were still less
visible in solution.
There is, therefore, but little prospect, with the present
arrangement of apparatus (especially with the use of
arc light), of being able to see the molecules of ordinary
dyestuffs which are dissolved as electrolytes and diffuse,
particularly if they exhibit slight or no fluorescence.
On the other hand it is very likely that in the colloidal
solution of dyestuffs 2 the individual particles can be
seen, if they are large enough; and it is not impossible
that the molecules of fluorescent dyestuffs can be made
visible, providing they are very large and suflBciently
fluorescent.
* Liebermann and Wiedermann, Ber. d. D. chem. Ges., 1901, Vol.
XXXIV, p. 2611.
' Benzopurpurin, for example. (See note, p. 206).
