QUANTITATIVE ANALYSIS BY MEANS OF THE MICROSCOPE 215 
It is evident therefore that the concentration of the unknown 
material lies between the concentrations of tubes number 4 and 
5, that is between 0.13 and 0.14 gram-molecule per liter. Hence, 
. , 25.02 25.02 . 
molecular weight =- = 179, or-= 192. ihat is, the 
^ o I ^ o 13 
molecular weight of the unknown lies between 179 and 192; the 
average = 185.5. Calculated for glucose, C6H12OG = 180. 
It appears from a very large number of experiments that this 
rtiethod is a simple and dependable one, apparently subject to 
errors no greater than those usually inherent in macroscopic 
molecular weight determinations. 
As small amounts as 25 to 50 milligrams may sucessfully be 
used. 
For special precautions, sources of error and suggestions as 
to the choice of solvents and standards, the student is referred 
to the original article. 
This method of Barger’s for the determination of molecular 
weights is another example of the manifold applications of the 
the microscope. The microscopist whose laboratory is seldom 
equipped with apparatus for the determination of molecular 
weights by the usual methods of boiling or freezing points, or 
by vapor densities, may nevertheless obtain sufficiently accurate 
results for all practical purposes by the procedure outlined above. 
The method is worthy of far more attention by analysts 
than it has been given. 
Micro-Colorimetry. — Accurate quantitative colorimetric deter¬ 
minations may be made by taking advantage of the divided 
field of a comparison eyepiece. Two compound microscopes 
serve to hold the tiny colorimeter cylinders upon their stages. 
A power is employed only just sufficient to magnify the bores 
of the tubes as to just fill the fields of the comparison eyepiece. 
The colorimeter tubes may consist of small bore glass tubes 
cut to any convenient length — pairs of tubes varying from 5 
to 50 mm. long and from i to 5 mm. in diameter will be found 
to answer all purposes. These tubes are ground smooth at the 
ends exactly at right angles to the axis of the tubes. They 
must be of uniform diameter throughout their lengths and each 
