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, 



molecular weight =- - = 179, or - - = 192. That is, the 

 0.14 0.13 



molecular weight of the unknown lies between 179 and 192; the 

 average = 185.5. Calculated for glucose, CeH^Oo = 180. 



It appears from a very large number of experiments that this 

 method 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 



