for the effects of changes of temperature on the quartz fiber, must, 
of course, be applied when the greatest accuracy is desired. 
This instrument was devised in order to follow the course of a 
separation of atmospheric gases by fractional distillation at low tem- 
perature, which Prof. KAMERLINGH ONNes wished to be made and to 
be eontrolled by density measurements; but it is evident that its use 
is not confined to this field. It might be used for determining the 
densities of gases or vapors under various conditions, and therefore, 
their compressibilities ; but it is especially useful as an indicator of 
minute changes of density. Professor KAMeRLINGH Onnus has already 
suggested its use to determine the composition of coexisting vapor 
and liquid phases in cases where a chemical analysis would be 
difficult or impossible, for example, in a mixture of two of the 
inert gases of the atmosphere. 
Constructional details and refinements, together with the results 
of more careful and more varied tests will be communicated in a 
later paper. 
(May 25, 1905). 
