THE 



AMERICAN JOURNAL OF SCIENCE 



[THIRD SERIES.] 



Art. X. — Isothermals, Isopiestics and Isometrics relative to 

 Viscosity ;* by C. Barus. 



1. Historical. — In the following paragraphs, I endeavor to 

 give a preliminary account of what may be cajled the isother- 

 mals, the isopiestics, and the isometrics with respect to vis- 

 cosity. Notwithstanding the great geological importancef of 

 these relations, nobody has' as yet attempted to represent them 

 systematically. 



2. The Material chosen.— in order to obtain pronounced 

 results for the effect of pressure on viscosity, substances must 

 be selected on which temperature has a similarly obvious effect. 

 For, in addition to the direct access to the molecule which is 

 beyond the reach of pressure, temperature has the same 

 marked influence on the expansion mechanism per unit of 

 volume increment as the other agency. Hence liquids like 

 marine glue, pitch, etc., which change continuously from solid 

 to liquid, • and in which this change takes place at an enor- 

 mously rapid rate and is complete within relatively few degrees, 

 are especially available for the present investigation. 



The following data refer to marine glue. Viscosity is con- 

 sidered as a physical quality, and apart from such chemical 

 considerations as are introduced in passing from one body to 

 another. I must state, however, that the marine glue can be 

 made to change its viscosity permanently, by cautiously heat- 

 ing it for different lengths of time. Thus I obtained charges 



* Enlarged from a note in the Proc. American Acad., January, 1892. 



f As has been indicated by Mr. Clarence King, in this Journal for January. 



Am. Jour. Sci.— Third Series, Vol. XLV, No. 266. — February, 1893. 



7 



