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IV. Physico-Chemical Determinations at High Pressures hy Optical Methods. 
{Reseai'ch executed in the Davy Faraday Research Lahoratory of the 
Royal Institution of Great Britain, London, W .) 
By Walter Waiil, Ph.D., Rosenberg Research Fellow of the University of 
Helsingfors. 
Communicated hy Sir James Dewar, F.R.S. 
(Received April 20,—Read May 23, 1912.) 
Contents. 
Historical 
Part I.— Apparatus for Optical Determinations at High Pressure. 
1. The plant for the production and measurement of high pressures. 
2. The pressure-bomb to hold the substance under investigation and the thermostat 
3. The optical installation..,. 
(1) The optical bench. 
(2) The windows of the pressure-bomb. 
Part II.— Optical Determination of Diagrams of State. 
4. General methods. 
(1) Method of observing the substance. 
(2) Isothermal melting and crystallizatiou. 
5. Diagram of state of carbon tetrabromide.. 
(1) Determinations of melting-points .. 
(2) Determinations of transition-points. 
6. Diagram of state of a/3-bibrompropionic acid. 
(1) The melting-point curve of the stable modihcation. 
(2) The melting-point curve of the unstable modification. 
7. Discussion of results. . 
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Historical. 
In the course of his well-known investigations concerning the compressibility of gases 
and liquids, Amagat made some series of measurements taking observations through 
VOL. CCXII.-A 487. Publislied separately, July 17, 1912. 
