416 Sir D. Brewster on Prof. Rudberg's Results. 



M. Arago at Paris, in the course of last summer (1814-), he 

 mentioned to him a series of experiments on the refractive 

 power of water at different temperatures, in order to deter- 

 mine if its maximum density was above 32°. He filled a 

 prism with water at the temperature of 32^, and observed the 

 angle of deviation produced by refraction, while its tempera- 

 ture rose from 32° to 212°. The angle of deviation was 

 greatest at 32°, and it gradually diminished to 212°, exhibit- 

 ing no marks whatever of a variation of refractive power at 

 40°, or at any point between 32° and 212°. Hence M. Arago 

 concluded, that since the refractive power always increases 

 with the density, the density of water must be a maximum at 

 32°. * * * It is assumed in this reasoning, that the refractive 

 jio-jocr of bodies increases voith their density, — a doctrine which 

 requires to be established by direct experiment, before it can 

 be admitted as a valid argument in favour of any other posi- 

 tion. Nay, it has actually been proved by Albert Euler, from 

 numerous experiments, that the I'efractive power of glass is 

 increased by heat. An augmentation of temperature of 60° of 

 Reaumur diminished the focal length gjth part, and an aug- 

 mentation of 33^ produced a diminution ot\j'yth. M. Euler 

 concludes, without sufficient evidence, that the refractive 

 power of fluids is increased with heat." 



Looking at all these facts together, the action of heat is 

 very anomalous : 



Heat increases the refractive power of glass. 

 Heat diminishes the X'efractive power of water, oils, &c. 

 Fleat increases the extraordinary refractive power of cal- 

 careous spar. 



Heat diminishes the extraordinary refractive power of 

 quartz. 



Heat does not affect the ordinary refractive power of cal' 

 careous spar. 



Heat diminishes the ordinary refractive power of quartz. 

 Hence there is reason to infer that heat produces some 

 6ther change in the state of a body than a mere change in the 

 relative distance of its particles. 



The difference between the action of heat on calcareous 

 spar and quartz is very extraordinary. The primitive form 

 of each is a rhomb ; and they differ only in the former having 

 negative, and the latter positive double refraction. It will, 

 therefore, be of importance to examine other negative and 

 positive crystals ; and if the difference of effect is not found 

 to depend upon this circumstance, it may possibly arise from 

 the peculiar structure of quartz in reference to circular pola- 

 rization. 



