Royal Society. ^^ 



Your second Medal has been adjudged to M. MItscherlich. 

 No aitempts of naturalists have been more unremitting or more 

 ardently pursued than those for connecting the external characters 

 of all bodies with their internal qualities, which really constitute 

 their natural families, their true classes, genera, and species. 



The labours of Linnaeus, so successful in the organized king- 

 doms, and so much the contrary in respect to minerals, may be cited 

 as an example. Since the great discovery by Mr. Dalton of De- 

 finite Proportions, by far the most important, I have always thought, 

 since the laws were demonstrated which bind together the universe, 

 the hope of obtaining such characters has been raised higher than 

 before ; while crystals measured by the goniometer of Dr. Wollas- 

 ton promised to rival, in the quality of scientific indices, the fructi- 

 fication of plants, which they had always emulated by their beauty. 

 But these hopes were not at once realized ; anomalies appeared, 

 and no lengthened clue could be found till M. Mitscherlich ascer- 

 tained,— That the chemical elements of bodies may be arranged 

 into classes or groups giving isomorphite crystallization ; so that 

 when compounds are formed by the unions of these substances with 

 equal atomic proportions, crystals are produced identical in their 

 primitive and in their secondary or modified forms. Hence it fol- 

 lows, that the mixture, in any proportions, of compound bodies 

 having this quality, will present crystals of the form common to 

 each tn its unmixed state ; as is said to take place in all the va- 

 rieties of magnesian limestone, however the relative proportion 

 between the two carbonates may change. The investigation has 

 been extended to other bodies found dimorphous under various 

 circumstances, explaining further apparent anomalies in the atomic 

 theory : but the subject is at once too novel and too intricate for 

 further discussion here. 



M. Mitscherlich has had the additional good fortune to detect a 

 very irregular and unexpected property of crystallized bodies, in 

 reference to heat. Their expansions in different directions, are 

 found to vary after different laws in respect to temperature; there- 

 by modifying their action, in giving polarity to the rays of light. 

 So extraordinary have the laws of expansion been found in Iceland 

 spar, that while in common with ail other substances it expands in 

 bulk by the accession of heat, and its lintar dimension is extended in 

 one direction, the sign of expansion is actually changed in the di- 

 rection perpendicular to the former, and it cimtracts. 



Perhaps 1 may be permitted to read the Report of a Committee 

 appointed by your Council of last year. 

 * ' ^ ^ November 26, 1828. 



" Your Committee have taken into consideration the subject re- 

 ferred to them ; and, bearing in mind the important object for 

 which it has been so referred, they desire that they may not be un- 

 derstood as pronouncing sentence on any disputed point, or as as- 

 suming any weight to be due to their judgement beyond what the 

 circumstances under which they meet may authorize, when they 

 stale it as their opinion that the doctrine of the equivalent forms of 



tlitmical 



