for the Advancement of' Science. 99 



but the progress of our knowledge here necessarily requires time^ 

 since the most delicate chemical analysis and the exact mea^ 

 surement of thirty or forty crystals are wanted for the satis- 

 factory establishment of the properties of each species*. In 

 Chemistry, besides the great object of isomorphism to which 

 I have referred, there are some other yet undecided questions, 

 as, for instance, those concerning the existence and relations of 

 the sulpho-salts and chloro-salts ; and these are not small points, 

 for they affect the whole aspect of chemical theory, and thus shew 

 us how erroneously we should judge, if we were to consider this 

 science as otherwise than in its infancy. 



In every science, Notation and Nomenclature are questions 

 subordinate to calculation and theory. The Notation of Crys- 

 tallography is such as to answer the purposes of calculation, 

 whether we take that of Mohs, Weiss, or Nauman. It appears 

 very desirable that the Notation of Chemistry also should be so 

 constructed as to answer the same purpose. Dr Turner in the 

 last edition of his Chemistry, and Mr Johnston in his Repgrt, 

 have used a notation which has this advantage, which that com- 

 monly employed by the continental chemists does not possess. 



I have elsewhere stated to the Association how little Hope 

 there appears at present to be of purifying and systematizing 



• Perhaps I shall not have a more favourable occasion than the present of 

 correcting a statement in my Report, which is not perfectly accurate, on a 

 point which has been a subject of controversy between Sir D. Brewster and 

 Mr Brooke. I have noticed (p. 338.) the sulphato-tri-carbonate of lead of 

 Mr Brooke, as a mineral which at first appeared to contradict Sir D. Brew- 

 ster's general law of the connexion of crystalline form with optical structure, 

 inasmuch as it appeared to be of the rhombohedral system, and was found to 

 have two axes of double refraction ; and which was afterwards found to con- 

 firm the law, the apparently rhombohedral forms being found by Mr Haid- 

 inger to be not simple but compound. It seems, however, that the solution 

 of the difficulty (for no one now will doubt that it has a solution) is some- 

 what different. There appear to have been included under this name two 

 different kinds of crystals belonging to different systems of crystallization. 

 Some which Mr Brooke found to be rhombuhedral, Sir D. Brewster foimd 

 to have a single optical axis with no trace of coiuposLtion : others were pris- 

 matic with two axes ; and thus Mr Brooke's original determinations were 

 probably correct. The high reputation of the parties in tliis controversy 

 does not need this explanation, but probably those who look with pleasure at 

 the manner in which the apparent exceptions to laws of nature gradually .dis« 

 appear, may not think, a moment or two lost in placing the matter oa it» pro- 

 per footing. 



g2 



