1826,] of Photometry, with Remarks. 381 



flames might be got rid of by receiving the light through similar 

 prisms ; and comparing in each spectrum the same coloured ray, 

 which, though it might be neither rigidly homogeneous, nor 

 precisely the same compound in the two cases, would be a suf- 

 ficiently near approximation to enable the eye to judge of the 

 equalization of mtensity; the relative space occupied by the 

 same colour in each spectrum should be measured ; and the sum 

 of the relative illuminations multiphed into the relative spaces, 

 will give the ratio of the resulting compound illuminations. 



But it must be confessed that to be obliged to have recourse 

 to such troublesome operations and calculations in every mea- 

 surement will probably make this method inapplicable for 

 general practical purposes. Nor indeed, according to a remark 

 before made, can the name of a photometer be properly applied 

 to an apparatus, in using which so much remains to be done 

 besides obtaining the simple indication. In saying this, I am of 

 course going upon the assumption that difference in colour inter- 

 feres to an injurious amount with the power of judging of the 

 equalization of lights. But, perhaps, this may not in practice 

 be found so insuperable a difficulty that skill and experience in 

 the operator may not sufficiently overcome it. At all events 

 there are many cases to which this method may be applied with- 

 out any of the additional precautions here mentioned. 



Article XI. 



Oh the Alterations that must necessarily be made in the System of 

 Chemical Mineralogy, in consequence of the Property of Isomor- 

 pkous Bodies to replace one another in indefinite Proportions. 

 By M. J. Berzelius.* 



Ever since chemistry took part in the classification of mine- 

 rals, and observation of their external characters consequently 

 ceased to be exclusively admitted in the determination of species, 

 the chemical method has met with a difficulty in the property 

 which certain oxides possess of mutually replacing each other, 

 without any accompanying change of crystalline form ; whence 

 it happens that when those oxides form colourless compounds, 

 of nearly equal specific gravities, no difference is perceptible in 

 the crystal; it can only be discovered by chemical analysis. 

 Notwithstanding their definition of what constitutes a mineralo- 

 gical species, the schools of Werner and Haiiy have ranged 

 crystals of different composition under the same species; and 

 to avoid this difficulty, Haiiy was obliged to admit accidental 

 mixtures, moulded iu the form peculiar to a species, by the 



• From the Annales dc Chimie. 



