348 On Mr, Skenevix* Attack [April, 



had not been arranged by nature into species. It was necessary^ 

 however, to divide them into species, for the sake of mineralogy. 

 Such species are merely conventional and artificial, and the basis 

 of their formation is utility. Some rule must be followed in 

 forming them. Now Werner's mode is this : and 1 confess that 

 to me at least it appears excellent. He examined all the 

 external characters of minerals, and divided them under various 

 heads. Some of these characters are susceptible ol a great 

 number of varieties: colour^ for example. He determined all 

 the different shades of colour which occur in the mineral king- 

 dom, and drew them up into a catalogue, in the order in which 

 the different shades run into one another. The same was done 

 with respect to hardness^ specific gravity^ and every other cha- 

 racter susceptible of variation. A certain range of each of these 

 characters was allowtd to mine rals belonging to the same species. 

 Thus two points were taken in the catalogue of colours, between 

 which all the colours belonging to a given species would be 

 found. In like manner two points were taken in S^q catalogue 

 of hardness^ specific gravity, &c. : and between these two points 

 lie the hardness, specific gravity^ &c. of every mineral belonging 

 to a given species. 



Thus Wei ner formed all the characters susceptible of it into 

 suites, gradually passing through a variety of changes from one 

 extremity of the suite to the other. Certain points were fixed 

 upon at some distance from each other in each of these suites, 

 and all the minerals possessed of the characters lying between 

 these two points were considered as belonging to the same 

 species. Thus with respect to colour, for example, it is not 

 necessary for all minerals belonging to the same .species to have 

 the same colour ; but they must possess some one of the colours 

 lying between the two points in tlie series of colours pitched 

 upon as including all the colours belonging to that srpecies. 

 Hence the colour suite between these two points characterizes 

 the particular species, and becomes as important and unvarying 

 a character as ^ny other character whatever. The same rule is 

 followed with respect to the hardness, the specific gravity, the 

 crystals, and all the other characters. So that the ruld for 

 forming species does not require perfect identity in all the cha- 

 racters 5 but a certain variation is allowed in each 5 and provided 

 this variation keeps within the prescribed limits, it does not 

 prevent minerals from being classed together. Exact identity is 

 not insisted on; but the minerals classed together under the 

 same specific name approach as nearly as possible to each other 

 in their properties. To form species according to this rule is by 

 no means an easy task. It requires, in the first place, a collection 

 of a considerable number of specimens, and a very careful 

 examination and comparison of all the characters. The extent 



