Editorial Comment. 53 
physical relations to surrounding atoms, but it is in the di- 
rection of this preciseness that these researches tend. The 
chemical method of distinguishing between these feldspars, 
which is the ultimate theoretical basis of the law of Tscher- 
mak, encounters the indefiniteness of chemical solutions, and 
the variations of chemical balances. It depends largely on 
the determination of chemical quantities when they are in 
unknown amounts and set free from their normal attinities. 
The exigences of evaporation, unequal heating at critical 
readings, loss by contact with the apparatus and with the re- 
agents — these impose on all chemical determinations certain 
limitations which render it impossible in this way to enter 
into the minutest mineralogical distinctions, and which puts 
before the law of Tschermak a priori an obstacle which 
seems to render it fundamentally unsatisfactory to the human 
mind. Physical methods, however, have only to do with fixed 
quantities, with solids which manifest always the same phys- 
ical characters, and if the refinement of the appliances be 
sufficiently nice, they are the best adapted for the examina- 
tion of the difi'erences that exist between the feldspars. A 
feldspar is a solid. On solution it is no longer a feldspar. 
On fusion it may contain the same chemical elements but it 
cannot be called a feldspar. A feldspar possesses definite ex- 
ternal form and crystalline interior. Its proper examination 
should, therefore, be physical and its specification should be 
dependent on the characters that crystalline solids exhibit. 
Now the physical examination of the plagioclases has been 
greatly advanced by the researches of Michel-Levy. American 
students are hardly aware of the state of progress to which 
this art-science has been carried. Michel-Levy has tabulated 
the properties of the plagioclases as derived from an exami- 
nation of thin sections cut parallel to the bases of the crystals 
and also in those cut parallel to their brachypinacoids. While 
much of this was known before, the full classification and ex- 
pression of the differences is due to the optical and mathe- 
matical skill of Michel-Levy. These tables are to be seen in 
Etude sur la determiiiatioti des feldspaths dans les p/a</aefi 
mince an point de cue dela cla.ssi/fcation des rot-hcs, published 
at Paris in 189i. Tliese tables consist of circular plates rep- 
resenting tlie stereographic projectidu of the crystal and its 
