APPENDIX.
A.—COMPOSITION OF SUBSTANCES.
In the preceding pages all calculations have been carefullyomitted, with a view to rendering the subject as plain and simpleas possible ; as, however, there are many things which cannot bewell explained without reference to numbers, they have beenplaced at the end, in the Appendix . In expressing the compositionof any substance, chemists are in the habit of saying, that itconsists of such and such proportions of its elements ; whateverquantity they may have taken for analysis, they generally cal-culate the proportion which a hundred or a thousand parts wouldconsist of. Thus, for example 550 grains of pure dry Carbonate ofLime contain 308 grains of Lime, and 242 grains of Carbonic Acid ; hence 1000 grains must contain 560 grains of Lime, and440 grains of Carbonic Acid , and 100 grains of Carbonate ofLime contain 56 grains of Lime, and 44 grains of Carbonic Acid.This is a very simple example, but it constantly happens
that the composition of substances is not so easily expressed.
1000 grains of dry Gypsum, or Sulphate of Lime, consist of 412grains of Lime, and 588 grains of Sulphuric Acid . The compo-sition of 100 grains of such a substance is represented thus :
Lime . .41*2
This merely means that one hundred grains contain 41 and 2-tenths of a grain of Lime, and 58 and 8-tenths of a grain of Sul phuric Acid , hence there is no real difference whether we put thedot or not; if it is used, the figures behind it are known to be