380 On definite Proportions. 
proportions whatever, has hitherto concealed from us what 
the proper chemical proportions are. Potassium, for ex- 
ample, crystallizes with quicksilver in two determinate pro- 
portions, one of which is twice as greatas the other. The 
arbor Diane is always the same combination of quicksilver 
and silver. If a mixture of zinc and copper is exposed to 
distillation at a high temperature, it loses a part of the zinc; 
but the remainder cannot be separated from it, as long as 
the air is excluded. When zinc is distilled, in order to 
purify it, it leaves some alloys behind, from which the zinc 
cannot he expelled. All this indicates some determinate 
proportions even in the combinations of the metals which 
may be mixed in all possible quantities. We shall here- 
after be able to compute these proportions according to 
those of the protoxides, for the metals must combine with 
each other either in such proportions as to be able to take 
up equal quantities of oxygen, or that the least strongly 
[positive] may be able to take vp 2, 3, or 4 times as much 
oxygen as the most strongly. I had proposed to make a 
series of experiments on this subject ; but since it is diffi- 
cult to perform such experiments with accuracy, and they 
are expensive and tedious, and since the truth of the law is 
sufficiently evident without them, 1 have desisted from my 
purpose. Since the earths must also be considered as me= 
tallic oxides, the same Jaw must be applicable to the com- 
binations of metals and earths in crystallized minerals, or 
in all such as are formed by the operation of chemical affi- 
nities, so that the oxygen must determine the proportion in 
this case also: and it will be necessary that all analyses of 
minerals should be repeated aud examined by this test. 
Protoxide of arsenic. J thought it probable that arsenic, 
which resembles sulphur in its stages of oxygenization, 
must also have a protoxide, containing one-sixth as much 
oxygen as the arsenic acid, that is 8°57 for 100 of the metal, 
I therefore mixed 10 gr. of melted muriate of the pro- 
toxide of Jead with 6 gr. of metallic arsenic, and ignited 
the mixture in a small glass retort, The arsenic sublimed 
in a metallic form, and the muriate remained undecom- 
posed. Jt appears, therefore, that arsenic affords no oxide 
capable of combination with the muriatic acid. 
But every chemist knows that arsenic, exposed to the 
air, falls into a black powder, which is no longer metallic, 
and must therefore be considered as an oxide. I therefore 
exposed two grammes of very finely powdered arsenic, ina 
smal] glass dish, covered with paper, for two months, on 
a stove, to a temperature between 30° and 40° [86° and 
104°], 
