434 A Comparison of the Old and New Theories [Junx, 
the accuracy of the old doctrine, as we know several other examples 
of the same kind, the explanation of which must remain till our 
knowledge be further advanced. Many other bodies susceptible of 
an higher degree of oxidation mutually decompose each other at 
different temperatures, as happens in the present case with azote 
and muriatic acid. When, for example, a cold solution of proto- 
sulphate of iron is poured into sulphate of silver, the silver is pre- 
cipitated by the oxide in the metallic state, and the ferruginous salt 
is converted into persulphate of iron. If the mixture be now boiled, 
the silver again takes oxygen from the iron, and we obtain a solution 
of proto-sulphate of iron and sulphate of silver. This phenomenon 
is still more striking when powder of silver is boiled in a somewhat 
concentrated solution of persulphate of iron. How far soever our 
knowledge shall advance, we must expect still to meet with inex- 
plicable facts. 
It is now asked, How can an anhydrous acid be accurately dis- 
tinguished from a hydrous acid? And how the anhydrous state 
contributes something to elucidate the nature of the exploding 
compound which differs from a compound of common acids? 
Though none of the anhydrous compounds of muriatic acid with 
phosphoric acid, phosphorous acid, &c. can be produced as an 
example, yet whoever will cast his eye over the whole class of these 
compounds will find many instances. 
It has been ascertained, both by my own experiments and those 
of other chemists, that many of the more powerful acids, which the 
older chemists considered as pure, are combined with water, which 
serves them as a basis, without, however (as is the case with most 
other bases), diminishing their acid properties. Several of these 
acids are of such a nature that chemistry knows no method of ex- 
hibiting them in an anhydrous state. Others of them may be 
brought to a state of purity, and then show very striking properties, 
which deserve more attention from chemists than has hitherto been 
bestowed on them. 
If we cast an eye upon phosphoric and boracic acids rendered 
free from water by heat, and upon dry carbonic acid gas, we per- 
ceive at once that in this state they have lost a good deal of their 
acid properties, which they only recover by the addition of water. 
When glassy phosphoric or boracic acid in powder is brought in 
contact with ammoniacal gas over mercury, no absorption of the 
gas takes place, and no ammoniacal salt is formed. But if a moist 
paper be introduced into the jar, and left on the surface of the 
mercury, the formation of phosphate or borate of ammonia in- 
stantly begins, and continues till the paper is thoroughly dry, and 
all the water has entered into combination with the salt. From 
this it seems to follow that no simple ammoniacal salt can exist 
without combined water. But as sub-ammoniacal salts exist with- 
out water, and as boracic acid is capable of forming sub-salts, a 
sub-borate of ammonia, in such a case, might be supposed capable 
of being formed. If well-burnt lime, free from water, be brought 
ee 
——— CO 
