1816.) —_ respecting the Nature of Oxymuriatic Acid. 439 
a sudden appearance of fire, and then the temperature again sinks 
to that of the surrounding combustibles. I made numerous expe- 
riments to elucidate the nature of this appearance, and ascertained 
that the weight of the salt was not altered, and that the appearance 
took place without the presence of oxygen. Before the appearance 
of fire, these salts are very easily decomposed, but afterwards they 
are neither attacked by acids nor alkaline leys—a proof that their 
constituents are now held together by a stronger affinity, or that 
they are more intimately combined. Their opposite electro-chemical 
polarity must, therefore, be more completely neutralized, by which 
these bodies are brought to a state of chemical indifference. The 
cause of the appearance of the fire, therefore, is that in a higher 
temperature a more intimate chemical combination, or a stronger 
electro-chemical discharge must take place between the bodies. 
Hence it follows that between the same bodies different degrees of 
intimacy in point of combination may subsist. From my disserta- 
tion it will be seen that I even at that time foresaw that these facts 
would furnish a key to explain the explosion of euchlorine. 
Some time after, during my short stay in London, I mentioned 
this appearance to Wollaston and Davy. The former observed that, 
to his great surprise, he had seen something similar in gadolinite.* 
Davy had observed the same appearance of fire on heating the 
hydrate of zirconia, which he ascribed to a contraction of the earth 
at the instant when the water separated, Since that time I have 
observed these appearances in many other bodies; as, for example, 
in green oxide of chromium, oxide of tantalum, and oxide of rho- 
dium. I shall take oxide of chromium as an example. 
Upon pulverized chromate of lead let a mixture of concentrated 
muriatic acid and alcohol be poured. Heat is produced, and ether, 
muriate of lead, and muriate of chromium, formed. Let the solu- 
tion be mixed with more alcohol, in order to separate all the lead 
salts. Then distil off the alcohol, dilute the solution again with 
water, and precipitate all the oxide of chromium with caustic am- 
monia, adding a slight excess of alkali. Let the greenish-grey pre- 
cipitate, which is a hydrated oxide of chromium, be separated, 
dried, and heated, in a crucible or small retort till it be slightly 
red-hot. Water is given out, and the oxide becomes greyish-black, 
or almost black. Let it now be taken from the fire, weighed, and 
placed in a strong heat. As soon as it becomes red-hot we shall 
perceive it all on a sudden become intensely ignited, and this igni- 
tion disappears almost as suddenly, Oxide of chromium, by being 
thus treated, loses no weight. Its colour is pale green; and instead 
of its easy solubility in the state of hydrate, it has become, when 
* The appearance of fire which gadolinite displays is very lively. The variety 
with a glassy fracture answers better than the splintery variety. It is to be heated 
hefore the blow-pipe, so that the whole piece becomes equally hot, At a red 
heatitcatches fire. The colour becomes greenish-grey, and the solubility in acids 
is destroyed, Two small pieces of gadolinite, one of which had been heated to 
redness, were put in aqua regia; the first was dissolved in a few hours; the second 
Was not attacked in two months, 
a 
