CHE M I 
late, poured a little diftilled water .upon it, to carry oft’ 
any uncombined acid which plight be adhering to it, and, 
having placed it to dige'ft in the Syrup of violets, he 
found, at the end of a'few hours, that the fyrup was 
quite green. We have, in this in fiance, the example of 
a compound uniting with an excels of one of its confti- 
tuenfts, though an excels of the other was prefent. 
II. Chemical affinity is influenced by temperature ; its 
aft ion being accelerated, retarded, prevented, or ren¬ 
dered efficacious, according to the degree of the tempe¬ 
rature, There are two cafes in which temperature mo¬ 
difies the aftion of chemical affinity. In the one, the 
composition of bodies is changed, in confequence of the 
different affinities of their condiment parts with caloric, 
at different temperatures. Let the affinity of A with 
B —6, and its affinity with C— 5 ; 'and let A and C be ve¬ 
ry fixed lubftances, or at lealtmuch lefs volatile than II; 
it is evident that A will unite with B at the common tem¬ 
perature of the atmolphere. But, if we continually aug¬ 
ment the temperature of the three bodies, the tendency 
of B to the Hate of vapour will increafe in a fuller ratio 
than that of the other two, till at length this tendency, 
in addition to the affinity of C with A, will overcome the 
oppofing affinities; B will efcape in the Hate of vapour, 
and. C will combine with A. Obvious examples of this 
mode of influencing affinity by means of temperature, 
occur in every part of chemiftry. 
2. But there are cafes where the accumulation of ca¬ 
loric appears to be a condition effentially requisite to de¬ 
termine bodies to unite, without the caloric combining 
with any principle feparated from thefe bodies at the in- 
ltant of their union, or remaining in combination with 
the new compound formed. This happens particularly 
in cafes of conibultion. Every one knows, that,either a 
very high temperature, or an ignited (park, is neceflary 
to dilpofe hydrogenous gas to combine with the oxygen 
of atmofpheric air or oxygenous gas. In the fame man¬ 
ner charcoal, l'ulphur, phofphorus, &c. require a certain 
temperature for their combuftion, and this temperature 
vaiies according to the condition of the oxygen preferr¬ 
ed to them. Some of thefe bodies, phofphorus in par¬ 
ticular, burn at different temperatures, but with very 
different phenomena. Thefe eftefts of temperature up¬ 
on the affinities have been expreffed in a general form by 
Morveau; but lie has not attempted to give any rati¬ 
onale of their caufe ; nor indeed has any fatisfaftory ex¬ 
planation of them yet been given. From fome curfory 
obfervations made by Berthollet, in an Efl'ay on the 
Combination of Metallic Oxyds with Alkalis and Lime, 
it would appear, that, together, with Monge, he con¬ 
ceives that this effieft of caloric may be generalized, by 
aflimilating it with the operation of mechanical preffiure 
in producing certain chemical combinations. Dr. Hig¬ 
gins has al(o propoled an hypothefis on the Subject. See 
Minutes of the Society for Philofophical Experiments 
and Converfations. 
Two lubftances may have an affinity for each other, 
and yet there may be lome condition wanting, which is 
neceflary for their chemical combination. Acids do not 
unite with alkalis, unlefs the one or the other be diflblved 
in water. Metals do not form alloys with one another, 
unlefs in the Hate of fufion. Metals are not 1 'oluble in 
acids, unlefs they be previously oxydated ; and fome me¬ 
tallic oxyds, thofe of iron and manganefe for example, 
are capable of uniting with an excels of oxygen, fo as* to 
be rendered a fecond time inl’oluble in acids. To pro¬ 
cure a chemical union in thefe cafes, it is neceflary to 
diffolve the acid or alkali in water; to reduce the metals 
to the llate of fufion ; to oxydate the metals that are to be 
combined with acids ; and to difoxygenate in part, thole 
oxyds which have received an.exceis of oxygen. But 
when fubllances have an affinity for one another, and 
are prevented from combining together, as in thefe in¬ 
stances, by the abfence of lome condition neceflary for 
Vql. IV, No. 187. 
S T R Y. T77 
their union, Morveau applies the term cf affimte difpoje i? 
to exprefs the change that takes place in the operation cl 
their affinities when the obstacle to their union is remo¬ 
ved. The ufe of this term may be illustrated from an 
explanation that ha’s been given of tlie facility with which 
iron dilfolves in the diluted'Sulphuric acid. It is necel- 
fary to mention that iron, like the other metals* is info-' 
luble in acids, unlefs previously oxydated. It is alio 
neceflary to obferve, that iron, decompofcs water Slowly 
at the common temperature of the atrriofphere; but when 
a little fulphuric acid is added to the mixture of water 
and iron, the water is then decompofed rapidly,- and an 
oxyd of iron formed, which is immediately diflblved by 
the acid. But upon what principle does the fulphuric 
acid accelerate the decomposition of the* water, Since it is 
incapable of uniting with the iron till the latter has al¬ 
ready de.compofed the water, and acquired the ftaie ot a 
black oxyd ? We anfwer, that the iron, in its metallic 
Slate, lids an affinity with the fulphuric acid; that this 
affinity is indeed incapable of producing combination, 
b.ecaufe the iron, in its metallic Slate, cannot unite che¬ 
mically with any acid; but this affinity, Superadded to 
the affinity which the iron has for the oxygen of the wa¬ 
ter, increafes the Sum of the divellent powers that concur 
in producing the fame feries of chemical effefts. The 
affinity of the acid with the iron, and the affinity of the 
iron with the oxygen of the water, teiidto the fame com¬ 
mon end, and therefore favour each other’s operation. 
The affinity cf the iron with the acid is unable to pro¬ 
duce combination; we muff have the affinity, rendered 
efficacious (the affinite difpofee of Morveau); we muff have 
the iron oxydated. This is accomplished by the con¬ 
curring affinities of the iron with the acid, of the iron 
with the oxygen of the water, and of the acid with the 
oxygen of the water, as forming a neceflary constituent 
of the metallic oxyd. 
It would appear, then, that the term of affinite difpofee 
may be retained in chemiftry, as the general expreffion 
for the effeft of thofe changes in the condition of bodies; 
in Qonfequence of which, their previously exifting affini¬ 
ties are enabled to unite them chemically, and without 
which their union is impoflible. The affinite difp fee in¬ 
cludes, as a more comprehenfive expreffion, what has 
been called the affinity of intermedium. It is true, that 
the fulphat of iron is ultimately formed in confequence 
of the affinity of the fulphuric acid with the oxyd of iron ; 
but how are we to explain the rapid oxydation of the 
iron, without a previously exifting affinity between the. 
acid andiron, in its metallic Slate; and why not a gene¬ 
ral expreffion, for the effeft produced by the removing 
of thofe obftacles in the prefent condition of a body, 
which prevent its affinity for another body from accom¬ 
plishing their union. The anomalous appearances which 
had fuggefted to Baume and Cornette the idea of reci¬ 
procal affinity, have been accounted for, on the received 
principles of chemiftry, by Bergman and Berthollet; 
and their explanations have been adopted by Morveau. 
The views entertained by the molt phiiofdphic of the 
modern chemifts, upon this interesting fubjeft, are too 
remote from immediate application, to find a place with 
any propriety, in an elementary work. They will be: 
found in Bergman’s OpuJ'cula, in Macquer’s Dittionncdre 
de Cbimie, article Fefanteur, and in tlie article affinity, of 
the chemical part of the Encyclopedic Methodique. 
OF THE PRINCIPLES OF BODIES. 
In every age philofophers have admitted, that all the 
variety of natural bodies are formed of primary Sub¬ 
stances, more Ample than themfely.es, which they have 
diftinguifhed by the name of principles. Chemifts, who 
have the llrongeft conviction of this leading truth from 
their analyles, have formed ideas, Sufficiently precife, of . 
the nature and difference of thele principles; and have 
even admitted Several daffies of them. It mult, how*, 
Zz > ever, 
