posal of Dr Irvine is inly ingenious ; but we 
ala od. yotige poles iy ich per 0. 0 very 
from bei e to solve with any degree of accura- 
| tbe farrdasnontal positien 
perf 
the subject by different philosophers, have been attend- 
xiddy po cha ped ony results, as to show, that 
Cog the omy | or practice must be extremely imper- 
Besides Irvine's, the principal experiments which 
were ed, are those of Crawford, Gadolin, 
Lavoisier in conjunction with Laplace, and Seguin. 
pid these, re see to vp ney ome dea 
greatest care and accuracy, approac 
aart Wediyl te these af Tevine.| His method was to as- 
-certain the capacity of common salt, and also of its so- 
lution in water, and then he observed the degree of cold 
‘which is produced daring the solution. » He afterwards 
Bramnited the quantity of heat which was absorbed by 
the mutual action of common salt and snow, and the heat 
-extricated by the mixture of sulphuric acid and water ; 
and, by comparing these with the capacity of the bodies 
before their union, he endeavours, as before, to deter- 
mine the p’ ion which their absolute bore to their 
specificheat. (Crawfordlon An. Heat, p.457, et seg.) Lavoi- 
sier and Laplace employed the calorimeter, to measure 
the quantity of heat disengaged during the mixture of 
certain substances, comparing this with the capacity of 
the bodies separately ; but it would appear, that their 
iments were more liable to inaccuracy than those 
of Gadolin, and their results are completely at variance 
“with each other: (Mem. Acad. Scien. 1780, p. 384, et seq.) 
Mr Dalton has attempted to discover the real zero, by a 
-caleulation depending upon his idea of the constitution 
of an aeriform fluid, cid the relation which subsists be- 
tween its basis, and the caloric which enters into its 
‘composition. Without attempting a detail of his hypo- 
thesis, we shall merely state his conclusion, that the 
absolute quantity of caloric in elastic fluids at different 
. temperatures, is in the direct ratio of the cube roots of 
their bulk at the same temperature: (Manch. Mem.vol. v. 
p- 601.) How far this hypothesis may have any foun- 
dation in fact, we do not feel at present competent. to 
decide; but it has been asserted, that experiments 
made upon this principle at different temperatures, 
~G upon various substances, do not correspond to each 
other. ; 
se of - From the remarks that have been made, it will ap- 
€ capacity ents that Dr Black’s discovery of the existence of what 
‘bodies. he called latent heat, or of a difference in the capacity 
of bodies for heat, is a fact completely established by 
-experiment, and one of the most important in the whole 
‘range of chemical science. Very different opinions, 
“however, have been entertained respecting the manner 
/of accounting for the fact, and especially the question 
has been warmly agitated, whether the absorption of 
heat when a solid is converted into a liquid, isto be re- 
gt ote deci diene Acpphapeacbenr ps of state. 
lack?s opi- Dr Black conceived that it was the cause of the change ; 
‘on. that the body in question, as for example, a portion of 
_ ice, being exposed to the influence of caloric, this prin- 
ciple necessarily enters into it, becomes combined with 
its particles, and is not oo of producing any effect 
‘until it be again extricated by the liquid changing its 
form, or entering into some new combination. Dr Ir- 
me's. Vine adopted the contrary opinion, and regarded the ab- 
sorption of heat as the consequence of a change of capa- 
‘city which the ice undergoes when it is converted into 
water, which change of capacity necessarily causes it to 
absorb, and render latent a quantity of heat: (Black’s 
HEAT. 
687 
Lect. vol. i. p. 194.) Dy Irvine's opinion seems to be. Effectsof 
supported by the fact, that there actually is this diffe- Hest. 
rence of capacity in the substance ; but then it does not 7 7" 
appear to assign any cause for the change of state which 
the body undergoes. If we adopt it, we are obliged to 
assume as a principle, that when the solid is so situated 
as to receive above a certain quantity of heat, its parti- 
cles are suddenly forced to a considerable distance from 
each other, in consequence of something peries in its 
nature and constitution ; and that when this separation 
has been effected, the capacity becomes increased, and 
the caloric of course absorbed. Dr Black conceived, 
that the caloric was united to the body by a power simi- 
lar to chemical attraction ; andif we adopt this opinion, 
it would follow, that at certain temperatures substances 
acquire different degrees of affinity for heat, and form 
with these quantities an intimate union, which renders 
it no longer cognizable by the ordinary means. ~ 
4. The last effect of heat, which yet remains for us to * Ignition. 
consider, is ignition. Ignition, or, as it is sometimes 
called, incandescence, is the property which some bo- 
dies possess, after being exposed to a high tem > 
of becoming luminous, without any ical change 
taking place in their composition. In this respect, it 
differs essentially from combustion, where the bod 
extricates light, but where it is found to have experi- 
enced a complete. change in its chemical nature. Com- 
bustion also differs from ignition in another particular, 
that, in the former process, the co-operation of the at- 
mosphere, or some other external substance, is neces- 
sary ; whereas the latter is capable of existing without 
the intervention of any other body. It has been con- 
ceived, that, by proper management, all solids, and 
even liquids, may be rendered luminous, or brought 
into the state of ignition ; and Mr Wedgewood per- 
formed some experiments, which lead to the conclu- 
sion, that they all undergo this change at the same 
temperature. Many philosophers have attempted to 
find out the exact degree at which ignition commences ; 
and although their processes do not furnish precisely 
the same result, yet they correspond so far, as to ren- 
der it probable, that it must be somewhere between 
the 800° or 1000° of Fahrenheit. Mr Wedgewood 
sup) , that a body becomes just luminous in the 
dark at 947°; and that a full red heat, visible in open 
daylight, takes place at 1077°._ According to the inten- 
sity of the temperature, the colour of the ignited body is 
altered, At first, it exhibits what has been called a cherry 
red ; afterwards, the red acquires a yellowish tinge ; 
and, lastly, all colour disappears, and we have only a 
brilliant white light: (Phil. Trans, 1784, p. 370.) It 
is generally supposed, that aeriform fluids are not ca- 
pable of being ignited, A quantity of air, heated to 
such a degree that a piece of metal, when suspended in 
it, was rendered luminous, was itself quite invisible. 
Respecting the cause of ignition, two opinions have 
prevailed ; first, that light is a modification of heat, 
and that, at a high temperature, the one becomes con- 
verted into the other; the second, that light actually 
exists as a component of different bodies, and that, 
when the body is strongly heated, the light is propel- 
led or disengaged from it. Jt is perhaps impossible to 
adduce any direct arguments in favour of either of these 
hypotheses ; but the general relations of heat and light 
are in many so different from each other, 
we are induced to adopt the opinion which supposes 
them to depend upon the agency of principles essen- 
tially dissimilar. This, however, we advance as a 
doctrine which must be confirmed or refuted by future 
observations and experiments. 
Wedge- 
wood’s ex- 
periments. 
Cause of 
Ieniti 
