ON THE SPECIFIC GRAVITY OP SULPHURIC ACID. 13 
error of the specific gravity s according to Table II. (A.), on which the cal¬ 
culation of the contractions of the volume is founded. 
Under all circumstances, according to the formulas (1.) and (3.), it may 
be t^en as a general rule, that when to sulphuric acid of different degrees of 
dilation so much water is added that the volume of the mixture reaches its 
maximum of contraction, then the value of this contraction can be repre¬ 
sented by the length of the ordinates of an equilateral hyperbola (the asymp¬ 
totes being parallel to the axes of dio co-ordinates), when the number of 
atoms of water, which in the acid employed are combined with 1 atom of dry 
acid, is taken as the abscisses. 
As it must constantly be taken as a rule that heat is evolveil by a dimi¬ 
nution or contraction of volume of a body, and it ia known that by mixing 
an already much diluted sulphuric acid with inon? water a wmsible quantity 
of heat is evolved, it was natural to believe that tlie heat so produced stood 
in some relation to the decrease of the volume. Such a relation in the mean¬ 
time has not been proved, and chemists have denied tin: possibility of any 
such direct relation between the contraetion and the heat evolved, for the 
reason that there are other bodies, for instance, alcohol of certain degrees of 
dilution, which by nu.\ing with water increase in volume instead of decreasing, 
and yet produce heat. But it appears to me that this objection is not deci¬ 
sive, It cannot well be said that the production of heat is a direct or immc’ 
dmie effect of the change of the volume, a» It woulii certainly be a paradox 
t^t an extension and a contraction of volume should have Just the same 
effect; but more correctly it may be supposed that the change of the volume 
as well as the production of heat are both the cfTocta of a higher cause, 
namely the endeavours of the chemical or molecular forces to obtain a new 
state of equilibrium; uud as the value of both these effects must be in pro¬ 
portion to the intensity of the acting force, it is not improbable that the 
increase of volume as well as the contraction may be expressed as a function 
ol the heat evolved, or the contrary. 
The consideration that tlio molecules of the fluid, when the volume of the 
mixture has reached its minimum, must bo supposed to be most syminctri- 
c> j arranged, and to have obtained a stable equilibrium, which they even 
wu a certain inertia try to retain, because the volume in the neighbour- 
ood ol jte minimum is subject only to excessively slow eliange upon the 
addition ol more water, in its proportion to the sum of the volumes of the 
mixed —this couaideration, in combination with the following experi- 
mcDt of Parkes, induced me several years since to endeavour to find a path 
discover the relation between the change of the volume and the boat 
evolved, which at iengtli, at least so far as sulphuric acid is concernctl, 
^Pl^ars to h.aveled to a satisfactory answer to the question. 
Parkes has tried several experiments of the temperature w hich is produced 
when concentrated sulphuric acid and water are mixed in severj propior- 
tions. ^ He found that when to a great quantity of water greater ami greater 
<|nantities of acid were addoil by degrees, the temperature of the mixture 
increased to a certain maximum, and decreased again with the further addi¬ 
tion of acid. This maximum tenqieratura (2l(> F.) took place when the 
quantity of water and acid was iei the proportion of 10:25, consequently 
'vhen the mixture contained 1 equivalent dry acid and 5 equivalents 
water, or it occurred with the same decree of dilution as the maximum of 
contraction. 
By his important thermo-chemical experiments Hess has endeavoured to 
when the several hydrates of sulphuric acid SOs H,.0, SO, 2H;0, 
bU, 3H^, &c. are mixed with an excess of water, then the quantities of heat 
