( 569 )

The explanation of Mr. Kirwan’s term Real Acid, ami the method of eftimating its quantityin any mixture of acid and water, whofe fpecific gravity is known, have been already explainedunder the article Nitric Acid , p. 145. and Suxphuric Acid, p. 273. of this volume, and neednot be here repeated at length. It may juIt be obferved, however, that by real fulphwlc acid theauthor means an acid of the fame denlity as that which forms one of the two conftituent partsof perfectly dry and ignited fulphat of potafj —by real nitric acul, he underftands the acid of per-fectly dry nitrat offoda —and by real muriatic acid , the acid of ignited muriat of potajh.

In the above 'Tables the differences between the individual numbers are fo final! as to anfwermoil practical purpofes without further calculation, fo that any fpecific gravity which is not fetdown in the Table, or extra-tabular, may generally be referred to that in the Table to which itis the neareft. If, however, a greater approximation to accuracy be required, the contents percent, in real acid of any extra-tabular fpecific gravity may be found by the Rule of Proportionin the following way, as given by Mr. Kirwan:

Find the difference between the next higher and lower tabular fpecific gravities — D, and alfothe difference between their acid contents = D'.

Then find the difference between the extra-tabular fpecific gravity and the next upper or lowertabular fpecific gravity, whichever it is neareft to — d.

Then the difference between the quantity of real acid of the extra-tabular, and that of theneareft tabular fpecific gravity (rr d ) will be found by the following fimple Rule of Proportion,

D ' d

D : D':: d : d', therefore d' =2-and by adding d' to the acid contents of the loweft tabular

D

fpecific gravity, or fubtradting it from the upper, according to whichever is taken, the quantityfought will be given.

To illuftrate this by a fingle example: What is the quantity of real acid in 100 parts of ful"phuric acid of 1.790 fpecific gravity?

By infpefting the Table I find that 1.790 is between 1.7959 and 1.7849, confequently thetabular numbers concerned, with their refpective differences, will be as follows:

D

Sp. Gr.17959

Real Acid.75.00

IIO

17849 74.11

D' = .89

and d in this cafe is the difference between 17900 and 17849 = 51. The given proportionstherefore are, As no :.89 :: 51 : dl, therefore d! — .413 nearly, which added to 74.11 makes74.523 as the quantity per cent, of real add in fulphuric acid of 1.790 fpecific gravity.

This method, however, is not perfectly accurate, fince it is found that the affinity of fulphuricacid to water proceeds in a decreafing ratio as the quantity of water increafes, and confequentlythat the increafe in fpecific gravity for equal additions of real acid purfues an increafing ratiofrom the bottom of the fcale to the top. Therefore the value of d as above eftimated is rated alittle too low.

By a fimilar method the fpecific gravity of any acid, whofe contents per cent, of real acid areknown, but extra-tabular, may be found. Find D and D' as before and d is already given.D d

Then -= d and d added to the lower fpecific gravity, or fubtra&ed from the upper,

D'

gives the fpecific gravity fought.

4 C

vox, II,