TRANSACTIONS OF THE SECTIONS. 59 
other. It is curious, that the clay in the neighbourhood answers 
perfectly for making crucibles for cast steel; but it does not answer so 
well as Stourbridge clay for making glasshouse pots. On analysing the 
two clays, it was found that the Garnkirk contained much more 
alumina and less silica than the Stourbridge ; showing that glass in 
fusion acts more powerfully on alumina than on silica. 
2. Another manufacture of importance, and which is indebted to 
Glasgow for the state of perfection which it has reached, is that of 
sulphuric acid. It was begun by Dr. Roebuck, at Preston Pans, about 
the year 1763, but it is not more than twenty years since his manu- 
factory was abandoned. The sulphuric acid works at St. Rollox, on 
the banks of the Monkland canal, were begun about forty-five yearsago. 
They were at first upon a very small scale, though they now probably 
are the largest of the kind in Europe. Dr. Roebuck’s method was to 
mix together sulphur and saltpetre, and after setting the mixture on 
fire, to introduce it into a leaden vessel or chamber, at the bottom of 
which there was a quantity of water. This method was not ceconomical. 
A portion of the sulphur would unite with the potash of the saltpetre, 
and form with it a sulphuret, and probably a portion of the sulphuric 
acid formed would also unite to the potash and form a sulphate. 
When Messrs. Knox, Tennent, and Macintosh established their works 
at St. Rollox, they separated the sulphur from the saltpetre ; the sul- 
phur was burnt over a stove, and an iron cup, containing the requisite 
quantity of saltpetre, mixed with the requisite quantity of sulphuric 
acid, was placed over the burning sulphur. By this contrivance the 
sulphur was completely converted into sulphurous acid, and the whole 
of the nitric acid carried along with it into the leaden chambers. The 
size of the leaden chambers was yradually increased, and the substitu- 
tion of steam for the water formerly placed at the bottom of the cham- 
bers, was a vast improvement. The acid which collects at the bottom 
of the chambers has now a specific gravity of 1°75, or it is a compound 
of one atom anhydrous acid, and two atoms water. This acid is con- 
centrated by heating it in a platinum still till the second atom of water 
is driven off. When this manufacture is at full work, the quantity of 
sulphuric acid made in it exceeds 300,000]bs. avoirdupois per week. 
When he first began to purchase sulphuric acid, about forty-five years 
ago, it cost 8d. per pound ; the present price is under a penny a pound. 
3. One of the great purposes to which sulphuric acid is applied at 
St. Rollox, is the manufacture of bleaching powder, or chloride of lime, 
as it is now called. When the mode of bleaching by chlorine was 
introduced into Great Britain, by Mr. Watt, in 1787, the very offensive 
smell and deleterious effects of that gas upon the workmen, was a 
formidable objection to its use. Various methods were tried to remove 
this objection. It was found that if potash or soda was dissolved in the 
water before it was impregnated with the chlorine gas, the disagreeable 
smell was destroyed ; but, unfortunately, this addition destroyed at the 
same time the bleaching power of the gas. At last, Messrs. Knox, 
Tennent, and Macintosh discovered that if lime were mixed with the 
water before it was mixed with the gas, the disagreeable smell was 
