edt. Jat? tt 
XXX. On a new Method of preparing a Teft Liguor to foew the 
_Prefence of Acids and Alkalses in chemical Mixtures. By Mr. 
_ James Watt, Engineer ; communicated by Sir Jofeph Banks, 
Y Bore, ER. 4S. 
| Read May 27, ane 
HE fyrop of violets was formerly the teft of the point of 
{aturation of mixtures of acids and alkalies, which was 
principally ufed; but fince the late improvements in chemiftry | 
it has been found not to be fufficiently accurate, and the infu- 
fion of tournefol, or of an artificial preparation called litmus, 
have been fubftituted in the place of it. 
_ The infufion of litmus is blue, and becomes. red ‘tiie acids. . 
It is fenfible to. the prefence of one grain of common oil of 
vitriol, though it be mixed with, roo000, grains of water ; 
but as this infufion does not change its colour on being mixed 
with alkaline liquors, in order to difcover whether a liquor be 
neutral or alkaline, it is neceflary to add fome vinegar to the 
litmus, fo as juft to turn the infufion red, which will then be 
reftored to its blue colour, by being mixed with any alkaline 
liquor. The blue infution of litmus is alfo a teft of the pre- 
- fence of fixed air in water, with which it turns red, as it does 
with other acids. | 
_ The great degree of fenfibility of this teft would leave very 
little reafon to fearch for any other, were there reafon to be- 
lieve that it is always a teft of the exact point of faturation of 
Pitz acids 
F od 
