Calculate in the following manner-- BaCO₃ + H₂SO₄ = BaSO₄ + H₂O + CO₂. BaCO₃ = 197. H₂SO₄ = 98. CO₂ = 44. In the above equation, 98 parts of H₂SO₄ take the place of 44 parts of CO₂. If, therefore, 100 grains of BaCO₃ renders the liquid neutral, the amount of free SO₄ present will be represented by the increased weight of the precipitate in the proportion of 54 to 98, the difference between 44 and 98 the equivalent of CO₂ + SO₄. To extract the acid from organic mixtures, digestion with alcohol is required: filter and neutralise the filtrate with caustic potash, evaporate to dryness, and dissolve the residue in distilled water acidulated with hydrochloric acid; the previous tests may then be applied. Extraction with alcohol leaves behind combined acid in the form of sulphates. The presence of free acid in the filtrate or original mixture may be detected by the change of tropæolin solution from yellow to crimson or ruby colour when added to it; or on adding a few drops of the mixture or filtrate to a solution of potassio-tartrate of iron B.P. in water, so as to make it of a yellow tinge, and to which potassium sulphocyanide has been added, the liquid changes from a yellow to a port-wine colour. IV. Stains on the Clothes, &c. 1. The strong acid changes the colour of black woollen cloth to a dirty brown, the edges of the spots assuming a reddish tint after a few days. The dilute acid on the same cloth produces a red stain, which in time becomes brown. 2. The spots made by the strong acid remain damp for some time--strong sulphuric acid, having a great affinity for water, continually absorbs moisture from the atmosphere. 3. The spot should be cut out, boiled in distilled water, or digested with alcohol, filtered and tested for free sulphuric acid. 4. A portion of the cloth not touched by the acid should be tested, in order to show that the sulphuric acid found is not due to sulphates present in the cloth. 5. An acid sulphate--bisulphate of potash--gives a reddish stain to black cloth like that produced by the dilute acid. Test for this salt by incineration. _Fatal Period._--Average time before death ensues is from two to twenty-four hours. The shortest time was one hour, but in children death may be instantaneous. Life, however, may be prolonged for some weeks, or even months. _Fatal Dose._--One drachm in a healthy adult has proved fatal; on the other hand, however, four ounces have been swallowed without being fatal. _Treatment._--As before mentioned. _N.B._--This acid in past years has given rise to several actions, it having been employed to disfigure the person by throwing it in the face. NITRIC ACID _Forms._--Strong nitric acid, and dilute nitric acid. _Characters._--This acid is commonly known as _aqua fortis_, or red spirit of nitre. It is seldom used as a poison.