Q. Why does lightning turn milk sour?
A. Lightning causes the gases of the air (through which it passes) to combine, and thus produces a poison, called nitric acid; some small portion of which, mixing with the milk, turns it sour.[2]
(N. B. Sometimes, the mere heat of the air, during the storm, turns milk sour.)
[2] The air is composed of two gases, called oxygen and hydrogen, mixed together, but not combined. If oxygen is combined with nitrogen, it produces five deadly poisons, viz.—nitrous oxide, nitric oxide, hyponitrous acid, nitrous acid, and nitric acid, according to the proportion of each gas in the combination.
Q. What is the difference between combining and mixing?
A. When different ingredients mingle without undergoing any chemical change, they are said to be mixed; but when the natural properties of each are altered by the union, then those ingredients are said to be combined.
Q. Give me an example.
A. If different coloured sands be shaken together in a bottle, the various grains will mix together, but not combine: but if water be poured on quick lime, the water will combine with the lime, and not mix with it.
Q. Why are the different grains of sand said to be mixed, when they are shaken together?
A. Because they are mingled together, but the property of each grain remains the same as it was before.