Water has an affinity to, and combines with, almost all natural substances, aerial, fluid, or solid; but most intimately with acids, alkalies, calcareous earth, and that calx of iron which is called _finery cinder_, from which the strongest heat will not expel it. It has been supposed by some, that by frequent distillation, and also by agitation, water may be converted into a kind of earth; but this does not appear to be the case. It has also of late been thought, that water is resolvable into dephlogisticated and inflammable air; but the experiments which have been alleged to prove this do not satisfy me; so that, for any thing that appeared till very lately, water might be considered as a simple element. By means of heat, however, it seems to be resolvable into such air as that of which the atmosphere consists, viz. dephlogisticated and phlogisticated, only with a greater proportion of the former. Water, with respect to specific gravity and temperature, has generally been made the standard to all other substances; its freezing and boiling points being the limits by means of which thermometers are graduated. Other substances have also been compared with water, as a standard, with respect to the capacity of receiving heat, and retaining it in a latent state, as will be shewn when we consider the subject of heat. LECTURE XII. _Of the Nitrous Acid._ Under the head of _liquids_ I shall consider _acids_ and _alkalis_, though some of them may be exhibited in the form of air, and others in a solid form. These two chemical principles are formed to unite with one another, and then they constitute what is called a _neutral salt_. Both acids and alkalis are distinguishable by their taste. Another test, and more accurate, is, that acids change the blue juices of vegetables red, and alkalis turn the syrup of violets green. Acids are generally distinguished according to the three kingdoms to which they belong, viz. _mineral_, _vegetable_, and _animal_. The mineral acids are three, the _nitrous_, the _vitriolic_, and the _marine_. The nitrous acid is formed by the union of the purest inflammable air, or the purest nitrous air, with dephlogisticated air. But it is usually procured from nitre by means of the vitriolic acid, which, seizing its base, expels the nitrous acid in a liquid form. On this account this acid is said to be weaker than the vitriolic. If the nitrous acid be made to pass through a red-hot earthen tube, it will be decomposed, and the greatest part of it be converted into dephlogisticated air.