Nitric Acid. Chili-nitrate is always used for making nitric acid. It is the more abundant of the two naturally occurring nitrates, and therefore cheaper; moreover, weight for weight, it yields more nitric acid than the corresponding potassium compound. A mixture of sodium nitrate and sulphuric acid is heated in a large cast-iron retort (C, Fig. 5). The retort is entirely surrounded by flame and hot gases to prevent the condensation of the acid on the upper parts. If this precaution were not taken, the acid would dissolve the iron and the life of the retort would not be long; moreover, the product would contain ferric nitrate as an impurity. The vapour of the acid is led away by the tube D into a series of two-necked earthenware receivers called _bonbonnes_ (E), and there condenses to a liquid. The lower figure shows how the leading tube of the retort is protected from corrosion by the clay tube _a_, _b_; and how it is connected to the first receiver by the glass tube _e_, which is luted on at _f_. The percentage strength of the acid which distils over depends upon that of the sulphuric acid used and on the purity of the sodium nitrate. Pure nitric acid is a colourless liquid 1·559 times as heavy as water, volume for volume. It fumes strongly in air, and is a very corrosive liquid. The pure acid of commerce is obtained by distillation of a less concentrated acid. It is 68 per cent. pure. It is rendered free from dissolved oxides of nitrogen by blowing air through it. When kept exposed to light, the colour changes at first to yellow and then to brown, because light causes a certain amount of decomposition. Red fuming nitric acid owes its colour to the great quantity of oxides of nitrogen dissolved in it. It is made by distilling sodium nitrate that has been thoroughly dried with the strongest sulphuric acid; the distillation is carried out at a high temperature, with the express purpose of decomposing some of the nitric acid to furnish the oxides of nitrogen. Sometimes a little powdered starch is also added to facilitate the formation of these oxides. This variety of nitric acid is particularly active and is used in many operations, especially in making dyes, explosives, and other organic chemicals. Nitric acid has all the general properties of an acid, that is, it has a sour taste even in very dilute solution, it changes the colour of litmus to red, and dissolves carbonates and many metals. When the vapour of nitric acid is passed through a red-hot tube, and also when a nitrate is strongly heated, oxygen gas is given off. Analysis shows that the oxygen combined in pure nitric acid amounts to 76 per cent. of its weight, while that in sodium and potassium nitrates is 56 and 50 per cent. respectively. Nitric acid and the nitrates are, therefore, highly oxygenated compounds; moreover, under favourable circumstances, they are rather easily broken up.