Sulphuric acid, the most useful of the acids, is not made directly from its salts, but has to be synthesized. Calcium hydrate is also made by an indirect process, as follows: CaCO3, i.e. limestone, marble, etc., is burnt in kilns with C, a process which separates the gas, CO2, according to the reaction: CaCO3 = CaO + CO2. CaO is unslaked lime, or quick-lime. On treating this with water, slaked lime, Ca(OH)2 is formed, with generation of great heat. CaO + H2O = Ca(OH)2. Its affinity for H2O is so great that it takes the latter from the air, if exposed. Experiment 65.--Saturate some unslaked lime with water, in an e.d., and look for the results stated above, leaving it as long as may be necessary. 105. Resume.--From the experiments in the last few chapters on the three divisions of chemical compounds, acids, bases and salts, we have seen (1) that acids and bases are the chemical opposites of each other; (2) that salts are formed by the union of acids and bases; (3) that some acids can be obtained from their salts by the action of a stronger acid; (4) that some bases can be got from salts by the similar action of other bases; (5) that the strongest acids and bases, as well as others, may be obtained in an indirect way by synthesis. CHAPTER XXII. OXIDES OF NITROGEN. 106. There are five oxides of N, only two of which are important. NITROGEN MONOXIDE (N2O). 107. Preparation. Experiment 66.--Put into a flask, holding 200cc, lOg of ammonium nitrate, NH4NO3; heat it over wire gauze or asbestus in an iron plate, having a d.t. connected with a large t.t., which is held in a receiver of water, and from this t.t., another d.t. passing into a pneumatic trough, so as to collect the gas over water (Fig. 30). Have all the bearings tight. The reaction is NH4NO3 = 2H2O + N2O. The t.t. is for collecting the H2O. [Fig. 30.] Note the color of the liquid in the t.t.; taste a drop, and test it with litmus. If the flask is heated too fast, some NO is formed, and this taking O from the air makes NO2, which liquefies and gives an acid reaction and a red color. Some NH4NO3 is also liable to be carried over. 108. Properties. Experiment 67.--Test the gas in the receiver with a burning stick and a glowing one, and compare the combustion with that in O. N20may also be tested with S and P, if desired. N is set free in each case. Write the reactions. Nitrogen monoxide or protoxide, the nitrous oxide of dentists, when inhaled, produces insensibility to pain,-- anaesthesia,-- and, if continued, death from suffocation. Birds die in half a minute from breathing it. Mixed with one-fourth O, and inhaled for a minute or two, it produces intoxication and laughter, and hence is called laughing gas. As made in Experiment 66, it contains Cl and NO, as impurities, and should not be breathed. NITROGEN DIOXIDE (NO, OR N2O2). 109. Preparation.