Experiment 68.--Into a t.t. or receiver put 5g Cu turnings, add 5 cc. H2O and 5 cc. HNO3. Collect the gas like H, over water. 3Cu + 8HNO3 = ? What two products will be left in the generator? Notice the color of the liquid. This color is characteristic of Cu salts. Notice also the red fumes of NO2. 110. Properties. Experiment 69.--Test the gas with a burning stick, admitting as little air as possible. Test it with burning S. NO is not a supporter of C and S combustion. Put a small bit of P in a deflagrating-spoon, and when it is vigorously burning, lower it into the gas. It should continue to burn. State the reaction. What combustion will NO support? Note that NO is half N, while N2O is two-thirds N, and account for the difference in supporting combustion. NITROGEN TETROXIDE (NO2 or N2O4). 111. Preparation. Experiment 70.--Lift from the water-pan a receiver of NO, and note the colored fumes. They are NO2, or N2O4, nitrogen tetroxide. NO + O = NO2. Is NO combustible? What is the source of O in the experiment?OXIDES OF NITROGEN. NITROGEN TRIOXIDE (N2O3). 112. Preparation. Experiment 71.--Put into a t.t. 1 g. of starch and 1 cc. of HNO3. Heat the mixture for a minute. The red fumes are N2O3 and NO2. Nitrogen pentoxide, N2O5, is an unimportant solid. United with water it forms HNO3. N2O5 + H2O = 2HNO3. CHAPTER XXIII. LAWS OF DEFINITE AND OF MULTIPLE PROPORTION. 113. Weight and Volume.--We have seen that water contains two parts of H by volume to one part of O; or, by weight, two parts of H to sixteen of O. These proportions are invariable, or no symbol for water would be possible. Every compound in the same way has an unvarying proportion of elements. 114. Law of Definite Proportion.--In a given compound the proportion of any element by weight, or, if a gas, by volume is always constant. Apply the law, by weight and by volume, to these: HCl, NH3, H2S, N2O. There is another law of equal importance in chemistry, which the compounds of N and O well illustrate. Weight. Volume. N. O. N. O. Nitrogen protoxide N2O 28 16 2 1 Nitrogen dioxide N2O2 28 32 2 2 Nitrogen trioxide. N2O3 28 48 2 3 Nitrogen tetroxide N2O4 28 64 2 4 Nitrogen pentoxide N2O5 28 80 2 5 Note that the proportion of O by weight is in each case a multiple of the first, 16. Also that the proportion by volume of O is a multiple of that in the first compound. In this example the N remains the same. If that had varied in the different compounds, it would also havevaried by a multiple of the smallest proportion. This is true in all compounds. 115. Law of Multiple Proportion.--Whenever one element combines with another in more than one proportion, it always combines in some multiple, one or more, of its least combining weight, or, if a gas, of its least combining volume.