0° and 760 mm., it is obvious that w grms. thereof under these conditions would occupy But the specific gravity of a vapour, s, is the ratio of the weight of a certain volume thereof to the weight of the same volume of air measured under the same conditions. Hence we [It will be noted that the value found for 8 is independent of the temperature of the bath; it matters not what this may be so long as it is high enough to rapidly vaporise the body experimented on, and is constant during the experiment.] The theoretical specific gravity of ether vapour (air=1) is 2·56. It is generally necessary to purify the ether. This is done by allowing it to stand for several hours over fused CaCl2; then pouring it off into a flask containing thin slices of sodium. The flask should be provided with a cork through which an open chloride of calcium tube is inserted; this allows of the escape of hydrogen, but prevents absorption of atmospheric moisture by the ether. When the hydrogen bubbles have entirely ceased to be disengaged, the ether is anhydrous and free from alcohol, and is ready for use. = -- (2) The specific gravity of alcohol vapour. Since alcohol has a much higher boiling point than ether [alcohol, B. P. 78°; ether, B. P. = 35.5°] it is advisable in determining its specific gravity to use an aniline-vapour bath instead of a steam bath. Aniline boils at 182°. At this temperature alcohol is rapidly vaporised; whereas at a temperature of 100° the vaporisation although complete would be much slower. But the more rapid the vaporisation the less chance is there of error due to the diffusion of the vapour into the colder parts of the apparatus and its condensation there. The alcohol used should first be rendered as nearly anhydrous as possible by letting it stand for several hours in a corked flask either over anhydrous copper sulphate or over freshly-burnt and coarsely powdered quicklime, and finally distilling off on a water bath. The experiment is executed, and the result calculated, exactly as in the case of ether. (3) The specific gravity of water vapour. Although an aniline bath would serve in this case, yet it is well that the student should familiarise himself with the use of the sulphur bath, which is much used when a high and constant temperature is desired. Coarsely powdered sulphur is at first cautiously heated in the bath A (Fig. 43) till it melts, and then strongly heated till it boils, which it does at a temperature of about 440°. The experiment is conducted, and the result calculated, as before. You now know that the molecular weight of water-gas is approximately 18. To find whether it is exactly 18 or not, you must determine the mass of oxygen which combines with 1 part by weight of hydrogen to form water. Exp. 2. Quantitative synthesis of water. Arrange an apparatus as shewn in Fig. 44. A is a piece of hard glass dry HE A Fig. 44. tubing about 15-20 centims. long; B and C are small light U tubes containing dry calcium chloride; each tube is thoroughly dried before the calcium chloride is put into it; the corks of B and C must fit very securely; these corks are Two little pieces of caoutdried in a steam bath before use. chouc tubing are provided, each fitted at one end with a small glass rod as a stopper, of such a bore that they will just pass over the ends of the tubes a and b. The tube A is thoroughly dried. A quantity of pure copper oxide, CuO (prepared from copper wire), is strongly heated over a lamp, and then quickly transferred to A, which should be about 2 filled with it; a good cork carrying a tube of calcium chloride is at once placed in one end of A, the other end is firmly closed by a good cork, and A is allowed to cool. The U tubes are closed by the caoutchouc caps placed over a and b. A gas-holder is now filled with hydrogen, prepared by the interaction of pure zinc and pure sulphuric acid diluted with about 20 volumes of water in a flask kept cold by being surrounded with cold water, the acid being added when it is quite cold and then in small successive quantities. The exit tube of the gas-holder is connected with 2 or 3 bottles containing concentrated sulphuric acid, and these are followed by a pair of large U tubes containing calcium chloride and a small tube containing phosphorus pentoxide. The hydrogen is caused to flow in a gentle stream through the drying bottles and tubes for a little time. When A is cold, it is carefully weighed; the tubes B and C are also weighed. The caoutchouc caps are withdrawn from the tubes a and b (the caps must be carefully preserved); the tube a is pushed through a good cork which is then tightly fitted into A. A very good, soft, tightly fitting, cork, carrying a short piece of glass tubing, is now placed in the other end of A, and this glass tubing is connected with the hydrogen apparatus. A slow stream of hydrogen is allowed to pass through the entire apparatus for about 5 or 10 minutes; the tube A is then cautiously heated beginning at the end nearest the calcium chloride tubes and gradually extending to the other end. It is advisable to have two Bunsen-lamps, so that while one is used in heating the main part of A the other may keep the end of A nearest the U tubes so hot that water cannot condense there. To prevent all risk of charring the cork, it is well to have a disc of asbestos cloth with a slit in it which can be slipped over the tube near the cork. The heating of A is continued until the colour shews that the greater part of the black copper oxide has been converted into red copper. Before this point is reached an apparatus for evolving a stream of carbon dioxide is prepared. The hydrogen-generator is removed, and the exit tube from the flask in which carbon dioxide is being prepared is attached to the drying bottles and tubes. The tube A is kept red hot for 10 or 15 minutes while the carbon dioxide stream passes through it; by this means the last traces of water are driven out of A into B and C. The carbon dioxide is then stopped, and the apparatus is allowed to cool. The parts are detached, and the corks &c. are adjusted as at the beginning of the experiment; A is weighed, and also B and C together. The decrease in the weight of A gives the weight of oxygen which has combined with hydrogen to form water; the increase in the weight of B and C gives the water formed; the difference between the weight of water and that of oxygen gives the weight of hydrogen which has combined with the oxygen to form water. 8 parts by weight of oxygen combine with 1 part of hydrogen to form 9 parts of water. The results of Exps. 1 and 2 in this Chap. shew that 16 parts by weight of oxygen combine with 2 parts by weight of hydrogen to form a molecule of water-gas. The atomic weight of oxygen is therefore not greater than 16, but it may be less than this number. In order to determine the value of the atomic weight of oxygen it would be necessary to find the composition of a number of compounds of oxygen and to determine the molecular weight of each (s. "ELEMENTARY CHEMISTRY," Chap. XV., par. 297). The following experiments (3 and 4) are performed with the object of finding whether the atomic weight of oxygen is 16 or a submultiple of 16; they also serve to establish a probable value for the atomic weight of carbon. Exp. 3. Determine the molecular weight and composition of carbon dioxide. Arrange an apparatus as shewn in Fig. 45. A A is a piece of hard glass tubing about 15-20 centims. long, communicating with the potash bulbs B, which again are in connection with the small light tube C; at b is a little dry pure copper oxide (to oxidise any carbon monoxide that might be produced in the combustion). The bulbs B are filled with a solution of potash (1 part KÖH in 2 parts water) to such an extent that a fairly rapid stream of gas may be sent through the bulbs without danger of any liquid being lost by spirting. Tube C contains loosely packed solid potash. The other end of A communicates with an apparatus for supplying a regulated stream of pure dry oxygen. The oxygen is stored in a gas-holder, from which it is passed through two bottles of potash solution to absorb carbon dioxide, and then through two bottles of concentrated sulphuric acid and a large U tube containing calcium chloride to absorb moisture. The bulbs B are closed by caoutchouc caps and weighed; the ends of C are also closed and the tube is weighed. A small porcelain boat (indicated by a in the fig.) is strongly heated, allowed to cool, and weighed; a little bit of diamond is then placed in the boat which is again weighed. The boat is now placed in the tube A and the apparatus is arranged as shewn in the figure. The copper oxide, b, is gradually raised to full redness, a slow stream of oxygen is passed through the apparatus, and the diamond is heated until it burns in the oxygen. The product of the burning is carbon dioxide, which is absorbed by the potash in B and C. When the burning is finished the apparatus is allowed to cool; B and C are weighed together, with the caoutchouc caps on; and the boat is withdrawn and weighed. A very little ash remains when diamond is burnt; the weight of ash found is deducted from the weight of diamond used, and the difference gives the weight of pure carbon burnt to carbon dioxide. The increase in the weight of B and C gives the weight of carbon dioxide formed. Assuming that the sole product of the burning was carbon dioxide, the results obtained enable the composition of this compound to be established. The results of this experiment, if very carefully conducted, shew that 8 parts by weight of oxygen combine with 3 parts by weight of carbon to form 11 parts by weight of carbon dioxide. The molecular weight of carbon dioxide is therefore n11, where n is a whole number. We must now determine the value of n. An approximately accurate determination of the specific gravity of the gas carbon dioxide will evidently suffice for this purpose. Clean and dry a light flask of about 250 c.c. capacity. Close the flask by a very good well-fitting cork fitted with a short exit tube closed by a piece of caoutchouc tubing and a screw-clamp, and weigh it. In weighing the flask it is advisable to place another similar flask in the other pan and then to add weights until equilibrium is established. Note the temperature and pressure of the air. Pass a fairly rapid |