tion: A molecule of KCIO, weighs 39.1+35.5+48 = 122.6 m.c., and two molecules will weigh 245.2 m.c. These yield 2KCl, weighing 2 (39.1+35.5) = 149.2 m.c., and 30-0, weighing 96 m.c. We must next find the weight of ten litres of oxygen gas. To find the weight of one litre we multiply the specific gravity of the gas, or half molecular weight, by 8. Now, 180 × 16 = 1.44 gramme. Hence, ten litres weigh 14.4 grammes. But, if 96 m.c. of gas are made from 245.2 m.c. of salt, then 14.4 grammes would be obtained from a quantity easily found from the proportion : 96: 245.2 14.4 : x = 36.78 grammes. I think, after this, we will assume that these quantitative relations are all right, and let them take care of themselves. Returning to the experiment, before I show that the products are those which I have described, let me give just a word of caution to any of my young friends present, who may like to repeat it. We find that it is best to mix our chlorate with a heavy black powder, known in commerce as black oxide of manganese. What the effect of the powder is we do not know, for it is wholly unchanged in the process. But, in some way or other, it eases off the decomposition, which is otherwise apt to be violent. In buying the black oxide of manganese you must take care that it has not been adulterated with coal dust-for a mixture of coal-dust and chlorate explodes with dangerous violence when heated, and serious accidents have resulted from the cupidity which led to such adulteration. Let me, moreover, say in general that, although I highly approve of chemical experiments, as a recreation for boys, they ought always to be made under proper oversight, and according to exact directions, and I would warmly recommend, as a trustworthy companion for all beginners, the abridgment of "Eliot and Storer's Manual of Chemistry," recently edited by Prof. Nichols, of the Institute of Technology. But how shall I show you that this gas we have obtained is oxygen? I know of no better way than to test it with one of our watch-spring matches. . . . In no other gas will iron burn like this. So much for the oxygen. Let us next turn to the other product, that I called potassic chloride. This is left in the retort, forming a solid residue, but, as it would take a long time to bring what we have just made into a presentable condition, we must be content to see some of the product of a former process, which I have in this bottle. At a distance, you cannot distinguish the white salt from the potassic chlorate with which we started, but, if you compared the two carefully, you would see that there was a very great difference between them. I can only show you that the crystals of the two salts have wholly different forms. For this purpose I have crystallized them on separate glass plates, and I will now project a magnified image of the crystals on the screen. There you see them beautifully exhibited on the two illuminated disks side by side. The square figures on the left-hand disk (Fig. 23) are the projections of the cubes of potassic chloride, which differ utterly in form from the rhombic plates of potassic chlorate that appear on the right (Fig. 24). The second example of an analytical process which I have to show you is also familiar to many of my audience, and cannot fail to be interesting to the rest ; for it is the process by which nitrous oxide is prepared, PREPARATION OF NITROUS OXIDE. 197 the gas now so much used by the dentists as an anæsthetic. It was formerly called laughing-gas, but the peculiar intoxication it causes, when inhaled under certain conditions, has been almost forgotten in its present FIG. 23.-Crystals of Potassic Chloride. FIG. 24.-Crystals of Potassic Chlorate. beneficent application in minor surgery. Nitrous oxide is made from a well-known white salt, prepared from one of the secondary products of the gas-works, and called nitrate of ammonia, or ammonic nitrate. When this salt is gently heated in a glass flask, its molecules split up into those of nitrous oxide and water. Again, let us make use of the time required for the experiment to explain the process. The molecules of ammonic nitrate have the constitution N2HO, and the change may be represented thus: N2H403 = 2H2O + N2O. Ammonic Nitrate. Water. The experiment has been arranged so as to show both of the products (Fig. 25). The water condenses in this test-tube, while the gas passes forward, and is collected over a pneumatic trough. But what evidence can I give you that these are, in fact, the products? As regards the water, you would readily recognize the fa miliar liquid, which has collected in the tube, could you examine and taste it. But, as I cannot offer you this evidence, I will seek for another. Most of you must be familiar with the remarkable action of the alkaline metals on water. You see how this lump of potassium inflames the moment it touches the liquid. FIG. 25.-Preparation of Nitrous Oxide and Water, from Ammonic Nitrate. Let us now see whether it will act in a similar way on the liquid which has condensed in our tube. . . . There can be no doubt that we are dealing with water. Next for the gas. Nitrous oxide has the remarkable quality, not only of producing anæsthesia, but also of sustaining the combustion of ordinary combustibles with great brilliancy-like oxygen gas. But there is a marked difference between nitrous oxide and oxygen, which an experiment will serve to illustrate, and this, at the same time, will show us that the gas we have obtained in our experiment is really nitrous oxide. Taking a lump of sulphur, I will, in the first place, ignite it, and when it is only burning at a few points I will immerse it in a jar of oxygen. once burns up with great brilliancy. As you see, it at Taking now a sim ANALYSIS OF NITROUS OXIDE. 199 ilar lump of sulphur, and waiting until you all admit that it is ignited more fully than before, I will plunge it into this jar of gas we have just prepared, and which we assume to be nitrous oxide. . . It at once goes out, and the reason is obvious. There is an abundance of oxygen in the nitrous oxide-relatively, more than twice as much as in the air; but, in the molecules of N2O, the oxygen atoms are bound to the atoms of nitrogen by a certain force, which the sulphur at this temperature is unable to overcome. Let me, however, heat the sulphur to a still higher temperature, until the whole surface is burning, and you see that it burns as brilliantly in the compound as it does in the element ary gas. In the experiment with ammonic nitrate, this salt is resolved, not into elementary substances, but only into simpler compounds, and it will be instructive to inquire how we can push our chemical analysis still further, and from the two products-water and nitrous oxideobtain the elementary substances of which ammonic nitrate is ultimately composed. We have already seen that, by an electric current, water may be changed into two elementary aëriform substances named oxygen and hydrogen, and under such conditions as to prove that water consists of these two chemical elements, and of these alone. The demonstration having once been given, it is unnecessary to repeat this simple analytical process; for in chemistry, as in geometry, we should make little progress if we were always retracing our first steps. Passing, then, at once to the nitrous-oxide gas, let me call your attention to two successive reactions which, although they liberate only one of the constituents of this compound, clearly point out what the other constituent is, and enable us to estimate its amount. |