8. The first contraction is 12 vols., and in making this, all the hydrogen has been combined; therefore The oxygen, however, is not all combined, since contraction follows the second addition of hydrogen. If the total residual volume of 3 (RV) had been used and 6 of hydrogen added, the residual would have been in the same ratio as that observed—that is, 7.5 vols. The total volume would then have been 10 + 5+ 6, and, the residual being 7.5, the contraction would have been 13.5 vols., and in this, all the oxygen is combined; hence Hence the coefficients are 1, 2, 1, and the formula is CH2O. Pupils will need to be guarded against the error of dropping the fractions in the first set of quotients, and therefore calling the coefficients 3, 6, and 3. Number 66.-Possibly instructors may find other methods of illustrating Boyle's Law and Charles's Law conveniently accessible in the equipment of the physical laboratory, and in some circumstances it may be thought unnecessary to study them in detail; or, to perform the experiments before the class may be deemed sufficient. It is much better, however, if practicable, to give every pupil the chance to perform them for himself. If the apparatus described in the text for Boyle's Law is used, it should be prepared beforehand by the instructor in such number as may be thought necessary. I use the gasometric tubes therein described, and for jar a 50 c.c. cylinder ungraduated. For linear measurement, a foot-rule is cheaply and easily obtained. For economy they may be broken in two and still serve. Pupils will often carelessly lift the tubes too high and let the air out. It is quite a puzzle to them to see how they are put into position. This is done by pouring mercury into the gasometric tube until one half or two thirds full, closing with the thumb, and inverting in a sufficiently wide and shallow dish of mercury. From this the tube is transferred by dipping a deflagrating spoon under its mouth and carrying it in this position to the cylinder previously filled with mercury. After the tube is in position the cylinder may be partly emptied for economy of mercury. The cost of the latter is the chief objection to this form of apparatus. This experiment gives another good opportunity to discuss the limit of accuracy. Inasmuch as the reading of volumes is limited to about 0.1 c.c., and this observed value is multiplied by about 30, a constancy of product within three or four units, or even better, is realized. The barometer, if not already understood, should be brought before the class and explained so that they may realize how the relative pressures are measured in units of length. Number 67.-The apparatus for Charles's Law should be prepared beforehand. In Fig. 4 the supporting stand is not shown. I use the ordinary iron stand, clamping the tube A to the upright rod by means of a universal clamp, and supporting the shallow dish of mercury as well as the boiling flask on the iron rings. The tube S is best made of rubber for flexibility. A smaller quantity of water than indicated by the cut is better. The tube A is 12 inches long and 1.5 inches in diameter. It can easily be obtained of dealers. The upper stopper is cork, the lower one of rubber. A foot-rule may be used for linear measurements. The calculation of results I have not given in full, deeming it better to let the pupil try, at least, to reason it out. It involves, besides Boyle's Law, only the application of the simplest arithmetical analysis, but it is surprising how often even bright students stumble in such a matter. The book value of the increment is 0.00367. The experiment realizes 0.0037 ± 0.0001. The following results are taken from a student's note book: 1.018 × 18 = 18.32 c.c. 287.5 = 18.32 269.18 c.c. Final vol. at 1 in. and 100°. = Average increase of vol. for 1°. = = 1.018269.18= 0.0038 c.c.= Decrease in vol. for a decrease from 18° to 0°. Initial vol. at 0° and 1 in. Average increase for 1° for unit vol. at 0° = coefficient of expansion. Number 71/a.-The ordinary test-tubes (9 X 1) are used. It is recommended that they and also the rubber stoppers and strips of asbestos board be prepared beforehand and distributed as needed. The loose plug is quite essential to prevent the fine dust being swept out of the tube by the current of gas. A source of error is the loss of moisture in addition to the oxygen. This comes chiefly from the oxide, hence the preliminary ignition. I have found the chlorate usually dry enough to serve. There is advantage in the use of the oxide because of the lower temperature of reaction, making unnecessary the employment of hard glass ignition tubes. The directions call for the collection of about two or two and a half liters of gas. Some would prefer a smaller scale in order to economize time. But I deem the larger scale preferable, as in 41/3 and 41/4, because the error in weighing has less effect on the final result. The book value for the weight of the liter of oxygen is 1.43 and for the specific gravity 15.9. The students realize for the latter about 16 ± 0.5. Number 81/-The pupils should get the plan of this experiment, first and second steps, before beginning. Of late I have been using the commercial precipitated chalk instead of marble dust for the calcium carbonate, and, on the whole, I believe, like it better. Wrapping in paper (and perhaps in addition binding with a rubber band) serves to reduce the danger of loss by spray. Nitric is preferable to hydrochloric acid, because it gives a more soluble salt, all of which remains in solution in the given conditions. The intermediate bottles called for in 81/2 are two-gallon packing bottles. They are fitted beforehand with rubber stoppers and connecting tubes, and are supplied in considerable number, serving from year to year. I have a few pairs of common hand-bellows hung about the room during the progress of this experiment. The weight of carbon dioxide in the 5 grams of the carbonate, assuming the purity of the latter, is 2.20 grams. The tendency is to get too large results. I judge that 75 per cent of the determinations will not exceed 2.25 grams, which is as good as can be expected, since five weighings are necessary. The measurement of volume is not so close. The volume of the gas at 0° and 760 mm., which weighs (2.20 × 2) 4.40 grams, is 2.226 liters. But the average class result is, according to my experience, between 2.0 and 2.1 liters. This brings the specific gravity nearer 24 than 22, which can not be regarded a very satisfactory determination, and yet I find that it serves the purpose of illustration helpfully. Numbers 72-92.-This law of Gay-Lussac may perhaps be called with propriety the keystone in the system of combining weights, and therefore in the system of expressing the quantitative relations of chemical phenomena. To understand it is therefore very important for beginning students. The reader will note that in the text the matter is presented and discussed without using the conception of atoms and molecules. Some teachers may think this too radical a departure from prevailing methods, but my conviction grows with every year's experience that it is the wiser method. It has been a satisfaction to me to realize |