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Of the two methods given in 41/1 and 41/6, the first has the advantage of directness and simplicity of reaction, but there is sometimes difficulty in carrying the reaction to completeness without loss of oxide or without prolonged heating. Then, too, the porcelain is likely to be corroded by reduction, and I have seen the nitride formed in some instances. The product is often very dark-colored. On the other hand, the second method gives a cleaner reaction, although less direct to the beginner, but it involves the use of iodine, is somewhat more difficult in manipulation, and it uses smaller quantities. However, I have found the two methods together very instructive, as one part of the larger experiment. It is not necessary that the magnesium should be entirely dissolved by the iodine, as small fragments will be oxidized in the final heating. The mixture does not become thoroughly dry on the water-bath. But little alcohol is needed. The tendency is to use an excess, which adds to the difficulty of evaporation.

I find that 75 per cent of the class results reach 1.59 ± 0.04 grams of oxygen for 2.40 grams of magnesium. As an error of 0.02 in the weight of oxygen is multiplied by 2.4 in the final result of the first method and by 4.8 in the second, the range is fully as small as should be expected. In the experiment with zinc about the same proportion of the class reach 1.59 ± 0.07. Since in this one the error of 0.02 in the weight of oxygen is multiplied by 3.3, a variation of ± 0.07 or even more is to be expected. The range of variation in both cases should be made evident and be discussed in the class room.

Numbers 41 and 41/4-The scale of these might be reduced with some economy of time, but the effect of error in the final result would be thus increased. On the whole, I prefer the quantities specified. For collecting bottles I use the ordinary half-gallon packing bottles with glass stopper. On these, both bottle and stopper, is placed a number corresponding with the number of the individual equip

ment of which they are a part. Some such expedient is important, in order to identify each bottle with its content.

For graduated flask, I have found latterly one of 1,200 c.c. capacity more convenient than one of 750 c.c. Common flasks of the right capacity are easily obtained, and it is a small matter to graduate these with sufficient accuracy, using graduated flasks, and placing a mark with a file or a diamond at the proper point of the neck. This effects a considerable economy over the commercial graduated flasks. Thus prepared they are distributed about the laboratory on the side tables. The small residue of water may be weighed as directed, or its volume may be measured in the small cylinder.

About 75 per cent of the class results come within 0.200 ± 0.006 for the hydrogen by magnesium and by zinc. By comparing the results for oxygen and for hydrogen in the individual pupil's work, the practical equivalence often appears even when the partial results are not satisfactory, and this, indeed, is the main point at issue. It is especially recommended that there be thorough quizzing in all the details of the manipulations, observations, and calculations before the corresponding text in Part I is taken up. Then there should be thorough drilling on the law, the corollaries, and the definitions, as they constitute the basis of the whole system which is to follow. It may be suggestive to recall the axiom, Two quantities equal to a third are equal to each other.

Number 47/a.-The dealers supply under the name of "dissolving-tubes" what is preferable to the common testtube, being of a little heavier glass and of smaller diameter for the length. The size 9 x inches is used. The unittube may, perhaps, be obtained of the proper size under the name "specimen tube," but it is easily made to order if not in stock. For rubber bands I use No. 8, getting them in quarter-pound packages. It is a great convenience to get the oxygen of dealers. If this source is not available, this

part of the experiment might be omitted, although the necessary quantity of oxygen could be made in the laboratory without much trouble. In generating the dioxide, too much dilution of the nitric acid interferes with the reaction.

Pupils, it is to be expected, will need some help in these first gas manipulations. In the discussion they should be guarded against the error of assuming that the residual gas is the new product formed. It is recommended that they be called upon to pick out illustrations, especially of the fourth and fifth laws, from the data of Number 49, Part I.

The volume measurements in this experiment are crude, but I have found it useful, as it makes practicable some illustration of volumetric proportions without the more expensive and complicated apparatus that would otherwise be needed. If the Hofmann lecture apparatus is available, it will be especially helpful to show the class experimentally the volume ratio between hydrogen, oxygen, and water.

Number 50%-Believing that the energy relations of chemical phenomena should not be ignored, even in an elementary course, I have introduced this experiment to illustrate heat of neutralization. The oxalic acid is the crystallized commercial article. In the conditions of the experiment the rise of temperature is about 5°, which gives a neutralization heat of 25,000 cal. This is about the book value. A variation of 0.5° in the reading makes a difference of 2,500 in the final result.

Number 62/2.-Problems solved: