73

ANALOGOUS COMPOSITION OF

The two last results, calculated on the assumption that sulphuretted and selenetted hydrogen are constructed on the type of water, coincide exactly with the results obtained by experiment.

But our previous inquiries (comp. p. 50) have also taught us that the weight of oxygen (16), which is combined, in water-gas, with 2 parts (i.e., with the weight of 2 vols.) of hydrogen, represents the volume-weight of oxygen. If sulphuretted hydrogen and selenetted hydrogen be analogously constituted, we are justified in expecting that the weights of sulphur (32), and of selenium (79), which enter into combination with 2 parts of hydrogen, express also the respective volume-weights of the sulphur and selenium gases. Quite recently the volumeweights of sulphur and selenium gases have been accurately determined by experiment; and the results show that, at temperatures at which these elements assume the perfectly gaseous condition, sulphur-gas is 32 times, and selenium-gas 79 times, heavier than hydrogen.

The analogy of the construction of water, of sulphuretted, and of selenetted hydrogen is thus most satisfactorily made out; and, if the volume-weights of sulphur-gas and selenium-gas be respectively represented by S = 32, and Se = 79, the volumetric information afforded, in these cases, by ponderal analysis, may be thus diagrammatically symbolized :—

WATER-GAS, SULPHURETTED AND SELENETTED HYDROGEN. 79

Dropping, as before, our diagrammatic squares, we obtain the following concise expressions:

It now only remains for us to ascertain, in the last place, whether the typical construction of ammonia be really reproduced, in conformity with our anticipation, in phosphoretted and arsenetted hydrogen; but this inquiry, together with some general remarks on chemical symbolization, must be reserved for a separate lecture.

LECTURE V.

Chemical symbols (continued)-phosphoretted and arsenetted hydrogentheir construction upon the type of ammonia-ponderal analysis and volume-weights of these compounds-exceptional volume-weights of phosphorus and arsenic gases-combining weights of phosphorus and arsenic-general remarks on chemical symbolization-chemical formulæ as instruments of classification-representation of chemical processes in equations-translation of formulæ into concrete weights and volumes— ponderal analysis of sodic and potassic chlorides, oxides, and nitridesdetermination of the combining weights of sodium and potassium. HAVING Considered the structure of the compounds included in two of our typical groups, those, namely, which are formed in the mould of hydrochloric acid and water, we have now lastly to examine, in like manner, our third typical group, that, to wit, at the head of which we have placed ammonia; our object being to ascertain whether the structural type of this body is really reproduced, as we conceive it to be, in phosphoretted and arsenetted hydrogen.

For this purpose we must bear in mind that, while 2 vols. of hydrochloric acid were found to contain 1 vol. of hydrogen combined with 1 vol. of chlorine; and while 2 vols. of water-gas were found to contain 2 vols. of hydrogen combined with 1 vol. of oxygen; we established that 2 vols. of ammonia contain 3 vols. of hydrogen united with 1 vol. of nitrogen. The question now before us is, whether phosphoretted and arsenetted hydrogen tally precisely with this last-mentioned type?

Here, again, chemists have had recourse to ponderal analysis for the desired information; and the composition of ammonia, phosphoretted and arsenetted hydrogen, as respectively determined by aid of the balance, is as follows:

AMMONIA PHOSPHORETTED AND ARSENETTED HYDROGEN. 81

We know, from our previous experiments, that, in the case of ammonia, these weight-results correspond to the relative volume-weight, or specific gravity, subjoined :

3 parts of hydogren + 14 parts of nitrogen

=

17 parts of

2 vols.; whence the volume-weight or sp. gr. of

If, therefore, the volumetric structure of phosphoretted and arsenetted hydrogen be identical with that of ammonia, we ought to obtain, as their relative volume-weights, the following respective values:

For phosphoretted hydrogen :

3 parts of hydrogen + 31 parts of phosphorus

34 parts of phosphoretted hydrogen 2 vols.; whence the volume

=

3 parts of hydrogen + 75 parts of arsenic = 78 parts of arsenetted hydrogen 2 vols.; whence the volume-weight of

These are, in very truth, the volume-weights of phosphoretted and arsenetted hydrogen, as furnished by experiment.

So far, therefore, as our inquiry bears upon the three compounds in question, when already formed, and not upon their elements while as yet uncombined, experiment seems to justify us in affirming the entire identity of the structural type exemplified in these three cases.

But when, from the compounds formed, we turn to consider the volumes of the elements which take part in their formation, we discover a most remarkable and curious discrepancy-the first of its kind that our studies have brought under our notice. To the nature of this discrepancy we must now pay particular attention.

82

EXCEPTIONAL VOLUME-WEIGHTS

The weight of nitrogen (14) which combines with 3 parts by weight of hydrogen to form ammonia, is, as we have seen, the In other words, the combining volume-weight of nitrogen.

weight of nitrogen coincides with its volume-weight. Assuming the chemical construction of phosphoretted and arsenetted hydrogen to be strictly analogous to that of ammonia, we should expect that the weights of phosphorus (31) and of arsenic (75) united with 3 parts of hydrogen, in phosphoretted and arsenetted hydrogen, would represent the volume-weights of phosphorus and arsenic respectively. In other words, we should expect, in their case, the same coincidence of the combining weight with the volume-weight as obtains in the case of nitrogen.

Here, however, we meet with the first exceptions to a rule hitherto unbroken.

The volume-weights of phosphorus and arsenic are not represented by the figures (31 and 75), which express their respective combining weights; though, as we shall presently find, the two values stand in a very simple ratio to each other. Upon experimentally comparing the weights of equal volumes of hydrogen, phosphorus, and arsenic, at the temperature at which these latter bodies (usually solid) become gaseous, we find that phosphorus gas is not 31 times, nor arsenic gas 75 times heavier than hydrogen, but that each of these figures has to be doubled to bring it into conformity with fact. In other words, the volume-weight of phosphorus gas is not, like its combining weight, 31, but 31 x 2 = 62; and the volume-weight of arsenic gas is not, like its combining weight, 75, but 75 x 2 = 150.

This striking and singular deviation from a coincidence hitherto constantly observed, stands before us at present unexplained. We are unable to suggest any end likely to be served by these exceptional volumetric relations of phosphorus and pro tanto deviation from the ammonia arsenic gases, and by the type thus occasioned in phosphoretted and arsenetted hydrogen. But though the purpose of this difference is as yet unknown to us, its nature and limits are most clearly made out.