NOMENCLATURE AND NOTATION, CHEMICAL. 



619 



ing condensation of volume, with increase in 

 the number of hydrogen atoms the product- 

 volume in all the above cases being that of a 

 single gaseous molecule = two atoms, the types 

 after the first are called "condensed" (and 

 " derived " also) ; while types still more con- 

 densed, and also " mixed " types, are admitted, 

 to embrace peculiar forms of chemical consti- 

 tution. 



Weight and Volume of Atoms, and of Mole- 

 cules. It is quite impossible, within the limited 

 space here allowed, to present in full either the 

 important facts in the growth of the modern 

 chemical philosophy, or the system of weight 

 and volume determinations, atomic and molec- 

 ular, and the notation corresponding, which 

 find their basis in it. An attempt will, how- 

 ever, be made to give a general outline of the 

 latter subjects, or at the least so far as to em- 

 brace the principles most essential to an under- 

 standing of them. 



Up to a very recent period, the works of 

 chemistry have, in the main, proceeded sub- 

 stantially on the assumption that the volume 

 of hydrogen combined, in forming water, with 

 oxygen, was to be taken as the unit-volume, or 

 at least as representing (if the volumes of oxy- 

 gen and hydrogen in the case were subdivided 

 an equal number of times, and until the sin- 

 gle atom of the former was reached) a single 

 atom of hydrogen. In accordance with this 

 view, the atomic weight of hydrogen being 

 taken as 1, those of O, S, C, and Ca, respec- 

 tively, would be 8, 16, 6, and 20; while the 

 equivalents of the other elements would, of 

 course, be made to accord with the same as- 

 sumption. But the half of the hydrogen vol- 

 ume in water is the quantity which combines 

 with a like volume of chlorine to form chlor- 

 hydric acid gas, and is the proportion also 

 which directly combines with many other ele- 

 ments ; while, besides this, other facts also go 

 to show that this half of the volume of hydro- 

 gen in one unit-volume (compound) of water, 

 is itself the true unit-volume for the elements, 

 including not only hydrogen, oxygen, chlorine, 

 etc., but in fact all, save four or five exceptions 

 yet to be named. 



Now, admitting the conclusion thus arrived 

 at, many of the elements, as chlorine, iodine, 

 nitrogen, potassium, etc., which like hydrogen 

 combine with oxygen and elements of its class in 

 the lowest usual proportion in double volumes, 

 still retain their previously determined atomic 

 weights unchanged ; while of oxygen, sulphur, 

 carbon, and calcium, above referred to, and of 

 certain other elements, in consequence of the 

 fact that their unit-volume (gaseous or volatil- 

 ized, or as having analogies with other bodies 

 which may be so) must be compared with two 

 unit-volumes of "hydrogen and bodies of its class, 

 the previously determined atomic weights re- 

 quire now to be doubled; thus, O, S, C, and 

 Ca are now, respectively, 16, 32, 12, and 40. 

 Berzelius had crossed or barred the symbols of 

 elements which he supposed to 'combine in tico 



atoms each ; but more recently the lar has been 

 turned to the new use of marking the symbols 

 of elements whose atom-weight, calling the 

 hydrogen in water 2 atoms, should be doubled. 

 "Williamson and Hofmann, however, have 

 finally dropped the bar, employing the bare 

 symbols as before, but now to indicate the new 

 or double equivalents, in cases in which such 

 have been determined. 



By the atom of an elementary body is to be 

 understood the smallest proportional weight of 

 the latter which is capable of entering into or 

 existing in chemical combination. It has al- 

 ready been intimated that, calling the ordinary 

 elementary atom one volume, a body in the 

 free state, and whether simple or compound, 

 appears to exist as a collection of molecules, 

 each consisting (as the rule) of two atoms of 

 the given kind. The molecule of a body may, 

 in general, be defined as the smallest propor- 

 tional weight of it capable of existing in tho 

 free or uncombined state. Molecules generally, 

 then, equal in volume two atoms each. Hof- 

 mann takes the cubic litre (equal to 1 cubic 

 decimetre, or -nnnrth of a cubic metre) as his 

 standard unit-volume ; and, as his unit of actual 

 weights, the weight of this volume of hydro- 

 gen at O C. of temperature and 760 mm. 

 pressure ; while, for this weight, which equals 

 0.0896 gramme, he has proposed the name of 

 crith (KpiQr), a barley-corn). He accordingly 

 terms the usual atom-volume monolitral, and 

 the usual molecule-volume dilitral. 



Exceptions to the rule just implied are, on 

 one side, P and As, which combine in half tho 

 usual atom-volume, or are hemi-litral ; and on 

 the other Hg and Cd (to which, also, some 

 authors add Zn), which combine in double the 

 usual atom-volume, or are dilitral ; so that, in 

 these latter, the atom equals in volume the 

 molecule. Exceptions to the general rule of 

 product-volumes, or molecules of compound 

 bodies, appear to exist also in certain instances ; 

 as in case of perchloride of phosphorus, chlo- 

 ride of ammonium,monohydrated sulphuric acid, 

 and a few other bodies, the molecules of which 

 seem to represent 4 volumes, in place of 2. 



Analysis of the four Leading Types. Ob- 

 serving, now, the combining proportions, both 

 by volume and weight, in the four leading 

 types, the following interesting results are ob- 

 tained: : 



Atoms, H+C1 TI ) n H 



H f +0 H 

 H 



Proportions by weight, 1+85.5 1 ^ + ^g 



1) 11 



ih I 



IT 16. 



Proportions by vol- I .... 1 ) +1 ij. + i 



ume, ) 1 ( 1 J 1 f 



Weights of compound ) 

 molecules, ) 



86.5 IS 



+12. 



+L 



Sums of volumes, 



Volume of product, . . . 



Here it is seen that the product-velum es of 



