August 8, 1889] 



NATURE 



357 



alcohol, or spirits of wine, C.lIsfOH), are oxidized till they 

 become vinegar or acetic acid, CaHjOlOH). In the same way 

 caustic ammonia, or the combination of ammonia with water, 

 NH3H0O, or NH4(0H), which contains a great deal of hydro- 

 gen, by oxidation exchanges four atoms of hydrogen for two 

 atoms of oxygen, and becomes converted into nitric acid, 

 N02(0H). This process of conversion of ammonia salts into 

 saltpetre goes on in the fields every summer, and with especial 

 rapidity in tropical countries. The method by which this is 

 accomplished, though complex, though involving the agency of 

 all-permeating micro-organisms, is, in substance, the same as 

 that by which alcohol is converted into acetic acid, or glycol, 

 CoH4(OH).2 into oxalic acid, if we view the process of oxidation 

 in the light of the Newtonian principle-. 



But while speaking of the application of the principle of 

 substitution to wa!er, we need not multiply instances, but must 

 turn our attention to two special circumstances which are closely 

 connected with the very mechanism of substitutions. 



In the first place, the replacement of two atoms of hydrogen 

 by one atom of oxygen may take place in two ways, because the 

 hydrogen molecule is composed of two atoms, and therefore, 

 under the influence of oxygen, the molecule forming water may 

 separate before the oxygen has time to take its place. It is for 

 this reason that we find, during the conversion of a]c")hol into 

 acetic acid, that there is an interval during which is formed 

 aldehyde, CgHjO, which, as its very name iuiplies, is "alcohol 

 dehydrogenatum," or alcohol deprived of hydrogen. Hence 

 aldehyde combined with hydrogen yields alcohol, and, united to 

 oxygen, acetic acid. 



For the same reason there should be, and there actually are, in- 

 termediate products between ammonia and nitric acid, NOo(HO), 

 containing either less hydrogen than ammonia, less oxygen than 

 nitric acid, or less water than caustic ammonia. Accordingly 

 we find, among the products of the de-oxidization of nitric acid 

 and the oxidization of ammonia, not only hydroxylamine, but 

 also nitrous oxide, nitrous and nitric anhydrides. Thus, the 

 production of nitrous acid results from the removal of two atoms 

 of hydrogen from caustic ammonia and the substitution of the 

 oxygen for the hydrogen, NO(OH) ; or by the substitution, in 

 ammonia, of three atoms of hydrogen by hydroxyl, N(0H)3, 

 and by the removal of water; N(OH)3— HoO == NO(OH). 

 The peculiarities and properties of nitrous acid, as, for instance, 

 its action on ammonia and its conversion, by oxidation, into 

 nitric acid, are thus clearly revealed. 



On the other hand, in speaking of the principle of substitution 

 as applied to water, it is necessary to observe that hydrogen and 

 hydroxyl, 11 and OH, are not only competent to unite, but also 

 to form combinations with themselves, and thus become Hj and 

 H.jO, ; and such are hydrogen and the peroxide thereof. In gene- 

 ral, if a molecule AB exists, then molecules A A and BB can exist 

 also. A direct reaction of this kind does not, however, take 

 place in water, therefore undoubtedly, at the moment of forftia- 

 tion hydrogen reacts on the peroxide of hydrogen, as we can 

 show at once by experiment ; and further, because the peroxide 

 of hydrogen, H.^Oa, exhibits a structure containing a molecule 

 of hydrogen, H,, and one of oxygen, O.,, either of which is 

 capable of separate existence. The fact, however, may now be 

 taken as thoroughly established, that, at the moment of com- 

 bustion of hydrogen or of the hydrogen compounds, peroxide of 

 hydrogen is always formed, and not only so, but in all prob- 

 ability its formation invariably precedes the formation of water. 

 This was to be expected as a consequence of the law of Avo- 

 gadro and Gerhardt, which leads us to expect this sequence in 

 the case of equal interactions of volumes of vapours and gases ; 

 and in the peroxide of hydrogen we actually have such equal 

 volumes of the elementary gases. 



The instability of peroxide of hydrogen — that is to say, the 

 ease with which it decomposes into water and oxygen, even at 

 the mere contact of porous bodies — accounts for the circumstance 

 that it does not form a permanent product of combustion, and is 

 not produced during the decomposition of water. I may men- 

 tion this additional consideration that, with respect to the per- 

 oxide of hydrogen, we may look for its effecting still further 

 substitutions of hydrogen by means of which we may expect to 

 obtain still more highly oxidized water-compounds, such as 

 H-jOj and \i,f)\. These, Schonbein and Bunsen have long been 

 seeking, and Berthelot is investigating them at this moment. It 

 is probable, however, that the reaction will stop at the last com- 

 pound, because we find that in a number of cases the addition 

 of four atoms of oxygen seems to form a limit. Thus, OsO^, 



KCIO4, KMn04, K2SO4, Na3P04, and such like, represent the 

 highest grades of oxidation.^ 



As for the last forty years, from the times of Berzelius, Dumas, 

 Liebig, Gerhardt, Williamson, Frankland, Kolbe, Kekule, and 

 Butlerowj most theoretical generalizations have centred round 

 organic or carbon compounds, so we will, for the sake of brevity, 

 leave out the discussion of ammonia derivatives, notwithstanding 

 their simplicity in respect to the doc'.rine of substitutions ; wc 

 will dwell n:ore especially on its application to carbon com- 

 pounds, starting from methane, CII4, as the simplest of the 

 hydrocarbons, containing in its molecule one atom of carbon. 

 According to the principles enumerated, we may derive frcm 

 CH4 every combination of the form CH3X, ClI-.Xj, CHX3, and 

 CX4, in which X is an element, or radical, equivalent to hydro- 

 gen — that is to say, competent to take its place or to combine 

 with it. Such are the chlorine sub.-titutes mentioned already, 

 such is wood-spirit, CIIj^OH), in which X is represented by tie 

 residue of water, and such are numerous other carbon deriva- 

 tives. If we continue, with the aid of hydroxyl, further substi- 

 tutions of the hydrogen of methane, we shall obtain successively 

 CH./OH)2, CH(OH)3, and C(0H)4. But if, in proceeding thus, 

 we bear in mind that CH^iOYi)., contains two hydroxyls in the 

 same form as peroxide of hydrogen, HjOa or (OfI).2, contains 

 them— and, moreover, not only in one molecule, but together, 

 attached to one and the same atom of carbon — so here we must 

 look for the same decomposition as that which we find in perox- 

 ide of hydrogen, and accompanied also by the formation of water 

 as an independently existing molecule ; therefore CH2(OH)2 

 should yield, as it actually does, immediately water and the 

 oxide of methylene, CH^O, which is methane with oxygen 

 substituted for two atoms of hydrogen. Exactly in the same 

 manner out of CH(0H)3 are formed water and formic acid, 

 CHO(OII), and out of C(0H)4 is produced water and carbonic 

 acid, or directly carbonic anhydride, COg. which will therefore 

 be nothing else than methane with the double replacement of 

 pairs of hydrogen by oxygen. As nothing leads to the supposi- 

 tion that the four atoms of hydrogen in methane differ one from 

 the other, so it-does not matter by what means we obtain any 

 one of the combinations indicated — they will be identical ; that 

 is to say, there will be no case of actual isomerism, although 

 there may easily be such cases of isomerism as have been 

 distinguished by the term metamerism. 



Formic acid, for example, has two atoms of hydrogen, one 

 attached to the carbon left from the methane, and the other 

 attached to the oxygen which has entered in the form of 

 hydroxyl, and if one of them be replaced by some substance, 

 X, it is evident that we shall obtain bodies of the same com- 

 position, but of different construction, or of dffferent orders of 

 movement among the molecules, and therefore endowed with 

 other properties and reactions. If X be methyl, CH3 — that is 

 to say, a group capable of replacing hydrogen because it is 

 actually contained with hydrogen in methane itself — then by 

 substituting this group for the original hydrogen, we obtain 

 acetic acid, CCH30(0H), out of formic, and by substitution of 

 the hydrogen in its oxide or hydroxyl, we obtain methyl for- 

 miate, CH0(0CII:j). These bodies differ so much from each 

 other physically and chemically that, at first sight, it is hardly 

 possible to admit that they contain the same atoms in identically 

 the same proportions. Acetic acid, for example, boils at a 

 higher temperature than water, and has a higher specific gravity 

 than it, while its metamer, formo-methylic ether, is lighter than 

 water, and boils at 30° — that is to say, it evaporates very easily. 



Let us now turn to carbon compounds containing two atoms 



' Because more than four atoms of hydrogen never unite with one atom of 

 the elements, and because the hydrogen compounds {e.g. HCI, HoS, H3P, 

 H4Si) always form their highest oxides with four atoms of oxygen, and as 

 the highest forms of oxides (OSO4RO4) also contain four of oxygen, and 

 eight groups of the periodic system, corresponding to the highest basic 

 oxides R...O, RO, R-P;). RO.j, R.jOr,, RO:j. R2O7, and RO4, imply the 

 above relationship, and because of the nearest analogues among the ele- 

 ments — such as Mg, Zn. Cd, and Hg; or Cr, Mo, W, and U ; or Si, Ge, 

 Sn, and Pt ; or F, CI, Br, and J, and so forth — not more thaa four are 

 kn'iwn, it seems to me that in these relationships there lies a deep interest 

 and meaning with regard to chemical mechanics. But because, to my ima- 

 gination, the idea of unity of design in Nature, either acting in complex 

 celestial systems or among chemical molecules, is very attractive, especially 

 because the atomic teaching at once acquires its true meaning, I will recall 

 the following facts relating to the solar system. There are eight major 

 planets, of which the four inner ones are not only separated from the four 

 outer by asteroids, but differ from them in many respects, as, for example, 

 in the smallness of their diameters and their greater density. Saturn with 

 his ring has eight satellites, Jupiter and Uranus have each four. It is evi- 

 dent that in the solar systems also we meet with these higher numbers, four 

 and eight, which appear in the combination of chemical molecules. 



