: I 



ATOMK THKOHV. 



ATOMIC THKoltY. 



hydrogen by weight, or on* atom, cod eight of oxygen by weight, or 

 uoe *u.tu. na in U ease* MI atom of hydrogen being represented by 

 1. an atom of oxygen will be represented by 8 ; and these being the 

 atomic weight of the element*, that of the compound will be obtained 

 by adding them together, thu 



Brincea > atom - 1 



Oxna > atom " 



: 



1 



= 9 



The weight, then, of a compound atom is obtained by adding together 

 the atomic weight* of iti constituent*. Although many elementary 

 bodies unite with hydrogen, there are some which do not, but there u 

 no one which doe* not combine either with hydrogen or with oxygen ; 

 when, therefore, the hydrogen itandard or unit fail* on this account, 

 we may refer to the atom of oxygen, and determine what weight of 

 the substance in queetion, supposing only one compound to be formed, 

 unite* with eight parts by weight, or one atom of oxygen. Now 

 cadmium U a metal of thi description ; it forma no compound with 

 hydrogen, and only one with oxygen, and as 8 parts of thin element 

 unite with 5574 of the metal, to form the only known oxide of it, we 

 ay that the atomic weight of cadmium is 5574, and that the oxide is 



:..; 



Cmdminm 



1 Atom 

 1 atom 



8 

 55-74 



Oxide of cadmium 1 atom = 63-74 



It U, however, possible, though by no means probable, that such an 

 inference may be incorrect, for the oxide in queetion may be composed 

 either of two or uio.e atoms of oxygen united with one atom of the 

 metal, or the contrary, instead of what it is presumed to be ; but the 

 error may be detected by examining the proportion in which the metal 

 unite* with other elements, whose atomic weight* ore already deter- 

 mined. The atomic weight* of sulphur, chlorine, ami selenium, are 

 respectively 16, 86, and 40 : now if, in a series of combinations with 

 then substance*, the compounds containing the largest proportion of 

 metal were constituted of 



exception, though a comparatively rare one, to this rule ; there are 

 two oxide* of thi* metal composed of 



I'v .-.-. 



, ...... 



S 



cs-s 



C'\>.'. 



1 ..;.,.. r 



8 

 SI -85 



..-, :/ r 



Metal 



16 

 55-74 



Chlorine 35-49 

 55-74 



Selenium 39-63 

 Metal 55-74 



we should then conclude, as these agree with the composition of the 

 oxide, as above given, that 5574 is the atomic weight of the metal. But 

 if it was found that the compounds in question containing the largest 

 proportion of metal were constituted of 



Sulphur 18 Chlorine 35-49 Selenium 39-62 



Metal 111-48 Mclal 111-48 McUl 111-48 



we should conclude that the atomic weight of the metal was 111'48, 

 and consequently that the oxide formed of 8 oxygen and 55-74 metal, 

 wa a peroxide, equivalent to 16 = 2 atoms of oxygen + 111'48 = 1 atom 

 of metal. 



If, on the other hand, it appeared that the compound containing the 

 argest proportion of metal consulted of 



Sulphur 16 Chlorine 35-49 Selenium 39-62 



Metal 27-87 Metal 17-87 Metal 27-87 



we must then consider the oxide composed of 8 oxygen and 5574 

 metal a* a nAoxidt, constituted of 1 atom of oxygen =8 + 2 atoms of 

 metal = 57 74. 



Thi* method of proceeding is according to the rule thus laid down 

 by Dr. Dalton : " It is neceraary not only to consider the combinations 

 of A with B, but also those of A with CUE, Ac., as well as those of B 

 with C I>, &c., before we can have good reason to be satisfied with our 

 determination as to the number of atoms which enter into the various 

 compounds." (' New System of Chemical Philosophy,' vol. ii. p. 300.) 



In fact, the protoxide of a metal, that ia, 1 atom oxygen + 1 atom 

 metal, may prisms* mich properties as to prevent it* composition from 

 being by direct mean* accurately ascertained; and it is likewise 

 possible that no protoxide may exist. 



We have alluded to the circumstance, that various compounds of the 

 same two dement* may exist, and supposing an elementary body, aa 

 tin or copper united with two proportion* of oxygen, various questions 

 may arise a* to the constitution of the resulting oxides ; as, whether 

 that which contain* lead oxygen i* a luboxide, or protoxide ; or whether 

 that which contain* most is a protoxide or a peroxide. These are point* 

 which can be determined only by comparison : for example, with respect 

 to oxygen and tin, that oxide which contains least oxygen consist* of 

 8 oxygen + 58 '8 2 metal; that which contain* most, of 16 oxygen 

 + (8~82 metal : now, in this caw, we consider that which contains lut 

 oxygen a* composed of 1 atom of each of it* element*, and that which con- 

 tains mof oxygen a* formed of 2 atoms of oxygen + 1 atom of tin, thus 

 Oxygen . . 1 atom = 8 Oxygen . . 2 atom* = 10 



Tin . 1 atom = 58-82 Tin . . . 1 atom = 58-81 



Protoxide of tin 1 atom = 66-82 Peroxide of tin 1 atom = 74-82 



This rule of amuming that oxide to be a protoxide which contains 

 least oxygen will be generally found correct, especially when confirmed, 

 a* it U in thi* instance, by the corresponding constitution of the two 

 hlorktes and two (ulpburet*. The oxide* of copper, however, form an 



Now in thi* cue, the oxide containing the largest proportion of 

 copper i* regarded as a suboxide, and that containing the smallest pro- 

 portion a* a protoxide, from certain considerations derived from the 

 chemical behaviour of these two oxide*. In general however, tin- 

 rule may be relied upon, that the metallic oxide which contains least 

 oxygen is the protoxide, and that weight of the metal which combines 

 with 8 by weight of oxygen, denote* the weight of it* atom, and their 

 united weight that of the oxide. 



It will be observed with respect to the compounds of oxygen ami 

 tin, that the second ]>rti"ii i that element which unites with the 

 same quantity of the metal, U double the tint. Now upon this and 

 numerous similar facts in foumli-d m- of the moat important and 

 beautiful peculiarities of Dr. Dalton 's theory, sometimes described a* 

 the doctrine of multiple proportion*. In the cam just alluded to, the 

 second portion of oxygen U precisely double the first ; but there are 

 some coses in which the greater proportions are not multiples of the 

 less, by any entire number : for example, there are two well-known 

 oxides of iron consisting of 



Oxygen 8 Oxygen 11 



Iron 18 Iron 18 



The first of these is the protoxide, and the second the peroxide ; but 

 it will be observed, the second portion of oxygen is only one half 

 greater than the first, instead of double, as happens with respect to t in. 

 In fact, the additional quantity is equal to only half an atom of oxygen ; 

 but as the idea of dividing an atom is absurd, the difficulty ia overcome 

 by multiplying both the oxygen and iron by 2, in which case we shall 

 have 12x2 = 24, or 3 atoms of oxygen, combined with 28 x 2=56, 

 2 atoms of iron, and these proportions are perfectly consistent witli the 

 theory. 



Other cases of apparent anomaly occur : thus there are throe oxides 

 of lead, viz., 



Protoxide. 

 Oxygen 8 

 Lead 103-57 



Red oxiclo, 

 Oxygen 10-66 

 Lead 103-57 



Peroxide. 

 Oxygen 1 

 Lead 103-57 



The first and last of these oxides are constituted exactly as the 

 oxides of tin are, the second portion of oxygen being double tli.it of 

 the first ; but the red oxide of lead is composed of an atom of im-t.il 

 and such a quantity of oxygen as is equal to one atom and a third. 

 If, then, both the oxygen and metal be multiplied l.y li. wi> .shall have 

 a compound of 4 atoms of oxygen and 3 atoms of lend, or 32 + 310-71 

 = 34271, and it is found if these 84271 parts of red lead be treated 

 with dilute nitric acid, they are separated into 2 atoms of protoxide 

 = 111-57, which are dissolved, and 1 atom of peroride = 119-57, which 

 remains unacted upon in the state of a brown powder. This case, 

 then, of apparent anomaly is explained by showing that the red oxide 

 of lead U equivalent to, or perhaps composed of, the other two oxides, 

 and is resolvable into them. 



The oxides of manganese offer a still more remarkable case of 

 apparent irregularity of combination, and of the disposition of metallic 

 oxides themselves to combine in definite proportions. 



While on the subject of multiples, it will be proper to adduce one of 

 the most remarkable and regular series of them presented to us. There 

 are five compounds of oxygen and nitrogen, viz., 



Oxygen. Nitroiren. 



Nitrous oxide, composed of 8 = 1 atom +14 = 1 atom. 

 Nitric oxide . . . 16 = 2 atoms + 14 = 

 Ilyponitroua acid ..21 = 3 + 14 = 



Nitrous acid . . .32 = 4,, + 14 = 

 Nitric acid . . . . 40 = 5 -f 14 = 



In these compounds, it will be observed, that to form a new com- 

 pound, 1 atom of oxygen is in every case added to the preceding 

 quantity, and that the atoms of oxygen combined with 1 atom of 

 nitrogen, are 1, 2, 8, 4, and 5. 



The case in which the second portion of oxygen in an oxide, instead 

 of being equal to the first, is only one half greater, has been ] 

 out in the instance of the oxides of iron, and the means by which the 

 absurdity of supposing the existence of half an atom is obviated have 

 been mentioned. Such oxides are, however, generally termed tetqui- 

 oridct, and there are also several instances in which secondary com- 

 pounds are similarly constituted; these are in like manner termed 

 tttquaaltt. The alkalies ammonia and soda, and some other bases, 

 form three compounds with the same acid : for example, we have 



Carbonate of icda, composed of 1 atom acid + 1 atom bate 

 Bicarbonate of Kxla, 1 atom* acid + 1 atom 



and a carbonate of aoda, 8 atoms acid -f 2 atoms 



It i* then evident that the last salt i* equivalent to a compound of 

 14 atom acid + 1 atom bate. Now if an atom of this salt, considered 

 a* a sesquisalt, be added to an atom of nitrate of lime, double decom- 

 position ensues, 1 atom of neutral nitrate of potash remains in s! 

 1 atom of neutral carbonate of lime is precipitated, and carUu 

 equal to half an atom i* expelled in the state of gas. With respect to 



