m 



ATOMIC 



ATOMIC VOLUME. 



7)1 



WiUi respect to Uw utility of tin* atomic theory, we cannot do 

 bettor, in ouncliidiug thi. account of it. than to state, in the word* of 

 Dr. Uaubsoy C Introduction to the Atomic Theory,' p. 87), that " it 

 would he superfluous to enlarge upon the proofs lrm.ly afforded, with 

 wpect to U greater precision it ha* introduced into the *cianoy 

 the wooderful curing of time and Ubour which U derived from it, not 

 only by the |.hil..pUcr in hi* more speculative inquiries, but .-veil by 

 the manufacturing chemist, in the every -day operation* of hU trade." 



It i* evident that, in the present aUte of our knowledge, no sooner 

 have we ascertained the eufct proportion in which a new substance 

 unite* with any one of thoM bodies whoee atomic weight U already 

 determined, than we are imahM to calculate in what quantities it 

 muet oomNn with all the remainder, ao that, intead of being com- 

 pelled, ae heretofore would have appeared necessary, to analyse every 

 existing combination, in order to determine the proportion of iU 

 ingredient*, we might rest contented, were it not for the sake of' 

 obviating the <A~vi- of error in any tingle ox|>eriiuent, with ascer- 

 Uining the compoaition of one out of the whole number of compounds, 

 into which the ingredient in queetion enter*. [CHEMICAL Equv*- 

 LKSTS ; MOLECULES ; CHEMICAL AFFINITY.] 



\ I oil l< - VULl'.MK. The equivalent or atomic weight expresses 

 the reUtion of weight in which bodies combine with each other. The 

 atomic volume, or, aa it U also called, the equivalent, specific, or 

 moUmilr volume, expresses the relation of rolunte in which mib- 

 stances combine. If the equivalent weights of two substance* are to 

 each other a* A : B, and their specific gravities as a : 6, then the spaces 



A "R 

 occupied by these equivalent weights are as _ . -. In other words, 



a ' 6 



the atomic volume of a body is the quotient obtained when the 

 equivalent of a body is divided by its specific gravity ; or atomic 



volume = e q" iv>lent wei ht 



specific gravity 



The atomic volume is not an absolute quantity; it only expresses a 

 relation. Depending on the equivalent, it will differ according as the 

 equivalent is based on the scale in which oxygen = 100, or on that in 

 which oxygen = 8 ; it will also differ according to the assumed unit of 

 specific gravity : only the atomic volumes of those substances are 

 comparable whose specific gravities are based on a common unit. 

 Gases and vapours, whose densities are based on that of atmospheric 

 air as unity, may be compared with each other ; and solids and 

 li.|iiuls, whoee densities are based on that of water as unity may be 

 compared ; but the atomic volumes of gases cannot be compared with 

 those of solids and liquids. 



The following is a list of the atomic volumes of some gases and 

 vapours: 



Atomic 



Weight. 



8 



Spec. Gravity. 



Oxygen . . 



Phosphorus . 



Anraic . . . 

 Hydrogen . 

 Nitrogen . 

 Chlorine. . . 

 Mercury . . 



Water 



Carbonic ociJ 

 Hydrochloric acid 

 Ammonia 

 Chloride of ethjrtc 

 Acetic acid . 

 Valeriaoate of rthyle 



32 



75 

 1 



14 



99-5 



100-1 



9 



22 



3G-5 



17 



C4-5 



60 

 190 



Air 

 1-108 

 4-294 



10-988 



I. -i l.!l 



0-969 

 2-458 

 6925 

 0-623 

 1-524 

 1-264 

 0-589 

 2-23$ 

 2-078 

 4-501 



Atomic 

 Volume. 

 7-22 

 7-22 

 7-22 

 14-44 

 14-44 

 14-44 

 14-44 

 11-44 

 14-44 

 28-88 

 28-88 

 It-Si 

 2888 

 28-88 



It will be seen that the atomic volumes of the element-try substances 

 in this table stand to each other in a very simple relation. They have 

 an atomic volume either equal to that of oxygen, as in the case of 

 phosphorus and arsenic, or twice aa great, as in the case of chlorine, 

 hydrogen, and nitrogen. 



When two gases combine, they do so in simple relations of volume, 

 and the volume of the resultant compound, considered in the state of 

 gas, is either equal to the sum of the volumes of the constituent gases, 

 or stands in a simple relation thereto. Thus, two volumes of hydrogen 

 combine with two volumes of chlorine to form four volumes of hydro- 

 chloric acid gas; two volumes of hydrogen anil <>! volume of oxygen 

 combine to form two volumes of steam ; two volumes of nitrogen and 

 one volume of oxygen combine to form one volume of nitrous oxide ; 

 two volumes of nitrogen and six volumes of hydrogen combine and are 

 condensed to four volumes of ammonia, 



A substance whose atomic volume in the gaseous or vaporous state 

 U equal to that of oxygen, or is two or four times as great, is said to 

 exhibit a condensation to one, two, or four volumes. It will be seen 

 that the organic compound* enumerated in the above table all exhibit 

 a condensation to four volumes j and it has been almost invariably 

 found that all organic bodies present the same regularity. Hence, in 

 fixing the equivalent of any new compound, a determination of the 

 vapour density U an almost indispensable preliminary, as the follow ing 

 example may serve to illustrate. The vapour density of the substance 

 furfural was found to be 8-875, and analysis gave for it a composition 

 whose simple* expression is C.H.O.; these data would give for fur 

 furol an atonic volume expressing a condensation to two volumes, an 



unusual fact with organic bodies. But the formula C, H.O., which 

 equally well expresses the com position, give* for the atomic volume a 

 condensation to four volume*, and thus brings it in accordance with 

 other organic substance*; and on this account, and also because it 

 better expresses the decompositions, the latter formula has been 



' : '' ' 



\ve do not notice in the atomic volumes of solids and of liquids 

 those regularities which prevail among the atomic volume* of gases 

 and vapours. Thus, the atomic volume of iron is 6'3 ; that of 

 mini, 4-6 ; of cadmium, 6'5 ; of tin, 8-0 ; of lead, 9*2 ; of arsenic, 18-3; 

 of antimony, 17'9; of sodium, 237 ; of iodine, 257; of potassium, 

 45-G. But it must be remarked, that these specific gravities (and 

 therewith the atomic volumes) have not been determined for uniform 

 otxlitionn. The specific gravity of a body may vary according as it 

 s crystalline or amorphous, or with its crystalline form if it be dimor- 

 phous. The various solid elements have very different melting ] 

 some of the specific gravities have been determined at points very 

 near the melting temperature, as phosphorus, sodium, ftc. ; others at 

 xiinU very distant, as platinum : the atomic volume is only known for 

 ihe mean temperature of the atmosphere, and hence very unequally 

 distant from the melting points. Probably if the specific gravities were 

 determined for really comparable temperatures, at or near the melting 

 j>oint8, for example, the atomic volumes would no doubt exhibit more 

 pimple relations. 



In certain groups of chemically similar elements the atomic volumes 

 do exhibit agreement : for example, iron 3-d, cobalt 3'5, manganese 3'5, 

 nickel 3'4, irklium 4'S, palladium 4'6, platinum 4'6, molybdenum 5-3, 

 and Tungsten 6-3; lithium 11 '8, sodium 23*7, and potassium 4515, 

 stand very nearly in the relation 1:2:4. Those elements have gene- 

 rally the same atomic volume which are either isomorphou* or can 

 replace each other in isomorphous compounds. 



It has been shown by Kopp that bodies with a similar composition 

 and the some crystalline form have also the same atomic volume. 

 Thus, carbonate of strontia (strontianite), SrOCO,, and carbonate of 

 lead (cerusite), PbOCO, , have the atomic volumes 20'5 and 207 ; sul- 

 phate of magnesia, MgOSO, + 7HO,'and sulphate of zinc, ZnOSO, + 7HO, 

 which have the same crystalline form, have the atomic volumes 70*2 

 and 70-5. 



Many carbonates, as of magnesia, MgOCO,, manganese, MnOCO,, 

 lime, CaOCO., and of iron, FeOCO,, crystallise in the same form, that 

 of the rhombohedron, but differ in the degree of inclination of the 

 corresponding angles; and it has been noticed that the greater the 

 agreement in the corresponding angles, the greater the agreement in 

 the atomic volume. 



The relation between the atomic volumes of a solid compound, and 

 of its conxtituents, has not yet been sufficiently investigated, but the 

 regularity has been observed which was first pointed out by 8chro<li-r. 

 That when from the atomic volumes of analogous compounds, the 

 atomic volumes of the corresponding constituents be subtracted, the 

 common remainder is in many cases the same. Thus the at oi nil- 

 volumes of the analogous oxides of copper, and of zinc, are respectively 

 62 and 7'2 ; U from these the atomic volumes (3'6 and 4-ti) of the 

 corresponding metals be subtracted, the atomic volume of the re- 

 mainder is found to be the same 2'6. The atomic volumes of lead and 

 -re respectively 9'2. By their conversion into the nitrates they 

 undergo the same increase of equivalent, and the atomic volumes of 

 the nitrate of lead, and of silver, are respectively 387 and 39'9 ; if 

 from these numbers the atomic volumes of the metals be subtracted, 

 the remainders 29 "5 and 297 will be seen to be sensibly equal. The 

 atomic volume of oxygen in binoxide of tin is 1'3, in the oxides of zinc, 

 lead, and copper it is 2'6, and in suboxidc of copper 5'2, numbers 

 which stand in a simple relation to each other ; but in many cases in 

 which the atomic volume of oxygen has been determined, such regu- 

 larities do not obtain. 



The atomic volumes of liquids appear to exhibit relations v.hrn 

 compared at their ImiliiiR points. Isomeric liquids, with unequal lioil- 

 ing points, when compared at the same temperatures have vnrioim 

 atomic volumes, but when the expansion of the liquids is known ; and 

 their specific gravities, and therewith their atomic volumes, be calculated 

 for tht- boiling points, the atomic volumes will be found to agree. 

 Thus acetic acid and formiate of methylo, both C, H. (),, which boil 

 respectively at 118 and 36, have at the boiling jxiints the atomic 

 volume OS'S ; at the atomic volume of the former in 557, and of the 

 latter 60-1. 



It has also been noticed that a common difference in the atomic 

 volumes corresponds to the common dilfi-n-iK-e in the members of an 

 homologous series. Thus a difference n x -I'l oarrespcmd* very closely 

 to a difference of n x C,H,. The atom if volumes of acetic acid C,H,<> ( , 

 propionic acid C e H O t , butyric acid C,H,O., and valerianic acid, C 10 

 H, 0,, are respectively 63-6, 85'4, 107'2, and 1807. 



Hydrogen may be replaced in a compound by an equivalent weight 

 of oxygen without any essential alteration in the atomic volume. 

 Alfolinl <',\i a O t , and C 4 H,O ( , have respectively the atomic volumes 

 62-3 and 68'S. Chloride of ethyle C.H.C1, and chloride of othyle C. 

 H,O,C1, have respectively the atomic volumes 72'8 and 74'8. A 

 increase on the atomic volume appears to be produced. Equivalent 

 weights of carbon and hydrogen may replace each other without any 

 essential alteration. Benzole acid C U U O,, and butyrate of nu.-thylo 



