ATOMIC THEORY. 



ATOMIC THEORY. 



710 



its Ixue then, sesquicarbonate of potash may be regarded as a neutral 

 carbonate, though, as to its acid, as a sesquicarbonate ; for if lime- 

 water be added to an atom of a sesquicarbonate, carbonate of lime is 

 precipitated equivalent in quantity to 1 4 atom. 



It may now be easily made to appear how it happens that when two 

 neutral salts decompose each other, the new salts obtained by the 

 operation are also neutral ; an atom of nitric acid weighs 54, and one 

 of baryta 76'64, forming when combined, 130'64 of neutral nitrate of 

 baryta ; 86'96 = an atom of neutral sulphate of potash is composed of 

 an atom of sulphuric acid = 40, and an atom of potash = 46'96. Now, 

 when 130-64, or an atom of nitrate of baryta, dissolved in water, is 

 mixed with a solution of 86'96, or an atom, of sulphate of potash, 

 double decomposition ensues, and two new and perfectly neutral salta 

 are formed, namely, 1 atym of nitrate of potash = 100'96, consisting of 

 an atom of nitric acid =54, and an atom of potash = 46-96; this 

 remains in solution ; and there is precipitated an atom of neutral 

 sulphate of baryta = 116'64, composed of 1 atom of sulphuric acid 

 = 40, and 1 atom of baryta = 76'64. The annexed diagram will show the 

 constitution of the salte employed, and those formed by their mutual 

 decomposition ; and it will be seen also, that the weight of the new 

 compounds is precisely equal to those of the original salts. 

 100-96 Nitrate of Potash. 



54 Nitric acid. 



Potash 46-96. 



130-64 

 Nitrate , 

 of 1 

 Baryta 



76-64 Baryta. 



86-96 

 Sulphate 



of 

 Potash. 



Sulphuric acid 40. 



116-64 Sulphate of Baryta. 



Although the atomic theory, thus developed by Dr. Dalton in 1808, 

 contained truths of the highest importance, quite independent of the 

 hypothesis by which they were illustrated, it was not until after the 

 appearance of Dr. Wollaston's Memoirs, ' On Super-acid and Sub-acid 

 Salts,' and ' On a Synoptic Scale of Chemical Equivalents,' that chemists 

 were fully impressed with the practical applications of which the theory 

 wa susceptible. In the first memoir (' Phil. Trans.' 1808), a memoir 

 equally remarkable for its conciseness and clearness, Dr. Wollaston 

 shows, that Dr. Dalton's theory, first applied to determining the con- 

 stitution of gaseous bodies, is applicable to that of acid and basic salts ; 

 and he proves that carbonate of potash contains exactly half the 

 quantity of carbonic acid existing hi the bi-carbonate, by showing that 

 if the latter be heated it loses half its acid, and is reduced to the 

 state of carbonate by the loss ; the same rule was found to exist with 

 the carbonate and bi-carbonate of soda, the sulphate and bi-sulphate 

 of potash, and with three oxalates of potash. 



The paper on the tynoptit tcale appeared in the ' Phil. Trans.' for 

 1814. By this instrument the practical utility of the doctrine of 

 definite proportions was most satisfactorily pointed out. 



This instrument consists of a moveable scale of numbers on the 

 principle of Gunter's scale, so that any number can be placed opposite 

 the names of a series of substances in adjoining columns, arranged in 

 the order of their combining weights, in such a manner that the 

 number denoting the combining weight of a body being placed opposite 

 to its name, 8, for example, opposite to oxygen the numbers express- 

 ing the combining quantities of others will appear opposite to their 

 names; thus copper will be found opposite to 31*65, showing that this 

 quantity of it combines with 8 of oxygen, and opposite to 39'65 will be 

 found oxide of copper. By mere inspection, a great number of im- 

 portant results are obtained. If the composition of a substance with 

 regard to the proportion of its elements is to be determined, the slider 

 is to be so placed that the number 100, or any required number, is oppo- 

 site to its name, and the respective quantities of the ingredients will be 

 found opposite to their names, and the quantities of other compounds 

 required to decompose them : for example, when 68'96 is placed oppo- 

 site to carbonate of potash, 22 will be opposite to carbonic acid, 46'96 

 to potash, 49 to oil of vitriol, 40 to dry sulphuric acid, and 9 to water. 

 Now it is well known that carbonate of potash is decomposed by 

 sulphuric acid ; and on further inspecting the scale, it will be observed 

 that sulphate of potash, the newly-formed salt, is opposite to 86'96, 

 showing the quantity formed by the union of 40 of dry sulphuric acid 

 and 48-96 of potash, while 22 of carbonic acid are expelled, and 9 of 

 water are et free. This simple example is sufficient to show the very 

 extensive use which by mere inspection, may be made of this instru- 

 ment, iu exhibiting the constitution of various oxides, acids, and salts, 

 and of the quantities of substances required to form or decompose 

 compound bodies. 



The use of the term atom has been objected to as hypothetical, 

 because it is said that we have no means of ascertaining or judging of the 

 weight or magnitude of an atom of any element, and that any sup- 

 posed relative weight of their atoms must therefore be a mere hypo- 

 thetical assumption, from which no satisfactory conclusion can be 

 drawn ; and by those who appear to entertain this opinion, other 

 terms, such as equivalent number, molecule, combining proportion, &c. 

 have been substituted for the word atom. 



In the year 1808, Berzelius, in consequence of a perusal of Ritcher's 

 work already alluded to, undertook an investigation of the numerical 

 proportions in which different bodies combine so as to neutralise each 

 other ; these investigations were accompanied by a series of analyses 

 which for number and accuracy have probably never been equalled. 

 As the results of these labours, he laid down certain laws relative to 

 chemical combinations, which, however, are in general only to be con- 

 sidered as corollaries from those determined by Dalton. From these 

 analyses subsequently extended and corrected, a large number of the 

 atomic weights given in the table below have been derived, but in all 

 cases where more recent researches have either indicated errors in 

 the numbers obtained by Berzelius, or have added new elements to the 

 list, the results of these researches have been embodied in the table. 



In constructing tables of atomic weights several standards have 

 been proposed by different chemists, but only two of these have been 

 retained in use, namely, the standard proposed by Dalton, Hydrogen 

 = 1 ; and that adopted by Berzelius, Oxygen = 100. The former is 

 almost universally employed in this country, whilst the latter is 

 almost as generally used on the continent. Both these standards are 

 used in the following table : 



TABLE OP THB ATOMIC WEIGHTS, OH COMBINING PKOPOBTIONS ov ELEMENTARY 



SCBSTANCES. 



Name of Element. 



Atomic Weight. 



Hydrogen = 1 Oxygen = 100. 



Aluminium 13-67 170-90 



Antimony .... 129-03 1612-90 



Arsenicum (Metallic arsenic) . 75-00 937-80 



Barium 68-64 858-03 



Bismuth 212-86 2660-75 



Boron 10-90 130-20 



Bromine 79-97 999-62 



Cadmium 55-74 696-76 



Calcium 20-12 251-31 



Carbon 6-00 75-00 



Cerium 46-00 575-00 



Chlorine 35-49 443-67 



Chromium .... 26-27 328-38 



Cobalt 29-49 368-65 



Copper 31-65 395-60 



Didymium ..... 48-00 600-00 

 Erbium ...... 



Fluorine 19.00 237-50 



Glucinum 6-97 87-12 



Gold 98-33 1229-16 



Hydrogen 1-00 12-50 



Ilmenium ..... 



Iodine 126-88 1585-99 



Iridium 98-56 1232-08 



Iron . .' . . . . 28-04 350-50 

 Lantanium .... 



Lead 103-57 1294-64 



Lithium 6-53 81-66 



Magnesium 12-65 158-14 



Manganese .... 27-57 344-68 



Mercury 100-10 1251-29 



Molybdenum .... 47-69 596-10 



Nickel 29-54 369-33 



Niobium 



Nitrogen H-00 175-06 



Osmium 99-41 1242-62 



Orygen 8-00 100-00 



Palladium 53-24 665-47 



Phosphorus . . ... 32-02 400-30 



Platinum 98-58 1232-08 



Potassium 38-96 487-00 



Rhodium 52-16 651-96 



Kuthcnium 62-11 651-39 



Selenium 39-62 495-28 



Silicon 21-36 267-00 



Silver 108-00 1350-00 



Sodium 22-97 287-17 



Strontium 43-84 548-02 



Sulphur 1600 200-00 



Tantalum, or Co'iumbium . . 



Tellurium 64-08 801-76 



Terbium 



Thorinum 59-50 743-86 



Tin 58-82 735-29 



Titanium 24-12 301-55 



Tungsten 94-64 118S-30 



Uranium 60-00 750-00 



Vanadium .... 08-46 855-84 



Yttrium 



Zinc 82-52 406-59 



Zirconium ..... 33-58 419-73 



