504 



NA TURE 



[February 24, 19 10 



These ideas havp, in the words of Ostwald, been the 

 scientific confession of faith of chemists throughout half 

 a century. They have had the greatest influence on scientific 

 thought in every branch of chemistry. It is on the basis 

 of them that we have arrived at a theory of affinity which 

 at the present moment is being developed into one of the 

 most perfect chemical theories. Lastly, it is due to these 

 ideas that the experimental material has been produced 

 which during all time will place the name of Julius Thomsen 

 in the first rank of men of science. 



To go through this material in detail is, as I have said, 

 impossible here. It may be stated generally that practically 

 every simple inorganic process has been investigated calori- 

 metrically by Thomsen, or can be calculated by means of 

 the calorimetric data furnished by him. In the case of 

 organic substances, data have been given for estimating the 

 heat of combustion of a large number of compounds. All 

 these estimations were made by Thomsen personallv, accord- 

 ing_ to a pre-arranged plan, and in systematic succession 

 during a period of more than thirty years. They comprise 

 more than 3500 calorimetrical estimations. It has been 

 truly said that this work is unique in the chemical history 

 of any country. 



Among the results of Thomsen 's thermochemical inquiries 

 which have special value for physical chemistry is his 

 investigation of the phenomena of neutraHsation, in which 

 he shows that the basicity of acids can be estimated thermo- 

 chemically, and that it can in this way be proved whether 

 or not a point of neutrality e.xists. His' observation that the 

 heat of neutralisation is the same for a long series of 

 inorganic acids, such as hydrochloric acid, hvdrobromic acid, 

 hydriodic acid, chloric acid, nitric acid, &c., supports the 

 theory of electrical dissociation, inasmuch as this requires 

 that the heat of neutralisation of the strong acids must in 

 all cases be independent of the nature of the acid, because 

 the process of neutralisation for all of them is the com- 

 bination of the ion of hydrogen in the acid with the ion of 

 hydroxyl of the base to' form water. These investigations 

 also led to the important thermochemical result that the 

 heat of neutralisation of acids (or the heat of their dis- 

 sociation) cannot be considered as a measure of the strength 

 of the acids. 



Another important result is the proof bv experiment of 

 the connection which exists between the changes of the 

 heat-effect with the temperature and the specific heat of 

 the reacting substances. The first law of thermo- 

 dynamics requires the relation indicated by Kirchhoff : 

 (iU/dT = C, — C„ where U is the heat-eflfect,' T the tem- 

 perature, and C, and C, are the heat capacities of the two 

 systems before and after the reaction, pnd Thomsen showed 

 by investigation of the heat of neutralisation, the heat of 

 solution, and the heat of dilution, that this relation was 

 satisfied. For the purpose of his inquiry, the specific heats 

 of a large number of solutions of salts were estimated by 

 an ingenious method, and with an exactness hitherto 

 unattained. 



Of no less importance are Thomsen 's thermochemical 

 investigations on the influence of mass. In the vear 1867 

 Guldberg and Wango published their theory of the' chemical 

 effect of mass. But they had onlv verified the theory to a 

 small extent and in particularly simple cases. They had not 

 mvestigated the complete homogeneous equilibrium, because 

 at that time no method existed for experimental investigation 

 of such homogeneous equilibrium. Thomsen showed that 

 the estimation could be made thermochemicallv. By allow- 

 ing, for instance, an acid to act on a salt of another acid in 

 an aqueous solution, the latter acid will be partlv replaced 

 by the first, which will form a salt. By mixin'g, for in- 

 stance, a solution of sodium sulphate and nitric acid, there 

 is formed sodium nitrate and sulphuric acid, but the process 

 will not proceed to completion. If we have estimated the 

 heat of neutralisation of the two acids with sodium 

 hydroxide, the difference between these two heat-phenomena 

 will give the amount of heat corresponding to the total 

 decomposition of the sodium sulphate, and the heat found 

 experimentally by mixing the two solutions will therefore 

 show to w-hat degree the transformation has taken place. 

 It would be possible to estimate thermochemicallv the 

 amount of the four substances in solution, and thereby, by 

 varying the concentration or the proportion between the 

 initial quantities of substances, to calculate whether the 

 NO. 2104, VOL. 82] 



Guldberg-Waage theory on the effect of mass was confirmed 

 in this case. 



Thomsen applied this method to a large number of 

 different acids and bases, and was enabled thereby to prove 

 the agreement with the law of the influence of mass in all 

 the cases which he examined. He found particularly that 

 the proportion of the one acid which remained combined 

 with the base was constant with mixtures of constant pro- 

 portion. On this basis he propounded the term avidity. 

 which he defined as the tendency of the acid to unite with 

 the base, and he showed that the avidity was independent of 

 the concentration, and only to a small extent varied with the 

 temperature. The term avidity has since acquired great 

 importance, particularly since other and more exact methods 

 for its estimation have been found. Concurrently with this, 

 its meaning has been made clear by the theory of electro- 

 lytic dissociation. 



On the basis of these estimations, Thomsen drew up the 

 first table, based on e.xperiments, of the relative strength 

 of the acids, and the numbers in this table have been found 

 to agree with the results obtained by examining the elec- 

 trical conductivity of the acids. 



It is worth noting that Thomsen not onlv produced the 

 experimental proof of the correctness of the Guldberg-Waage 

 theory of the effect of mass soon after the appearance of 

 tliis theory, but also that he was the first to acknowledge 

 and adopt it. It is remarkable that this work of Thomsen 

 received so little attention, although it appeared in a widely 

 circulated German journal, and it was not until ten years 

 later that the law of the effect of mass was generally 

 recognised, as the result of the work of Ostw-ald and 

 van 't Hoff. 



Although Thomsen 's title to scientific fame rests mainly 

 upon his thermochemical work, his interests extended 

 beyond this particular department of physical chemistry. 

 He woYkcd on chloral hydrate, selenic acid on ammonlacal 

 platinum compounds, and on glucinum platinum chloride, 

 on iodic acid and periodic acid, on hydrogen peroxide, hypo- 

 phosphorous acid, and hydrogenium. He early recognised 

 the importance of Mendeleeff's great generalisation, and 

 contributed to the abundant literature it produced. His 

 paper of 1S95, " On the Probability of the Existence of a 

 Group of Inactive Elements," may be said to have -fore- 

 shadowed the discovery of the congeners of argon. He 

 pointed out that in periodic functions the change from 

 negative to positive value, or the reverse, can only take 

 place by a passage through zero or through infinity ; in the 

 first case, the change is gradual, and in the second case it 

 is sudden. The first case corresponds with the gradual 

 change in electrical character with rising atomic weight in 

 the separate series of the periodic system, and the second 

 case corresponds with a passage from one series to the next. 

 It therefore appears that the passage from one series to 

 the next in the periodic system should take phace through 

 an element which is electrically indifferent. The valency 

 of such an element would be zero, and therefore in this 

 respect also it would represent a transitional stage in the 

 passage from the univalent electronegative elements of the 

 seventh to the univalent electropositive elements of the first 

 group. This indicates the possible existence of a group 

 of inactive elements with the atomic weights 4, 20, 36, 84, 

 132, the first five numbers corresponding fairly closely with 

 the atomic weights respectively of helium, neon, argon, 

 krypton, and xenon {Zeitsch. anorg. Chem., 1805, ix., 283 ; 

 Journ. Chem. Soc, 1896, l.xx., ii., 16). He subsequently 

 made knowm the existence of helium in the red fluorite from 

 Ivigtut. 



As evidence of Thomsen's manipulative ability and his 

 power of accurate work may be mentjoned his determination 

 of the atomic weights of oxygen and hydrogen, and inci- 

 dentally of aluminium. For the atomic weight of hydrogen 

 he obtained the value 1-00825 when = i6, which is prac- 

 tically Identical with that of Morley and Noyes. He 

 further made most accurate estimations of the relative 

 densities of these gases, and of the volumetric ratios in 

 which they enter into the composition of water. His value 

 for the atomic weight of aluminium is nearly identical with 

 that adopted in the last Report of the International Com- 

 mittee on .'Vtomic Weights. 



Thomsen maintained his interest in thermochemical 

 problems up to the end, and w^as a keen and »lear-slghted 

 critic of the work which appeared from time to time during 



