442 PRINCIPLES OF CHEMISTRY 



now, thanks more especially to Stas, be placed in a region void of any 

 experimental support whatever, and therefore not subject to the dis- 

 cipline of the positive data of science. 



Among the platinum metals ruthenium, rhodium, and palladium, 

 by their atomic weights and properties, approach silver, just as iron 

 and its analogues (cobalt and nickel) approach copper in all respects. 

 Gold stands in exactly the sarnie position in relation to the heavy 

 platinum metals, osmium, iridium, and platinum, as copper and 

 silver do to the two preceding series. The atomic weight of gold is 

 nearly equal to their atomic weights ; 28 it is dense like these metals. 

 It also gives various grades of oxidation, which are feeble, both in 

 a basic and an acid sense. Whilst near to osmium, iridium, and pla- 

 tinum, gold at the same time is able, like copper and silver, to form 

 compounds which answer to the type RX that is, oxides of the compo- 

 sition R 2 0. Cuprous chloride, CuCl, silver chloride, AgCl, and aurous 

 chloride, AuCl, are substances which are very much alike in their 

 physical and chemical properties. 28 bis They are insoluble in water, 

 but dissolve in hydrochloric acid and ammonia, in potassium cyanide, 



88 It might be expected from the periodic law and analogies with the series iron, cobalt, 

 nickel, copper, zinc, that the atomic weights of the elements of the series osmium, 

 iridium, platinum, gold, mercury, would rise in this order, and at the time of the esta- 

 blishment of the periodic law (1869), the determinations of Berzelius, Rose, and ofchera 

 gave the following values for the atomic weights : Os = 200, Ir = 197, Pt = 198, Au = 196, 

 Hg = 200. The fulfilment of the expectations of the periodic law was given in the first 

 place by the fresh determinations (Seubert, Dittmar, and Arthur) of the atomic weight of 

 platinum, which proved to be nearly 196, if O = 16 (as Marjgnac, Brauner, and others 

 propose) ; in the second place, by the fact that Seubert proved that the atomic weight of 

 osmium is really less than that of platinum, and approximately Os = 191 ; and, in the 

 third place, by the fact that after the researches of Kriiss, Thorpe, and Laurie there was 

 no doubt that, the atomic weight- of gold is greater than that of platinum namely, 

 nearly 197. 



88 t>u in Chapter XXII., Note 40, we gave the thermal data for certain of the com- 

 pounds of copper of the type CuXa ; we will now cite certain data for the cuprous 

 compounds of the type CuX, which present an analogy to the . corresponding compounds 

 AgX and AuX, some of which were investigated by Thomson in his classical work, 

 ' Thermochemische Untersuchungen ' (Vol. iii., 1883). The data are given in the same 

 manner as in the above-mentioned note : 



R = Cu Ag Au 



R + C1 +33 +29 +6 



RfBi +25 +23 



R + I +16 +14 -6 



R + O +41 +6 -? 



Thus we see in the first place that gold, which possesses a much smaller affinity than Ag, 

 evolves far less heat than an equivalent amount of copper, giving the same compound, and 

 in the second place that the combination of copper with one atom of oxygen disengages 

 more heat than its combination with one atom of a halogen, whilst with silver the reverse 

 is the case. This is connected with the fact that Cu 2 O is more stable under the action 

 of beat than Ag 2 O. 



